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[thirdparty/git.git] / merge-ort.c

/*
 * "Ostensibly Recursive's Twin" merge strategy, or "ort" for short.  Meant
 * as a drop-in replacement for the "recursive" merge strategy, allowing one
 * to replace
 *
 *   git merge [-s recursive]
 *
 * with
 *
 *   git merge -s ort
 *
 * Note: git's parser allows the space between '-s' and its argument to be
 * missing.  (Should I have backronymed "ham", "alsa", "kip", "nap, "alvo",
 * "cale", "peedy", or "ins" instead of "ort"?)
 */

#include "cache.h"
#include "merge-ort.h"

#include "alloc.h"
#include "attr.h"
#include "blob.h"
#include "cache-tree.h"
#include "commit.h"
#include "commit-reach.h"
#include "diff.h"
#include "diffcore.h"
#include "dir.h"
#include "entry.h"
#include "ll-merge.h"
#include "object-store.h"
#include "promisor-remote.h"
#include "revision.h"
#include "strmap.h"
#include "submodule.h"
#include "tree.h"
#include "unpack-trees.h"
#include "xdiff-interface.h"

/*
 * We have many arrays of size 3.  Whenever we have such an array, the
 * indices refer to one of the sides of the three-way merge.  This is so
 * pervasive that the constants 0, 1, and 2 are used in many places in the
 * code (especially in arithmetic operations to find the other side's index
 * or to compute a relevant mask), but sometimes these enum names are used
 * to aid code clarity.
 *
 * See also 'filemask' and 'dirmask' in struct conflict_info; the "ith side"
 * referred to there is one of these three sides.
 */
enum merge_side {
        MERGE_BASE = 0,
        MERGE_SIDE1 = 1,
        MERGE_SIDE2 = 2
};

static unsigned RESULT_INITIALIZED = 0x1abe11ed; /* unlikely accidental value */

struct traversal_callback_data {
        unsigned long mask;
        unsigned long dirmask;
        struct name_entry names[3];
};

struct deferred_traversal_data {
        /*
         * possible_trivial_merges: directories to be explored only when needed
         *
         * possible_trivial_merges is a map of directory names to
         * dir_rename_mask.  When we detect that a directory is unchanged on
         * one side, we can sometimes resolve the directory without recursing
         * into it.  Renames are the only things that can prevent such an
         * optimization.  However, for rename sources:
         *   - If no parent directory needed directory rename detection, then
         *     no path under such a directory can be a relevant_source.
         * and for rename destinations:
         *   - If no cached rename has a target path under the directory AND
         *   - If there are no unpaired relevant_sources elsewhere in the
         *     repository
         * then we don't need any path under this directory for a rename
         * destination.  The only way to know the last item above is to defer
         * handling such directories until the end of collect_merge_info(),
         * in handle_deferred_entries().
         *
         * For each we store dir_rename_mask, since that's the only bit of
         * information we need, other than the path, to resume the recursive
         * traversal.
         */
        struct strintmap possible_trivial_merges;

        /*
         * trivial_merges_okay: if trivial directory merges are okay
         *
         * See possible_trivial_merges above.  The "no unpaired
         * relevant_sources elsewhere in the repository" is a single boolean
         * per merge side, which we store here.  Note that while 0 means no,
         * 1 only means "maybe" rather than "yes"; we optimistically set it
         * to 1 initially and only clear when we determine it is unsafe to
         * do trivial directory merges.
         */
        unsigned trivial_merges_okay;

        /*
         * target_dirs: ancestor directories of rename targets
         *
         * target_dirs contains all directory names that are an ancestor of
         * any rename destination.
         */
        struct strset target_dirs;
};

struct rename_info {
        /*
         * All variables that are arrays of size 3 correspond to data tracked
         * for the sides in enum merge_side.  Index 0 is almost always unused
         * because we often only need to track information for MERGE_SIDE1 and
         * MERGE_SIDE2 (MERGE_BASE can't have rename information since renames
         * are determined relative to what changed since the MERGE_BASE).
         */

        /*
         * pairs: pairing of filenames from diffcore_rename()
         */
        struct diff_queue_struct pairs[3];

        /*
         * dirs_removed: directories removed on a given side of history.
         *
         * The keys of dirs_removed[side] are the directories that were removed
         * on the given side of history.  The value of the strintmap for each
         * directory is a value from enum dir_rename_relevance.
         */
        struct strintmap dirs_removed[3];

        /*
         * dir_rename_count: tracking where parts of a directory were renamed to
         *
         * When files in a directory are renamed, they may not all go to the
         * same location.  Each strmap here tracks:
         *      old_dir => {new_dir => int}
         * That is, dir_rename_count[side] is a strmap to a strintmap.
         */
        struct strmap dir_rename_count[3];

        /*
         * dir_renames: computed directory renames
         *
         * This is a map of old_dir => new_dir and is derived in part from
         * dir_rename_count.
         */
        struct strmap dir_renames[3];

        /*
         * relevant_sources: deleted paths wanted in rename detection, and why
         *
         * relevant_sources is a set of deleted paths on each side of
         * history for which we need rename detection.  If a path is deleted
         * on one side of history, we need to detect if it is part of a
         * rename if either
         *    * the file is modified/deleted on the other side of history
         *    * we need to detect renames for an ancestor directory
         * If neither of those are true, we can skip rename detection for
         * that path.  The reason is stored as a value from enum
         * file_rename_relevance, as the reason can inform the algorithm in
         * diffcore_rename_extended().
         */
        struct strintmap relevant_sources[3];

        struct deferred_traversal_data deferred[3];

        /*
         * dir_rename_mask:
         *   0: optimization removing unmodified potential rename source okay
         *   2 or 4: optimization okay, but must check for files added to dir
         *   7: optimization forbidden; need rename source in case of dir rename
         */
        unsigned dir_rename_mask:3;

        /*
         * callback_data_*: supporting data structures for alternate traversal
         *
         * We sometimes need to be able to traverse through all the files
         * in a given tree before all immediate subdirectories within that
         * tree.  Since traverse_trees() doesn't do that naturally, we have
         * a traverse_trees_wrapper() that stores any immediate
         * subdirectories while traversing files, then traverses the
         * immediate subdirectories later.  These callback_data* variables
         * store the information for the subdirectories so that we can do
         * that traversal order.
         */
        struct traversal_callback_data *callback_data;
        int callback_data_nr, callback_data_alloc;
        char *callback_data_traverse_path;

        /*
         * merge_trees: trees passed to the merge algorithm for the merge
         *
         * merge_trees records the trees passed to the merge algorithm.  But,
         * this data also is stored in merge_result->priv.  If a sequence of
         * merges are being done (such as when cherry-picking or rebasing),
         * the next merge can look at this and re-use information from
         * previous merges under certain circumstances.
         *
         * See also all the cached_* variables.
         */
        struct tree *merge_trees[3];

        /*
         * cached_pairs_valid_side: which side's cached info can be reused
         *
         * See the description for merge_trees.  For repeated merges, at most
         * only one side's cached information can be used.  Valid values:
         *   MERGE_SIDE2: cached data from side2 can be reused
         *   MERGE_SIDE1: cached data from side1 can be reused
         *   0:           no cached data can be reused
         *   -1:          See redo_after_renames; both sides can be reused.
         */
        int cached_pairs_valid_side;

        /*
         * cached_pairs: Caching of renames and deletions.
         *
         * These are mappings recording renames and deletions of individual
         * files (not directories).  They are thus a map from an old
         * filename to either NULL (for deletions) or a new filename (for
         * renames).
         */
        struct strmap cached_pairs[3];

        /*
         * cached_target_names: just the destinations from cached_pairs
         *
         * We sometimes want a fast lookup to determine if a given filename
         * is one of the destinations in cached_pairs.  cached_target_names
         * is thus duplicative information, but it provides a fast lookup.
         */
        struct strset cached_target_names[3];

        /*
         * cached_irrelevant: Caching of rename_sources that aren't relevant.
         *
         * If we try to detect a rename for a source path and succeed, it's
         * part of a rename.  If we try to detect a rename for a source path
         * and fail, then it's a delete.  If we do not try to detect a rename
         * for a path, then we don't know if it's a rename or a delete.  If
         * merge-ort doesn't think the path is relevant, then we just won't
         * cache anything for that path.  But there's a slight problem in
         * that merge-ort can think a path is RELEVANT_LOCATION, but due to
         * commit 9bd342137e ("diffcore-rename: determine which
         * relevant_sources are no longer relevant", 2021-03-13),
         * diffcore-rename can downgrade the path to RELEVANT_NO_MORE.  To
         * avoid excessive calls to diffcore_rename_extended() we still need
         * to cache such paths, though we cannot record them as either
         * renames or deletes.  So we cache them here as a "turned out to be
         * irrelevant *for this commit*" as they are often also irrelevant
         * for subsequent commits, though we will have to do some extra
         * checking to see whether such paths become relevant for rename
         * detection when cherry-picking/rebasing subsequent commits.
         */
        struct strset cached_irrelevant[3];

        /*
         * redo_after_renames: optimization flag for "restarting" the merge
         *
         * Sometimes it pays to detect renames, cache them, and then
         * restart the merge operation from the beginning.  The reason for
         * this is that when we know where all the renames are, we know
         * whether a certain directory has any paths under it affected --
         * and if a directory is not affected then it permits us to do
         * trivial tree merging in more cases.  Doing trivial tree merging
         * prevents the need to run process_entry() on every path
         * underneath trees that can be trivially merged, and
         * process_entry() is more expensive than collect_merge_info() --
         * plus, the second collect_merge_info() will be much faster since
         * it doesn't have to recurse into the relevant trees.
         *
         * Values for this flag:
         *   0 = don't bother, not worth it (or conditions not yet checked)
         *   1 = conditions for optimization met, optimization worthwhile
         *   2 = we already did it (don't restart merge yet again)
         */
        unsigned redo_after_renames;

        /*
         * needed_limit: value needed for inexact rename detection to run
         *
         * If the current rename limit wasn't high enough for inexact
         * rename detection to run, this records the limit needed.  Otherwise,
         * this value remains 0.
         */
        int needed_limit;
};

struct merge_options_internal {
        /*
         * paths: primary data structure in all of merge ort.
         *
         * The keys of paths:
         *   * are full relative paths from the toplevel of the repository
         *     (e.g. "drivers/firmware/raspberrypi.c").
         *   * store all relevant paths in the repo, both directories and
         *     files (e.g. drivers, drivers/firmware would also be included)
         *   * these keys serve to intern all the path strings, which allows
         *     us to do pointer comparison on directory names instead of
         *     strcmp; we just have to be careful to use the interned strings.
         *
         * The values of paths:
         *   * either a pointer to a merged_info, or a conflict_info struct
         *   * merged_info contains all relevant information for a
         *     non-conflicted entry.
         *   * conflict_info contains a merged_info, plus any additional
         *     information about a conflict such as the higher orders stages
         *     involved and the names of the paths those came from (handy
         *     once renames get involved).
         *   * a path may start "conflicted" (i.e. point to a conflict_info)
         *     and then a later step (e.g. three-way content merge) determines
         *     it can be cleanly merged, at which point it'll be marked clean
         *     and the algorithm will ignore any data outside the contained
         *     merged_info for that entry
         *   * If an entry remains conflicted, the merged_info portion of a
         *     conflict_info will later be filled with whatever version of
         *     the file should be placed in the working directory (e.g. an
         *     as-merged-as-possible variation that contains conflict markers).
         */
        struct strmap paths;

        /*
         * conflicted: a subset of keys->values from "paths"
         *
         * conflicted is basically an optimization between process_entries()
         * and record_conflicted_index_entries(); the latter could loop over
         * ALL the entries in paths AGAIN and look for the ones that are
         * still conflicted, but since process_entries() has to loop over
         * all of them, it saves the ones it couldn't resolve in this strmap
         * so that record_conflicted_index_entries() can iterate just the
         * relevant entries.
         */
        struct strmap conflicted;

        /*
         * pool: memory pool for fast allocation/deallocation
         *
         * We allocate room for lots of filenames and auxiliary data
         * structures in merge_options_internal, and it tends to all be
         * freed together too.  Using a memory pool for these provides a
         * nice speedup.
         */
        struct mem_pool pool;

        /*
         * output: special messages and conflict notices for various paths
         *
         * This is a map of pathnames (a subset of the keys in "paths" above)
         * to strbufs.  It gathers various warning/conflict/notice messages
         * for later processing.
         */
        struct strmap output;

        /*
         * renames: various data relating to rename detection
         */
        struct rename_info renames;

        /*
         * attr_index: hacky minimal index used for renormalization
         *
         * renormalization code _requires_ an index, though it only needs to
         * find a .gitattributes file within the index.  So, when
         * renormalization is important, we create a special index with just
         * that one file.
         */
        struct index_state attr_index;

        /*
         * current_dir_name, toplevel_dir: temporary vars
         *
         * These are used in collect_merge_info_callback(), and will set the
         * various merged_info.directory_name for the various paths we get;
         * see documentation for that variable and the requirements placed on
         * that field.
         */
        const char *current_dir_name;
        const char *toplevel_dir;

        /* call_depth: recursion level counter for merging merge bases */
        int call_depth;
};

struct version_info {
        struct object_id oid;
        unsigned short mode;
};

struct merged_info {
        /* if is_null, ignore result.  otherwise result has oid & mode */
        struct version_info result;
        unsigned is_null:1;

        /*
         * clean: whether the path in question is cleanly merged.
         *
         * see conflict_info.merged for more details.
         */
        unsigned clean:1;

        /*
         * basename_offset: offset of basename of path.
         *
         * perf optimization to avoid recomputing offset of final '/'
         * character in pathname (0 if no '/' in pathname).
         */
        size_t basename_offset;

         /*
          * directory_name: containing directory name.
          *
          * Note that we assume directory_name is constructed such that
          *    strcmp(dir1_name, dir2_name) == 0 iff dir1_name == dir2_name,
          * i.e. string equality is equivalent to pointer equality.  For this
          * to hold, we have to be careful setting directory_name.
          */
        const char *directory_name;
};

struct conflict_info {
        /*
         * merged: the version of the path that will be written to working tree
         *
         * WARNING: It is critical to check merged.clean and ensure it is 0
         * before reading any conflict_info fields outside of merged.
         * Allocated merge_info structs will always have clean set to 1.
         * Allocated conflict_info structs will have merged.clean set to 0
         * initially.  The merged.clean field is how we know if it is safe
         * to access other parts of conflict_info besides merged; if a
         * conflict_info's merged.clean is changed to 1, the rest of the
         * algorithm is not allowed to look at anything outside of the
         * merged member anymore.
         */
        struct merged_info merged;

        /* oids & modes from each of the three trees for this path */
        struct version_info stages[3];

        /* pathnames for each stage; may differ due to rename detection */
        const char *pathnames[3];

        /* Whether this path is/was involved in a directory/file conflict */
        unsigned df_conflict:1;

        /*
         * Whether this path is/was involved in a non-content conflict other
         * than a directory/file conflict (e.g. rename/rename, rename/delete,
         * file location based on possible directory rename).
         */
        unsigned path_conflict:1;

        /*
         * For filemask and dirmask, the ith bit corresponds to whether the
         * ith entry is a file (filemask) or a directory (dirmask).  Thus,
         * filemask & dirmask is always zero, and filemask | dirmask is at
         * most 7 but can be less when a path does not appear as either a
         * file or a directory on at least one side of history.
         *
         * Note that these masks are related to enum merge_side, as the ith
         * entry corresponds to side i.
         *
         * These values come from a traverse_trees() call; more info may be
         * found looking at tree-walk.h's struct traverse_info,
         * particularly the documentation above the "fn" member (note that
         * filemask = mask & ~dirmask from that documentation).
         */
        unsigned filemask:3;
        unsigned dirmask:3;

        /*
         * Optimization to track which stages match, to avoid the need to
         * recompute it in multiple steps. Either 0 or at least 2 bits are
         * set; if at least 2 bits are set, their corresponding stages match.
         */
        unsigned match_mask:3;
};

/*** Function Grouping: various utility functions ***/

/*
 * For the next three macros, see warning for conflict_info.merged.
 *
 * In each of the below, mi is a struct merged_info*, and ci was defined
 * as a struct conflict_info* (but we need to verify ci isn't actually
 * pointed at a struct merged_info*).
 *
 * INITIALIZE_CI: Assign ci to mi but only if it's safe; set to NULL otherwise.
 * VERIFY_CI: Ensure that something we assigned to a conflict_info* is one.
 * ASSIGN_AND_VERIFY_CI: Similar to VERIFY_CI but do assignment first.
 */
#define INITIALIZE_CI(ci, mi) do {                                           \
        (ci) = (!(mi) || (mi)->clean) ? NULL : (struct conflict_info *)(mi); \
} while (0)
#define VERIFY_CI(ci) assert(ci && !ci->merged.clean);
#define ASSIGN_AND_VERIFY_CI(ci, mi) do {    \
        (ci) = (struct conflict_info *)(mi);  \
        assert((ci) && !(mi)->clean);        \
} while (0)

static void free_strmap_strings(struct strmap *map)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;

        strmap_for_each_entry(map, &iter, entry) {
                free((char*)entry->key);
        }
}

static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
                                          int reinitialize)
{
        struct rename_info *renames = &opti->renames;
        int i;
        void (*strmap_clear_func)(struct strmap *, int) =
                reinitialize ? strmap_partial_clear : strmap_clear;
        void (*strintmap_clear_func)(struct strintmap *) =
                reinitialize ? strintmap_partial_clear : strintmap_clear;
        void (*strset_clear_func)(struct strset *) =
                reinitialize ? strset_partial_clear : strset_clear;

        strmap_clear_func(&opti->paths, 0);

        /*
         * All keys and values in opti->conflicted are a subset of those in
         * opti->paths.  We don't want to deallocate anything twice, so we
         * don't free the keys and we pass 0 for free_values.
         */
        strmap_clear_func(&opti->conflicted, 0);

        if (opti->attr_index.cache_nr) /* true iff opt->renormalize */
                discard_index(&opti->attr_index);

        /* Free memory used by various renames maps */
        for (i = MERGE_SIDE1; i <= MERGE_SIDE2; ++i) {
                strintmap_clear_func(&renames->dirs_removed[i]);
                strmap_clear_func(&renames->dir_renames[i], 0);
                strintmap_clear_func(&renames->relevant_sources[i]);
                if (!reinitialize)
                        assert(renames->cached_pairs_valid_side == 0);
                if (i != renames->cached_pairs_valid_side &&
                    -1 != renames->cached_pairs_valid_side) {
                        strset_clear_func(&renames->cached_target_names[i]);
                        strmap_clear_func(&renames->cached_pairs[i], 1);
                        strset_clear_func(&renames->cached_irrelevant[i]);
                        partial_clear_dir_rename_count(&renames->dir_rename_count[i]);
                        if (!reinitialize)
                                strmap_clear(&renames->dir_rename_count[i], 1);
                }
        }
        for (i = MERGE_SIDE1; i <= MERGE_SIDE2; ++i) {
                strintmap_clear_func(&renames->deferred[i].possible_trivial_merges);
                strset_clear_func(&renames->deferred[i].target_dirs);
                renames->deferred[i].trivial_merges_okay = 1; /* 1 == maybe */
        }
        renames->cached_pairs_valid_side = 0;
        renames->dir_rename_mask = 0;

        if (!reinitialize) {
                struct hashmap_iter iter;
                struct strmap_entry *e;

                /* Release and free each strbuf found in output */
                strmap_for_each_entry(&opti->output, &iter, e) {
                        struct strbuf *sb = e->value;
                        strbuf_release(sb);
                        /*
                         * While strictly speaking we don't need to free(sb)
                         * here because we could pass free_values=1 when
                         * calling strmap_clear() on opti->output, that would
                         * require strmap_clear to do another
                         * strmap_for_each_entry() loop, so we just free it
                         * while we're iterating anyway.
                         */
                        free(sb);
                }
                strmap_clear(&opti->output, 0);
        }

        mem_pool_discard(&opti->pool, 0);

        /* Clean out callback_data as well. */
        FREE_AND_NULL(renames->callback_data);
        renames->callback_data_nr = renames->callback_data_alloc = 0;
}

__attribute__((format (printf, 2, 3)))
static int err(struct merge_options *opt, const char *err, ...)
{
        va_list params;
        struct strbuf sb = STRBUF_INIT;

        strbuf_addstr(&sb, "error: ");
        va_start(params, err);
        strbuf_vaddf(&sb, err, params);
        va_end(params);

        error("%s", sb.buf);
        strbuf_release(&sb);

        return -1;
}

static void format_commit(struct strbuf *sb,
                          int indent,
                          struct commit *commit)
{
        struct merge_remote_desc *desc;
        struct pretty_print_context ctx = {0};
        ctx.abbrev = DEFAULT_ABBREV;

        strbuf_addchars(sb, ' ', indent);
        desc = merge_remote_util(commit);
        if (desc) {
                strbuf_addf(sb, "virtual %s\n", desc->name);
                return;
        }

        format_commit_message(commit, "%h %s", sb, &ctx);
        strbuf_addch(sb, '\n');
}

__attribute__((format (printf, 4, 5)))
static void path_msg(struct merge_options *opt,
                     const char *path,
                     int omittable_hint, /* skippable under --remerge-diff */
                     const char *fmt, ...)
{
        va_list ap;
        struct strbuf *sb = strmap_get(&opt->priv->output, path);
        if (!sb) {
                sb = xmalloc(sizeof(*sb));
                strbuf_init(sb, 0);
                strmap_put(&opt->priv->output, path, sb);
        }

        va_start(ap, fmt);
        strbuf_vaddf(sb, fmt, ap);
        va_end(ap);

        strbuf_addch(sb, '\n');
}

static struct diff_filespec *pool_alloc_filespec(struct mem_pool *pool,
                                                 const char *path)
{
        /* Similar to alloc_filespec(), but allocate from pool and reuse path */
        struct diff_filespec *spec;

        spec = mem_pool_calloc(pool, 1, sizeof(*spec));
        spec->path = (char*)path; /* spec won't modify it */

        spec->count = 1;
        spec->is_binary = -1;
        return spec;
}

static struct diff_filepair *pool_diff_queue(struct mem_pool *pool,
                                             struct diff_queue_struct *queue,
                                             struct diff_filespec *one,
                                             struct diff_filespec *two)
{
        /* Same code as diff_queue(), except allocate from pool */
        struct diff_filepair *dp;

        dp = mem_pool_calloc(pool, 1, sizeof(*dp));
        dp->one = one;
        dp->two = two;
        if (queue)
                diff_q(queue, dp);
        return dp;
}

/* add a string to a strbuf, but converting "/" to "_" */
static void add_flattened_path(struct strbuf *out, const char *s)
{
        size_t i = out->len;
        strbuf_addstr(out, s);
        for (; i < out->len; i++)
                if (out->buf[i] == '/')
                        out->buf[i] = '_';
}

static char *unique_path(struct strmap *existing_paths,
                         const char *path,
                         const char *branch)
{
        struct strbuf newpath = STRBUF_INIT;
        int suffix = 0;
        size_t base_len;

        strbuf_addf(&newpath, "%s~", path);
        add_flattened_path(&newpath, branch);

        base_len = newpath.len;
        while (strmap_contains(existing_paths, newpath.buf)) {
                strbuf_setlen(&newpath, base_len);
                strbuf_addf(&newpath, "_%d", suffix++);
        }

        return strbuf_detach(&newpath, NULL);
}

/*** Function Grouping: functions related to collect_merge_info() ***/

static int traverse_trees_wrapper_callback(int n,
                                           unsigned long mask,
                                           unsigned long dirmask,
                                           struct name_entry *names,
                                           struct traverse_info *info)
{
        struct merge_options *opt = info->data;
        struct rename_info *renames = &opt->priv->renames;
        unsigned filemask = mask & ~dirmask;

        assert(n==3);

        if (!renames->callback_data_traverse_path)
                renames->callback_data_traverse_path = xstrdup(info->traverse_path);

        if (filemask && filemask == renames->dir_rename_mask)
                renames->dir_rename_mask = 0x07;

        ALLOC_GROW(renames->callback_data, renames->callback_data_nr + 1,
                   renames->callback_data_alloc);
        renames->callback_data[renames->callback_data_nr].mask = mask;
        renames->callback_data[renames->callback_data_nr].dirmask = dirmask;
        COPY_ARRAY(renames->callback_data[renames->callback_data_nr].names,
                   names, 3);
        renames->callback_data_nr++;

        return mask;
}

/*
 * Much like traverse_trees(), BUT:
 *   - read all the tree entries FIRST, saving them
 *   - note that the above step provides an opportunity to compute necessary
 *     additional details before the "real" traversal
 *   - loop through the saved entries and call the original callback on them
 */
static int traverse_trees_wrapper(struct index_state *istate,
                                  int n,
                                  struct tree_desc *t,
                                  struct traverse_info *info)
{
        int ret, i, old_offset;
        traverse_callback_t old_fn;
        char *old_callback_data_traverse_path;
        struct merge_options *opt = info->data;
        struct rename_info *renames = &opt->priv->renames;

        assert(renames->dir_rename_mask == 2 || renames->dir_rename_mask == 4);

        old_callback_data_traverse_path = renames->callback_data_traverse_path;
        old_fn = info->fn;
        old_offset = renames->callback_data_nr;

        renames->callback_data_traverse_path = NULL;
        info->fn = traverse_trees_wrapper_callback;
        ret = traverse_trees(istate, n, t, info);
        if (ret < 0)
                return ret;

        info->traverse_path = renames->callback_data_traverse_path;
        info->fn = old_fn;
        for (i = old_offset; i < renames->callback_data_nr; ++i) {
                info->fn(n,
                         renames->callback_data[i].mask,
                         renames->callback_data[i].dirmask,
                         renames->callback_data[i].names,
                         info);
        }

        renames->callback_data_nr = old_offset;
        free(renames->callback_data_traverse_path);
        renames->callback_data_traverse_path = old_callback_data_traverse_path;
        info->traverse_path = NULL;
        return 0;
}

static void setup_path_info(struct merge_options *opt,
                            struct string_list_item *result,
                            const char *current_dir_name,
                            int current_dir_name_len,
                            char *fullpath, /* we'll take over ownership */
                            struct name_entry *names,
                            struct name_entry *merged_version,
                            unsigned is_null,     /* boolean */
                            unsigned df_conflict, /* boolean */
                            unsigned filemask,
                            unsigned dirmask,
                            int resolved          /* boolean */)
{
        /* result->util is void*, so mi is a convenience typed variable */
        struct merged_info *mi;

        assert(!is_null || resolved);
        assert(!df_conflict || !resolved); /* df_conflict implies !resolved */
        assert(resolved == (merged_version != NULL));

        mi = mem_pool_calloc(&opt->priv->pool, 1,
                             resolved ? sizeof(struct merged_info) :
                                        sizeof(struct conflict_info));
        mi->directory_name = current_dir_name;
        mi->basename_offset = current_dir_name_len;
        mi->clean = !!resolved;
        if (resolved) {
                mi->result.mode = merged_version->mode;
                oidcpy(&mi->result.oid, &merged_version->oid);
                mi->is_null = !!is_null;
        } else {
                int i;
                struct conflict_info *ci;

                ASSIGN_AND_VERIFY_CI(ci, mi);
                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        ci->pathnames[i] = fullpath;
                        ci->stages[i].mode = names[i].mode;
                        oidcpy(&ci->stages[i].oid, &names[i].oid);
                }
                ci->filemask = filemask;
                ci->dirmask = dirmask;
                ci->df_conflict = !!df_conflict;
                if (dirmask)
                        /*
                         * Assume is_null for now, but if we have entries
                         * under the directory then when it is complete in
                         * write_completed_directory() it'll update this.
                         * Also, for D/F conflicts, we have to handle the
                         * directory first, then clear this bit and process
                         * the file to see how it is handled -- that occurs
                         * near the top of process_entry().
                         */
                        mi->is_null = 1;
        }
        strmap_put(&opt->priv->paths, fullpath, mi);
        result->string = fullpath;
        result->util = mi;
}

static void add_pair(struct merge_options *opt,
                     struct name_entry *names,
                     const char *pathname,
                     unsigned side,
                     unsigned is_add /* if false, is_delete */,
                     unsigned match_mask,
                     unsigned dir_rename_mask)
{
        struct diff_filespec *one, *two;
        struct rename_info *renames = &opt->priv->renames;
        int names_idx = is_add ? side : 0;

        if (is_add) {
                assert(match_mask == 0 || match_mask == 6);
                if (strset_contains(&renames->cached_target_names[side],
                                    pathname))
                        return;
        } else {
                unsigned content_relevant = (match_mask == 0);
                unsigned location_relevant = (dir_rename_mask == 0x07);

                assert(match_mask == 0 || match_mask == 3 || match_mask == 5);

                /*
                 * If pathname is found in cached_irrelevant[side] due to
                 * previous pick but for this commit content is relevant,
                 * then we need to remove it from cached_irrelevant.
                 */
                if (content_relevant)
                        /* strset_remove is no-op if strset doesn't have key */
                        strset_remove(&renames->cached_irrelevant[side],
                                      pathname);

                /*
                 * We do not need to re-detect renames for paths that we already
                 * know the pairing, i.e. for cached_pairs (or
                 * cached_irrelevant).  However, handle_deferred_entries() needs
                 * to loop over the union of keys from relevant_sources[side] and
                 * cached_pairs[side], so for simplicity we set relevant_sources
                 * for all the cached_pairs too and then strip them back out in
                 * prune_cached_from_relevant() at the beginning of
                 * detect_regular_renames().
                 */
                if (content_relevant || location_relevant) {
                        /* content_relevant trumps location_relevant */
                        strintmap_set(&renames->relevant_sources[side], pathname,
                                      content_relevant ? RELEVANT_CONTENT : RELEVANT_LOCATION);
                }

                /*
                 * Avoid creating pair if we've already cached rename results.
                 * Note that we do this after setting relevant_sources[side]
                 * as noted in the comment above.
                 */
                if (strmap_contains(&renames->cached_pairs[side], pathname) ||
                    strset_contains(&renames->cached_irrelevant[side], pathname))
                        return;
        }

        one = pool_alloc_filespec(&opt->priv->pool, pathname);
        two = pool_alloc_filespec(&opt->priv->pool, pathname);
        fill_filespec(is_add ? two : one,
                      &names[names_idx].oid, 1, names[names_idx].mode);
        pool_diff_queue(&opt->priv->pool, &renames->pairs[side], one, two);
}

static void collect_rename_info(struct merge_options *opt,
                                struct name_entry *names,
                                const char *dirname,
                                const char *fullname,
                                unsigned filemask,
                                unsigned dirmask,
                                unsigned match_mask)
{
        struct rename_info *renames = &opt->priv->renames;
        unsigned side;

        /*
         * Update dir_rename_mask (determines ignore-rename-source validity)
         *
         * dir_rename_mask helps us keep track of when directory rename
         * detection may be relevant.  Basically, whenver a directory is
         * removed on one side of history, and a file is added to that
         * directory on the other side of history, directory rename
         * detection is relevant (meaning we have to detect renames for all
         * files within that directory to deduce where the directory
         * moved).  Also, whenever a directory needs directory rename
         * detection, due to the "majority rules" choice for where to move
         * it (see t6423 testcase 1f), we also need to detect renames for
         * all files within subdirectories of that directory as well.
         *
         * Here we haven't looked at files within the directory yet, we are
         * just looking at the directory itself.  So, if we aren't yet in
         * a case where a parent directory needed directory rename detection
         * (i.e. dir_rename_mask != 0x07), and if the directory was removed
         * on one side of history, record the mask of the other side of
         * history in dir_rename_mask.
         */
        if (renames->dir_rename_mask != 0x07 &&
            (dirmask == 3 || dirmask == 5)) {
                /* simple sanity check */
                assert(renames->dir_rename_mask == 0 ||
                       renames->dir_rename_mask == (dirmask & ~1));
                /* update dir_rename_mask; have it record mask of new side */
                renames->dir_rename_mask = (dirmask & ~1);
        }

        /* Update dirs_removed, as needed */
        if (dirmask == 1 || dirmask == 3 || dirmask == 5) {
                /* absent_mask = 0x07 - dirmask; sides = absent_mask/2 */
                unsigned sides = (0x07 - dirmask)/2;
                unsigned relevance = (renames->dir_rename_mask == 0x07) ?
                                        RELEVANT_FOR_ANCESTOR : NOT_RELEVANT;
                /*
                 * Record relevance of this directory.  However, note that
                 * when collect_merge_info_callback() recurses into this
                 * directory and calls collect_rename_info() on paths
                 * within that directory, if we find a path that was added
                 * to this directory on the other side of history, we will
                 * upgrade this value to RELEVANT_FOR_SELF; see below.
                 */
                if (sides & 1)
                        strintmap_set(&renames->dirs_removed[1], fullname,
                                      relevance);
                if (sides & 2)
                        strintmap_set(&renames->dirs_removed[2], fullname,
                                      relevance);
        }

        /*
         * Here's the block that potentially upgrades to RELEVANT_FOR_SELF.
         * When we run across a file added to a directory.  In such a case,
         * find the directory of the file and upgrade its relevance.
         */
        if (renames->dir_rename_mask == 0x07 &&
            (filemask == 2 || filemask == 4)) {
                /*
                 * Need directory rename for parent directory on other side
                 * of history from added file.  Thus
                 *    side = (~filemask & 0x06) >> 1
                 * or
                 *    side = 3 - (filemask/2).
                 */
                unsigned side = 3 - (filemask >> 1);
                strintmap_set(&renames->dirs_removed[side], dirname,
                              RELEVANT_FOR_SELF);
        }

        if (filemask == 0 || filemask == 7)
                return;

        for (side = MERGE_SIDE1; side <= MERGE_SIDE2; ++side) {
                unsigned side_mask = (1 << side);

                /* Check for deletion on side */
                if ((filemask & 1) && !(filemask & side_mask))
                        add_pair(opt, names, fullname, side, 0 /* delete */,
                                 match_mask & filemask,
                                 renames->dir_rename_mask);

                /* Check for addition on side */
                if (!(filemask & 1) && (filemask & side_mask))
                        add_pair(opt, names, fullname, side, 1 /* add */,
                                 match_mask & filemask,
                                 renames->dir_rename_mask);
        }
}

static int collect_merge_info_callback(int n,
                                       unsigned long mask,
                                       unsigned long dirmask,
                                       struct name_entry *names,
                                       struct traverse_info *info)
{
        /*
         * n is 3.  Always.
         * common ancestor (mbase) has mask 1, and stored in index 0 of names
         * head of side 1  (side1) has mask 2, and stored in index 1 of names
         * head of side 2  (side2) has mask 4, and stored in index 2 of names
         */
        struct merge_options *opt = info->data;
        struct merge_options_internal *opti = opt->priv;
        struct rename_info *renames = &opt->priv->renames;
        struct string_list_item pi;  /* Path Info */
        struct conflict_info *ci; /* typed alias to pi.util (which is void*) */
        struct name_entry *p;
        size_t len;
        char *fullpath;
        const char *dirname = opti->current_dir_name;
        unsigned prev_dir_rename_mask = renames->dir_rename_mask;
        unsigned filemask = mask & ~dirmask;
        unsigned match_mask = 0; /* will be updated below */
        unsigned mbase_null = !(mask & 1);
        unsigned side1_null = !(mask & 2);
        unsigned side2_null = !(mask & 4);
        unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
                                        names[0].mode == names[1].mode &&
                                        oideq(&names[0].oid, &names[1].oid));
        unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
                                        names[0].mode == names[2].mode &&
                                        oideq(&names[0].oid, &names[2].oid));
        unsigned sides_match = (!side1_null && !side2_null &&
                                names[1].mode == names[2].mode &&
                                oideq(&names[1].oid, &names[2].oid));

        /*
         * Note: When a path is a file on one side of history and a directory
         * in another, we have a directory/file conflict.  In such cases, if
         * the conflict doesn't resolve from renames and deletions, then we
         * always leave directories where they are and move files out of the
         * way.  Thus, while struct conflict_info has a df_conflict field to
         * track such conflicts, we ignore that field for any directories at
         * a path and only pay attention to it for files at the given path.
         * The fact that we leave directories were they are also means that
         * we do not need to worry about getting additional df_conflict
         * information propagated from parent directories down to children
         * (unlike, say traverse_trees_recursive() in unpack-trees.c, which
         * sets a newinfo.df_conflicts field specifically to propagate it).
         */
        unsigned df_conflict = (filemask != 0) && (dirmask != 0);

        /* n = 3 is a fundamental assumption. */
        if (n != 3)
                BUG("Called collect_merge_info_callback wrong");

        /*
         * A bunch of sanity checks verifying that traverse_trees() calls
         * us the way I expect.  Could just remove these at some point,
         * though maybe they are helpful to future code readers.
         */
        assert(mbase_null == is_null_oid(&names[0].oid));
        assert(side1_null == is_null_oid(&names[1].oid));
        assert(side2_null == is_null_oid(&names[2].oid));
        assert(!mbase_null || !side1_null || !side2_null);
        assert(mask > 0 && mask < 8);

        /* Determine match_mask */
        if (side1_matches_mbase)
                match_mask = (side2_matches_mbase ? 7 : 3);
        else if (side2_matches_mbase)
                match_mask = 5;
        else if (sides_match)
                match_mask = 6;

        /*
         * Get the name of the relevant filepath, which we'll pass to
         * setup_path_info() for tracking.
         */
        p = names;
        while (!p->mode)
                p++;
        len = traverse_path_len(info, p->pathlen);

        /* +1 in both of the following lines to include the NUL byte */
        fullpath = mem_pool_alloc(&opt->priv->pool, len + 1);
        make_traverse_path(fullpath, len + 1, info, p->path, p->pathlen);

        /*
         * If mbase, side1, and side2 all match, we can resolve early.  Even
         * if these are trees, there will be no renames or anything
         * underneath.
         */
        if (side1_matches_mbase && side2_matches_mbase) {
                /* mbase, side1, & side2 all match; use mbase as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+0, mbase_null, 0 /* df_conflict */,
                                filemask, dirmask, 1 /* resolved */);
                return mask;
        }

        /*
         * If the sides match, and all three paths are present and are
         * files, then we can take either as the resolution.  We can't do
         * this with trees, because there may be rename sources from the
         * merge_base.
         */
        if (sides_match && filemask == 0x07) {
                /* use side1 (== side2) version as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+1, side1_null, 0,
                                filemask, dirmask, 1);
                return mask;
        }

        /*
         * If side1 matches mbase and all three paths are present and are
         * files, then we can use side2 as the resolution.  We cannot
         * necessarily do so this for trees, because there may be rename
         * destinations within side2.
         */
        if (side1_matches_mbase && filemask == 0x07) {
                /* use side2 version as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+2, side2_null, 0,
                                filemask, dirmask, 1);
                return mask;
        }

        /* Similar to above but swapping sides 1 and 2 */
        if (side2_matches_mbase && filemask == 0x07) {
                /* use side1 version as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+1, side1_null, 0,
                                filemask, dirmask, 1);
                return mask;
        }

        /*
         * Sometimes we can tell that a source path need not be included in
         * rename detection -- namely, whenever either
         *    side1_matches_mbase && side2_null
         * or
         *    side2_matches_mbase && side1_null
         * However, we call collect_rename_info() even in those cases,
         * because exact renames are cheap and would let us remove both a
         * source and destination path.  We'll cull the unneeded sources
         * later.
         */
        collect_rename_info(opt, names, dirname, fullpath,
                            filemask, dirmask, match_mask);

        /*
         * None of the special cases above matched, so we have a
         * provisional conflict.  (Rename detection might allow us to
         * unconflict some more cases, but that comes later so all we can
         * do now is record the different non-null file hashes.)
         */
        setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                        names, NULL, 0, df_conflict, filemask, dirmask, 0);

        ci = pi.util;
        VERIFY_CI(ci);
        ci->match_mask = match_mask;

        /* If dirmask, recurse into subdirectories */
        if (dirmask) {
                struct traverse_info newinfo;
                struct tree_desc t[3];
                void *buf[3] = {NULL, NULL, NULL};
                const char *original_dir_name;
                int i, ret, side;

                /*
                 * Check for whether we can avoid recursing due to one side
                 * matching the merge base.  The side that does NOT match is
                 * the one that might have a rename destination we need.
                 */
                assert(!side1_matches_mbase || !side2_matches_mbase);
                side = side1_matches_mbase ? MERGE_SIDE2 :
                        side2_matches_mbase ? MERGE_SIDE1 : MERGE_BASE;
                if (filemask == 0 && (dirmask == 2 || dirmask == 4)) {
                        /*
                         * Also defer recursing into new directories; set up a
                         * few variables to let us do so.
                         */
                        ci->match_mask = (7 - dirmask);
                        side = dirmask / 2;
                }
                if (renames->dir_rename_mask != 0x07 &&
                    side != MERGE_BASE &&
                    renames->deferred[side].trivial_merges_okay &&
                    !strset_contains(&renames->deferred[side].target_dirs,
                                     pi.string)) {
                        strintmap_set(&renames->deferred[side].possible_trivial_merges,
                                      pi.string, renames->dir_rename_mask);
                        renames->dir_rename_mask = prev_dir_rename_mask;
                        return mask;
                }

                /* We need to recurse */
                ci->match_mask &= filemask;
                newinfo = *info;
                newinfo.prev = info;
                newinfo.name = p->path;
                newinfo.namelen = p->pathlen;
                newinfo.pathlen = st_add3(newinfo.pathlen, p->pathlen, 1);
                /*
                 * If this directory we are about to recurse into cared about
                 * its parent directory (the current directory) having a D/F
                 * conflict, then we'd propagate the masks in this way:
                 *    newinfo.df_conflicts |= (mask & ~dirmask);
                 * But we don't worry about propagating D/F conflicts.  (See
                 * comment near setting of local df_conflict variable near
                 * the beginning of this function).
                 */

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        if (i == 1 && side1_matches_mbase)
                                t[1] = t[0];
                        else if (i == 2 && side2_matches_mbase)
                                t[2] = t[0];
                        else if (i == 2 && sides_match)
                                t[2] = t[1];
                        else {
                                const struct object_id *oid = NULL;
                                if (dirmask & 1)
                                        oid = &names[i].oid;
                                buf[i] = fill_tree_descriptor(opt->repo,
                                                              t + i, oid);
                        }
                        dirmask >>= 1;
                }

                original_dir_name = opti->current_dir_name;
                opti->current_dir_name = pi.string;
                if (renames->dir_rename_mask == 0 ||
                    renames->dir_rename_mask == 0x07)
                        ret = traverse_trees(NULL, 3, t, &newinfo);
                else
                        ret = traverse_trees_wrapper(NULL, 3, t, &newinfo);
                opti->current_dir_name = original_dir_name;
                renames->dir_rename_mask = prev_dir_rename_mask;

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++)
                        free(buf[i]);

                if (ret < 0)
                        return -1;
        }

        return mask;
}

static void resolve_trivial_directory_merge(struct conflict_info *ci, int side)
{
        VERIFY_CI(ci);
        assert((side == 1 && ci->match_mask == 5) ||
               (side == 2 && ci->match_mask == 3));
        oidcpy(&ci->merged.result.oid, &ci->stages[side].oid);
        ci->merged.result.mode = ci->stages[side].mode;
        ci->merged.is_null = is_null_oid(&ci->stages[side].oid);
        ci->match_mask = 0;
        ci->merged.clean = 1; /* (ci->filemask == 0); */
}

static int handle_deferred_entries(struct merge_options *opt,
                                   struct traverse_info *info)
{
        struct rename_info *renames = &opt->priv->renames;
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        int side, ret = 0;
        int path_count_before, path_count_after = 0;

        path_count_before = strmap_get_size(&opt->priv->paths);
        for (side = MERGE_SIDE1; side <= MERGE_SIDE2; side++) {
                unsigned optimization_okay = 1;
                struct strintmap copy;

                /* Loop over the set of paths we need to know rename info for */
                strset_for_each_entry(&renames->relevant_sources[side],
                                      &iter, entry) {
                        char *rename_target, *dir, *dir_marker;
                        struct strmap_entry *e;

                        /*
                         * If we don't know delete/rename info for this path,
                         * then we need to recurse into all trees to get all
                         * adds to make sure we have it.
                         */
                        if (strset_contains(&renames->cached_irrelevant[side],
                                            entry->key))
                                continue;
                        e = strmap_get_entry(&renames->cached_pairs[side],
                                             entry->key);
                        if (!e) {
                                optimization_okay = 0;
                                break;
                        }

                        /* If this is a delete, we have enough info already */
                        rename_target = e->value;
                        if (!rename_target)
                                continue;

                        /* If we already walked the rename target, we're good */
                        if (strmap_contains(&opt->priv->paths, rename_target))
                                continue;

                        /*
                         * Otherwise, we need to get a list of directories that
                         * will need to be recursed into to get this
                         * rename_target.
                         */
                        dir = xstrdup(rename_target);
                        while ((dir_marker = strrchr(dir, '/'))) {
                                *dir_marker = '\0';
                                if (strset_contains(&renames->deferred[side].target_dirs,
                                                    dir))
                                        break;
                                strset_add(&renames->deferred[side].target_dirs,
                                           dir);
                        }
                        free(dir);
                }
                renames->deferred[side].trivial_merges_okay = optimization_okay;
                /*
                 * We need to recurse into any directories in
                 * possible_trivial_merges[side] found in target_dirs[side].
                 * But when we recurse, we may need to queue up some of the
                 * subdirectories for possible_trivial_merges[side].  Since
                 * we can't safely iterate through a hashmap while also adding
                 * entries, move the entries into 'copy', iterate over 'copy',
                 * and then we'll also iterate anything added into
                 * possible_trivial_merges[side] once this loop is done.
                 */
                copy = renames->deferred[side].possible_trivial_merges;
                strintmap_init_with_options(&renames->deferred[side].possible_trivial_merges,
                                            0,
                                            &opt->priv->pool,
                                            0);
                strintmap_for_each_entry(&copy, &iter, entry) {
                        const char *path = entry->key;
                        unsigned dir_rename_mask = (intptr_t)entry->value;
                        struct conflict_info *ci;
                        unsigned dirmask;
                        struct tree_desc t[3];
                        void *buf[3] = {NULL,};
                        int i;

                        ci = strmap_get(&opt->priv->paths, path);
                        VERIFY_CI(ci);
                        dirmask = ci->dirmask;

                        if (optimization_okay &&
                            !strset_contains(&renames->deferred[side].target_dirs,
                                             path)) {
                                resolve_trivial_directory_merge(ci, side);
                                continue;
                        }

                        info->name = path;
                        info->namelen = strlen(path);
                        info->pathlen = info->namelen + 1;

                        for (i = 0; i < 3; i++, dirmask >>= 1) {
                                if (i == 1 && ci->match_mask == 3)
                                        t[1] = t[0];
                                else if (i == 2 && ci->match_mask == 5)
                                        t[2] = t[0];
                                else if (i == 2 && ci->match_mask == 6)
                                        t[2] = t[1];
                                else {
                                        const struct object_id *oid = NULL;
                                        if (dirmask & 1)
                                                oid = &ci->stages[i].oid;
                                        buf[i] = fill_tree_descriptor(opt->repo,
                                                                      t+i, oid);
                                }
                        }

                        ci->match_mask &= ci->filemask;
                        opt->priv->current_dir_name = path;
                        renames->dir_rename_mask = dir_rename_mask;
                        if (renames->dir_rename_mask == 0 ||
                            renames->dir_rename_mask == 0x07)
                                ret = traverse_trees(NULL, 3, t, info);
                        else
                                ret = traverse_trees_wrapper(NULL, 3, t, info);

                        for (i = MERGE_BASE; i <= MERGE_SIDE2; i++)
                                free(buf[i]);

                        if (ret < 0)
                                return ret;
                }
                strintmap_clear(&copy);
                strintmap_for_each_entry(&renames->deferred[side].possible_trivial_merges,
                                         &iter, entry) {
                        const char *path = entry->key;
                        struct conflict_info *ci;

                        ci = strmap_get(&opt->priv->paths, path);
                        VERIFY_CI(ci);

                        assert(renames->deferred[side].trivial_merges_okay &&
                               !strset_contains(&renames->deferred[side].target_dirs,
                                                path));
                        resolve_trivial_directory_merge(ci, side);
                }
                if (!optimization_okay || path_count_after)
                        path_count_after = strmap_get_size(&opt->priv->paths);
        }
        if (path_count_after) {
                /*
                 * The choice of wanted_factor here does not affect
                 * correctness, only performance.  When the
                 *    path_count_after / path_count_before
                 * ratio is high, redoing after renames is a big
                 * performance boost.  I suspect that redoing is a wash
                 * somewhere near a value of 2, and below that redoing will
                 * slow things down.  I applied a fudge factor and picked
                 * 3; see the commit message when this was introduced for
                 * back of the envelope calculations for this ratio.
                 */
                const int wanted_factor = 3;

                /* We should only redo collect_merge_info one time */
                assert(renames->redo_after_renames == 0);

                if (path_count_after / path_count_before >= wanted_factor) {
                        renames->redo_after_renames = 1;
                        renames->cached_pairs_valid_side = -1;
                }
        } else if (renames->redo_after_renames == 2)
                renames->redo_after_renames = 0;
        return ret;
}

static int collect_merge_info(struct merge_options *opt,
                              struct tree *merge_base,
                              struct tree *side1,
                              struct tree *side2)
{
        int ret;
        struct tree_desc t[3];
        struct traverse_info info;

        opt->priv->toplevel_dir = "";
        opt->priv->current_dir_name = opt->priv->toplevel_dir;
        setup_traverse_info(&info, opt->priv->toplevel_dir);
        info.fn = collect_merge_info_callback;
        info.data = opt;
        info.show_all_errors = 1;

        parse_tree(merge_base);
        parse_tree(side1);
        parse_tree(side2);
        init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
        init_tree_desc(t + 1, side1->buffer, side1->size);
        init_tree_desc(t + 2, side2->buffer, side2->size);

        trace2_region_enter("merge", "traverse_trees", opt->repo);
        ret = traverse_trees(NULL, 3, t, &info);
        if (ret == 0)
                ret = handle_deferred_entries(opt, &info);
        trace2_region_leave("merge", "traverse_trees", opt->repo);

        return ret;
}

/*** Function Grouping: functions related to threeway content merges ***/

static int find_first_merges(struct repository *repo,
                             const char *path,
                             struct commit *a,
                             struct commit *b,
                             struct object_array *result)
{
        int i, j;
        struct object_array merges = OBJECT_ARRAY_INIT;
        struct commit *commit;
        int contains_another;

        char merged_revision[GIT_MAX_HEXSZ + 2];
        const char *rev_args[] = { "rev-list", "--merges", "--ancestry-path",
                                   "--all", merged_revision, NULL };
        struct rev_info revs;
        struct setup_revision_opt rev_opts;

        memset(result, 0, sizeof(struct object_array));
        memset(&rev_opts, 0, sizeof(rev_opts));

        /* get all revisions that merge commit a */
        xsnprintf(merged_revision, sizeof(merged_revision), "^%s",
                  oid_to_hex(&a->object.oid));
        repo_init_revisions(repo, &revs, NULL);
        rev_opts.submodule = path;
        /* FIXME: can't handle linked worktrees in submodules yet */
        revs.single_worktree = path != NULL;
        setup_revisions(ARRAY_SIZE(rev_args)-1, rev_args, &revs, &rev_opts);

        /* save all revisions from the above list that contain b */
        if (prepare_revision_walk(&revs))
                die("revision walk setup failed");
        while ((commit = get_revision(&revs)) != NULL) {
                struct object *o = &(commit->object);
                if (in_merge_bases(b, commit))
                        add_object_array(o, NULL, &merges);
        }
        reset_revision_walk();

        /* Now we've got all merges that contain a and b. Prune all
         * merges that contain another found merge and save them in
         * result.
         */
        for (i = 0; i < merges.nr; i++) {
                struct commit *m1 = (struct commit *) merges.objects[i].item;

                contains_another = 0;
                for (j = 0; j < merges.nr; j++) {
                        struct commit *m2 = (struct commit *) merges.objects[j].item;
                        if (i != j && in_merge_bases(m2, m1)) {
                                contains_another = 1;
                                break;
                        }
                }

                if (!contains_another)
                        add_object_array(merges.objects[i].item, NULL, result);
        }

        object_array_clear(&merges);
        return result->nr;
}

static int merge_submodule(struct merge_options *opt,
                           const char *path,
                           const struct object_id *o,
                           const struct object_id *a,
                           const struct object_id *b,
                           struct object_id *result)
{
        struct commit *commit_o, *commit_a, *commit_b;
        int parent_count;
        struct object_array merges;
        struct strbuf sb = STRBUF_INIT;

        int i;
        int search = !opt->priv->call_depth;

        /* store fallback answer in result in case we fail */
        oidcpy(result, opt->priv->call_depth ? o : a);

        /* we can not handle deletion conflicts */
        if (is_null_oid(o))
                return 0;
        if (is_null_oid(a))
                return 0;
        if (is_null_oid(b))
                return 0;

        if (add_submodule_odb(path)) {
                path_msg(opt, path, 0,
                         _("Failed to merge submodule %s (not checked out)"),
                         path);
                return 0;
        }

        if (!(commit_o = lookup_commit_reference(opt->repo, o)) ||
            !(commit_a = lookup_commit_reference(opt->repo, a)) ||
            !(commit_b = lookup_commit_reference(opt->repo, b))) {
                path_msg(opt, path, 0,
                         _("Failed to merge submodule %s (commits not present)"),
                         path);
                return 0;
        }

        /* check whether both changes are forward */
        if (!in_merge_bases(commit_o, commit_a) ||
            !in_merge_bases(commit_o, commit_b)) {
                path_msg(opt, path, 0,
                         _("Failed to merge submodule %s "
                           "(commits don't follow merge-base)"),
                         path);
                return 0;
        }

        /* Case #1: a is contained in b or vice versa */
        if (in_merge_bases(commit_a, commit_b)) {
                oidcpy(result, b);
                path_msg(opt, path, 1,
                         _("Note: Fast-forwarding submodule %s to %s"),
                         path, oid_to_hex(b));
                return 1;
        }
        if (in_merge_bases(commit_b, commit_a)) {
                oidcpy(result, a);
                path_msg(opt, path, 1,
                         _("Note: Fast-forwarding submodule %s to %s"),
                         path, oid_to_hex(a));
                return 1;
        }

        /*
         * Case #2: There are one or more merges that contain a and b in
         * the submodule. If there is only one, then present it as a
         * suggestion to the user, but leave it marked unmerged so the
         * user needs to confirm the resolution.
         */

        /* Skip the search if makes no sense to the calling context.  */
        if (!search)
                return 0;

        /* find commit which merges them */
        parent_count = find_first_merges(opt->repo, path, commit_a, commit_b,
                                         &merges);
        switch (parent_count) {
        case 0:
                path_msg(opt, path, 0, _("Failed to merge submodule %s"), path);
                break;

        case 1:
                format_commit(&sb, 4,
                              (struct commit *)merges.objects[0].item);
                path_msg(opt, path, 0,
                         _("Failed to merge submodule %s, but a possible merge "
                           "resolution exists:\n%s\n"),
                         path, sb.buf);
                path_msg(opt, path, 1,
                         _("If this is correct simply add it to the index "
                           "for example\n"
                           "by using:\n\n"
                           "  git update-index --cacheinfo 160000 %s \"%s\"\n\n"
                           "which will accept this suggestion.\n"),
                         oid_to_hex(&merges.objects[0].item->oid), path);
                strbuf_release(&sb);
                break;
        default:
                for (i = 0; i < merges.nr; i++)
                        format_commit(&sb, 4,
                                      (struct commit *)merges.objects[i].item);
                path_msg(opt, path, 0,
                         _("Failed to merge submodule %s, but multiple "
                           "possible merges exist:\n%s"), path, sb.buf);
                strbuf_release(&sb);
        }

        object_array_clear(&merges);
        return 0;
}

static void initialize_attr_index(struct merge_options *opt)
{
        /*
         * The renormalize_buffer() functions require attributes, and
         * annoyingly those can only be read from the working tree or from
         * an index_state.  merge-ort doesn't have an index_state, so we
         * generate a fake one containing only attribute information.
         */
        struct merged_info *mi;
        struct index_state *attr_index = &opt->priv->attr_index;
        struct cache_entry *ce;

        attr_index->initialized = 1;

        if (!opt->renormalize)
                return;

        mi = strmap_get(&opt->priv->paths, GITATTRIBUTES_FILE);
        if (!mi)
                return;

        if (mi->clean) {
                int len = strlen(GITATTRIBUTES_FILE);
                ce = make_empty_cache_entry(attr_index, len);
                ce->ce_mode = create_ce_mode(mi->result.mode);
                ce->ce_flags = create_ce_flags(0);
                ce->ce_namelen = len;
                oidcpy(&ce->oid, &mi->result.oid);
                memcpy(ce->name, GITATTRIBUTES_FILE, len);
                add_index_entry(attr_index, ce,
                                ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
                get_stream_filter(attr_index, GITATTRIBUTES_FILE, &ce->oid);
        } else {
                int stage, len;
                struct conflict_info *ci;

                ASSIGN_AND_VERIFY_CI(ci, mi);
                for (stage = 0; stage < 3; stage++) {
                        unsigned stage_mask = (1 << stage);

                        if (!(ci->filemask & stage_mask))
                                continue;
                        len = strlen(GITATTRIBUTES_FILE);
                        ce = make_empty_cache_entry(attr_index, len);
                        ce->ce_mode = create_ce_mode(ci->stages[stage].mode);
                        ce->ce_flags = create_ce_flags(stage);
                        ce->ce_namelen = len;
                        oidcpy(&ce->oid, &ci->stages[stage].oid);
                        memcpy(ce->name, GITATTRIBUTES_FILE, len);
                        add_index_entry(attr_index, ce,
                                        ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
                        get_stream_filter(attr_index, GITATTRIBUTES_FILE,
                                          &ce->oid);
                }
        }
}

static int merge_3way(struct merge_options *opt,
                      const char *path,
                      const struct object_id *o,
                      const struct object_id *a,
                      const struct object_id *b,
                      const char *pathnames[3],
                      const int extra_marker_size,
                      mmbuffer_t *result_buf)
{
        mmfile_t orig, src1, src2;
        struct ll_merge_options ll_opts = {0};
        char *base, *name1, *name2;
        int merge_status;

        if (!opt->priv->attr_index.initialized)
                initialize_attr_index(opt);

        ll_opts.renormalize = opt->renormalize;
        ll_opts.extra_marker_size = extra_marker_size;
        ll_opts.xdl_opts = opt->xdl_opts;

        if (opt->priv->call_depth) {
                ll_opts.virtual_ancestor = 1;
                ll_opts.variant = 0;
        } else {
                switch (opt->recursive_variant) {
                case MERGE_VARIANT_OURS:
                        ll_opts.variant = XDL_MERGE_FAVOR_OURS;
                        break;
                case MERGE_VARIANT_THEIRS:
                        ll_opts.variant = XDL_MERGE_FAVOR_THEIRS;
                        break;
                default:
                        ll_opts.variant = 0;
                        break;
                }
        }

        assert(pathnames[0] && pathnames[1] && pathnames[2] && opt->ancestor);
        if (pathnames[0] == pathnames[1] && pathnames[1] == pathnames[2]) {
                base  = mkpathdup("%s", opt->ancestor);
                name1 = mkpathdup("%s", opt->branch1);
                name2 = mkpathdup("%s", opt->branch2);
        } else {
                base  = mkpathdup("%s:%s", opt->ancestor, pathnames[0]);
                name1 = mkpathdup("%s:%s", opt->branch1,  pathnames[1]);
                name2 = mkpathdup("%s:%s", opt->branch2,  pathnames[2]);
        }

        read_mmblob(&orig, o);
        read_mmblob(&src1, a);
        read_mmblob(&src2, b);

        merge_status = ll_merge(result_buf, path, &orig, base,
                                &src1, name1, &src2, name2,
                                &opt->priv->attr_index, &ll_opts);

        free(base);
        free(name1);
        free(name2);
        free(orig.ptr);
        free(src1.ptr);
        free(src2.ptr);
        return merge_status;
}

static int handle_content_merge(struct merge_options *opt,
                                const char *path,
                                const struct version_info *o,
                                const struct version_info *a,
                                const struct version_info *b,
                                const char *pathnames[3],
                                const int extra_marker_size,
                                struct version_info *result)
{
        /*
         * path is the target location where we want to put the file, and
         * is used to determine any normalization rules in ll_merge.
         *
         * The normal case is that path and all entries in pathnames are
         * identical, though renames can affect which path we got one of
         * the three blobs to merge on various sides of history.
         *
         * extra_marker_size is the amount to extend conflict markers in
         * ll_merge; this is neeed if we have content merges of content
         * merges, which happens for example with rename/rename(2to1) and
         * rename/add conflicts.
         */
        unsigned clean = 1;

        /*
         * handle_content_merge() needs both files to be of the same type, i.e.
         * both files OR both submodules OR both symlinks.  Conflicting types
         * needs to be handled elsewhere.
         */
        assert((S_IFMT & a->mode) == (S_IFMT & b->mode));

        /* Merge modes */
        if (a->mode == b->mode || a->mode == o->mode)
                result->mode = b->mode;
        else {
                /* must be the 100644/100755 case */
                assert(S_ISREG(a->mode));
                result->mode = a->mode;
                clean = (b->mode == o->mode);
                /*
                 * FIXME: If opt->priv->call_depth && !clean, then we really
                 * should not make result->mode match either a->mode or
                 * b->mode; that causes t6036 "check conflicting mode for
                 * regular file" to fail.  It would be best to use some other
                 * mode, but we'll confuse all kinds of stuff if we use one
                 * where S_ISREG(result->mode) isn't true, and if we use
                 * something like 0100666, then tree-walk.c's calls to
                 * canon_mode() will just normalize that to 100644 for us and
                 * thus not solve anything.
                 *
                 * Figure out if there's some kind of way we can work around
                 * this...
                 */
        }

        /*
         * Trivial oid merge.
         *
         * Note: While one might assume that the next four lines would
         * be unnecessary due to the fact that match_mask is often
         * setup and already handled, renames don't always take care
         * of that.
         */
        if (oideq(&a->oid, &b->oid) || oideq(&a->oid, &o->oid))
                oidcpy(&result->oid, &b->oid);
        else if (oideq(&b->oid, &o->oid))
                oidcpy(&result->oid, &a->oid);

        /* Remaining rules depend on file vs. submodule vs. symlink. */
        else if (S_ISREG(a->mode)) {
                mmbuffer_t result_buf;
                int ret = 0, merge_status;
                int two_way;

                /*
                 * If 'o' is different type, treat it as null so we do a
                 * two-way merge.
                 */
                two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));

                merge_status = merge_3way(opt, path,
                                          two_way ? null_oid() : &o->oid,
                                          &a->oid, &b->oid,
                                          pathnames, extra_marker_size,
                                          &result_buf);

                if ((merge_status < 0) || !result_buf.ptr)
                        ret = err(opt, _("Failed to execute internal merge"));

                if (!ret &&
                    write_object_file(result_buf.ptr, result_buf.size,
                                      blob_type, &result->oid))
                        ret = err(opt, _("Unable to add %s to database"),
                                  path);

                free(result_buf.ptr);
                if (ret)
                        return -1;
                clean &= (merge_status == 0);
                path_msg(opt, path, 1, _("Auto-merging %s"), path);
        } else if (S_ISGITLINK(a->mode)) {
                int two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));
                clean = merge_submodule(opt, pathnames[0],
                                        two_way ? null_oid() : &o->oid,
                                        &a->oid, &b->oid, &result->oid);
                if (opt->priv->call_depth && two_way && !clean) {
                        result->mode = o->mode;
                        oidcpy(&result->oid, &o->oid);
                }
        } else if (S_ISLNK(a->mode)) {
                if (opt->priv->call_depth) {
                        clean = 0;
                        result->mode = o->mode;
                        oidcpy(&result->oid, &o->oid);
                } else {
                        switch (opt->recursive_variant) {
                        case MERGE_VARIANT_NORMAL:
                                clean = 0;
                                oidcpy(&result->oid, &a->oid);
                                break;
                        case MERGE_VARIANT_OURS:
                                oidcpy(&result->oid, &a->oid);
                                break;
                        case MERGE_VARIANT_THEIRS:
                                oidcpy(&result->oid, &b->oid);
                                break;
                        }
                }
        } else
                BUG("unsupported object type in the tree: %06o for %s",
                    a->mode, path);

        return clean;
}

/*** Function Grouping: functions related to detect_and_process_renames(), ***
 *** which are split into directory and regular rename detection sections. ***/

/*** Function Grouping: functions related to directory rename detection ***/

struct collision_info {
        struct string_list source_files;
        unsigned reported_already:1;
};

/*
 * Return a new string that replaces the beginning portion (which matches
 * rename_info->key), with rename_info->util.new_dir.  In perl-speak:
 *   new_path_name = (old_path =~ s/rename_info->key/rename_info->value/);
 * NOTE:
 *   Caller must ensure that old_path starts with rename_info->key + '/'.
 */
static char *apply_dir_rename(struct strmap_entry *rename_info,
                              const char *old_path)
{
        struct strbuf new_path = STRBUF_INIT;
        const char *old_dir = rename_info->key;
        const char *new_dir = rename_info->value;
        int oldlen, newlen, new_dir_len;

        oldlen = strlen(old_dir);
        if (*new_dir == '\0')
                /*
                 * If someone renamed/merged a subdirectory into the root
                 * directory (e.g. 'some/subdir' -> ''), then we want to
                 * avoid returning
                 *     '' + '/filename'
                 * as the rename; we need to make old_path + oldlen advance
                 * past the '/' character.
                 */
                oldlen++;
        new_dir_len = strlen(new_dir);
        newlen = new_dir_len + (strlen(old_path) - oldlen) + 1;
        strbuf_grow(&new_path, newlen);
        strbuf_add(&new_path, new_dir, new_dir_len);
        strbuf_addstr(&new_path, &old_path[oldlen]);

        return strbuf_detach(&new_path, NULL);
}

static int path_in_way(struct strmap *paths, const char *path, unsigned side_mask)
{
        struct merged_info *mi = strmap_get(paths, path);
        struct conflict_info *ci;
        if (!mi)
                return 0;
        INITIALIZE_CI(ci, mi);
        return mi->clean || (side_mask & (ci->filemask | ci->dirmask));
}

/*
 * See if there is a directory rename for path, and if there are any file
 * level conflicts on the given side for the renamed location.  If there is
 * a rename and there are no conflicts, return the new name.  Otherwise,
 * return NULL.
 */
static char *handle_path_level_conflicts(struct merge_options *opt,
                                         const char *path,
                                         unsigned side_index,
                                         struct strmap_entry *rename_info,
                                         struct strmap *collisions)
{
        char *new_path = NULL;
        struct collision_info *c_info;
        int clean = 1;
        struct strbuf collision_paths = STRBUF_INIT;

        /*
         * entry has the mapping of old directory name to new directory name
         * that we want to apply to path.
         */
        new_path = apply_dir_rename(rename_info, path);
        if (!new_path)
                BUG("Failed to apply directory rename!");

        /*
         * The caller needs to have ensured that it has pre-populated
         * collisions with all paths that map to new_path.  Do a quick check
         * to ensure that's the case.
         */
        c_info = strmap_get(collisions, new_path);
        if (c_info == NULL)
                BUG("c_info is NULL");

        /*
         * Check for one-sided add/add/.../add conflicts, i.e.
         * where implicit renames from the other side doing
         * directory rename(s) can affect this side of history
         * to put multiple paths into the same location.  Warn
         * and bail on directory renames for such paths.
         */
        if (c_info->reported_already) {
                clean = 0;
        } else if (path_in_way(&opt->priv->paths, new_path, 1 << side_index)) {
                c_info->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &c_info->source_files);
                path_msg(opt, new_path, 0,
                         _("CONFLICT (implicit dir rename): Existing file/dir "
                           "at %s in the way of implicit directory rename(s) "
                           "putting the following path(s) there: %s."),
                       new_path, collision_paths.buf);
                clean = 0;
        } else if (c_info->source_files.nr > 1) {
                c_info->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &c_info->source_files);
                path_msg(opt, new_path, 0,
                         _("CONFLICT (implicit dir rename): Cannot map more "
                           "than one path to %s; implicit directory renames "
                           "tried to put these paths there: %s"),
                       new_path, collision_paths.buf);
                clean = 0;
        }

        /* Free memory we no longer need */
        strbuf_release(&collision_paths);
        if (!clean && new_path) {
                free(new_path);
                return NULL;
        }

        return new_path;
}

static void get_provisional_directory_renames(struct merge_options *opt,
                                              unsigned side,
                                              int *clean)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        struct rename_info *renames = &opt->priv->renames;

        /*
         * Collapse
         *    dir_rename_count: old_directory -> {new_directory -> count}
         * down to
         *    dir_renames: old_directory -> best_new_directory
         * where best_new_directory is the one with the unique highest count.
         */
        strmap_for_each_entry(&renames->dir_rename_count[side], &iter, entry) {
                const char *source_dir = entry->key;
                struct strintmap *counts = entry->value;
                struct hashmap_iter count_iter;
                struct strmap_entry *count_entry;
                int max = 0;
                int bad_max = 0;
                const char *best = NULL;

                strintmap_for_each_entry(counts, &count_iter, count_entry) {
                        const char *target_dir = count_entry->key;
                        intptr_t count = (intptr_t)count_entry->value;

                        if (count == max)
                                bad_max = max;
                        else if (count > max) {
                                max = count;
                                best = target_dir;
                        }
                }

                if (max == 0)
                        continue;

                if (bad_max == max) {
                        path_msg(opt, source_dir, 0,
                               _("CONFLICT (directory rename split): "
                                 "Unclear where to rename %s to; it was "
                                 "renamed to multiple other directories, with "
                                 "no destination getting a majority of the "
                                 "files."),
                               source_dir);
                        *clean = 0;
                } else {
                        strmap_put(&renames->dir_renames[side],
                                   source_dir, (void*)best);
                }
        }
}

static void handle_directory_level_conflicts(struct merge_options *opt)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        struct string_list duplicated = STRING_LIST_INIT_NODUP;
        struct rename_info *renames = &opt->priv->renames;
        struct strmap *side1_dir_renames = &renames->dir_renames[MERGE_SIDE1];
        struct strmap *side2_dir_renames = &renames->dir_renames[MERGE_SIDE2];
        int i;

        strmap_for_each_entry(side1_dir_renames, &iter, entry) {
                if (strmap_contains(side2_dir_renames, entry->key))
                        string_list_append(&duplicated, entry->key);
        }

        for (i = 0; i < duplicated.nr; i++) {
                strmap_remove(side1_dir_renames, duplicated.items[i].string, 0);
                strmap_remove(side2_dir_renames, duplicated.items[i].string, 0);
        }
        string_list_clear(&duplicated, 0);
}

static struct strmap_entry *check_dir_renamed(const char *path,
                                              struct strmap *dir_renames)
{
        char *temp = xstrdup(path);
        char *end;
        struct strmap_entry *e = NULL;

        while ((end = strrchr(temp, '/'))) {
                *end = '\0';
                e = strmap_get_entry(dir_renames, temp);
                if (e)
                        break;
        }
        free(temp);
        return e;
}

static void compute_collisions(struct strmap *collisions,
                               struct strmap *dir_renames,
                               struct diff_queue_struct *pairs)
{
        int i;

        strmap_init_with_options(collisions, NULL, 0);
        if (strmap_empty(dir_renames))
                return;

        /*
         * Multiple files can be mapped to the same path due to directory
         * renames done by the other side of history.  Since that other
         * side of history could have merged multiple directories into one,
         * if our side of history added the same file basename to each of
         * those directories, then all N of them would get implicitly
         * renamed by the directory rename detection into the same path,
         * and we'd get an add/add/.../add conflict, and all those adds
         * from *this* side of history.  This is not representable in the
         * index, and users aren't going to easily be able to make sense of
         * it.  So we need to provide a good warning about what's
         * happening, and fall back to no-directory-rename detection
         * behavior for those paths.
         *
         * See testcases 9e and all of section 5 from t6043 for examples.
         */
        for (i = 0; i < pairs->nr; ++i) {
                struct strmap_entry *rename_info;
                struct collision_info *collision_info;
                char *new_path;
                struct diff_filepair *pair = pairs->queue[i];

                if (pair->status != 'A' && pair->status != 'R')
                        continue;
                rename_info = check_dir_renamed(pair->two->path, dir_renames);
                if (!rename_info)
                        continue;

                new_path = apply_dir_rename(rename_info, pair->two->path);
                assert(new_path);
                collision_info = strmap_get(collisions, new_path);
                if (collision_info) {
                        free(new_path);
                } else {
                        CALLOC_ARRAY(collision_info, 1);
                        string_list_init_nodup(&collision_info->source_files);
                        strmap_put(collisions, new_path, collision_info);
                }
                string_list_insert(&collision_info->source_files,
                                   pair->two->path);
        }
}

static char *check_for_directory_rename(struct merge_options *opt,
                                        const char *path,
                                        unsigned side_index,
                                        struct strmap *dir_renames,
                                        struct strmap *dir_rename_exclusions,
                                        struct strmap *collisions,
                                        int *clean_merge)
{
        char *new_path = NULL;
        struct strmap_entry *rename_info;
        struct strmap_entry *otherinfo = NULL;
        const char *new_dir;

        if (strmap_empty(dir_renames))
                return new_path;
        rename_info = check_dir_renamed(path, dir_renames);
        if (!rename_info)
                return new_path;
        /* old_dir = rename_info->key; */
        new_dir = rename_info->value;

        /*
         * This next part is a little weird.  We do not want to do an
         * implicit rename into a directory we renamed on our side, because
         * that will result in a spurious rename/rename(1to2) conflict.  An
         * example:
         *   Base commit: dumbdir/afile, otherdir/bfile
         *   Side 1:      smrtdir/afile, otherdir/bfile
         *   Side 2:      dumbdir/afile, dumbdir/bfile
         * Here, while working on Side 1, we could notice that otherdir was
         * renamed/merged to dumbdir, and change the diff_filepair for
         * otherdir/bfile into a rename into dumbdir/bfile.  However, Side
         * 2 will notice the rename from dumbdir to smrtdir, and do the
         * transitive rename to move it from dumbdir/bfile to
         * smrtdir/bfile.  That gives us bfile in dumbdir vs being in
         * smrtdir, a rename/rename(1to2) conflict.  We really just want
         * the file to end up in smrtdir.  And the way to achieve that is
         * to not let Side1 do the rename to dumbdir, since we know that is
         * the source of one of our directory renames.
         *
         * That's why otherinfo and dir_rename_exclusions is here.
         *
         * As it turns out, this also prevents N-way transient rename
         * confusion; See testcases 9c and 9d of t6043.
         */
        otherinfo = strmap_get_entry(dir_rename_exclusions, new_dir);
        if (otherinfo) {
                path_msg(opt, rename_info->key, 1,
                         _("WARNING: Avoiding applying %s -> %s rename "
                           "to %s, because %s itself was renamed."),
                         rename_info->key, new_dir, path, new_dir);
                return NULL;
        }

        new_path = handle_path_level_conflicts(opt, path, side_index,
                                               rename_info, collisions);
        *clean_merge &= (new_path != NULL);

        return new_path;
}

static void apply_directory_rename_modifications(struct merge_options *opt,
                                                 struct diff_filepair *pair,
                                                 char *new_path)
{
        /*
         * The basic idea is to get the conflict_info from opt->priv->paths
         * at old path, and insert it into new_path; basically just this:
         *     ci = strmap_get(&opt->priv->paths, old_path);
         *     strmap_remove(&opt->priv->paths, old_path, 0);
         *     strmap_put(&opt->priv->paths, new_path, ci);
         * However, there are some factors complicating this:
         *     - opt->priv->paths may already have an entry at new_path
         *     - Each ci tracks its containing directory, so we need to
         *       update that
         *     - If another ci has the same containing directory, then
         *       the two char*'s MUST point to the same location.  See the
         *       comment in struct merged_info.  strcmp equality is not
         *       enough; we need pointer equality.
         *     - opt->priv->paths must hold the parent directories of any
         *       entries that are added.  So, if this directory rename
         *       causes entirely new directories, we must recursively add
         *       parent directories.
         *     - For each parent directory added to opt->priv->paths, we
         *       also need to get its parent directory stored in its
         *       conflict_info->merged.directory_name with all the same
         *       requirements about pointer equality.
         */
        struct string_list dirs_to_insert = STRING_LIST_INIT_NODUP;
        struct conflict_info *ci, *new_ci;
        struct strmap_entry *entry;
        const char *branch_with_new_path, *branch_with_dir_rename;
        const char *old_path = pair->two->path;
        const char *parent_name;
        const char *cur_path;
        int i, len;

        entry = strmap_get_entry(&opt->priv->paths, old_path);
        old_path = entry->key;
        ci = entry->value;
        VERIFY_CI(ci);

        /* Find parent directories missing from opt->priv->paths */
        cur_path = mem_pool_strdup(&opt->priv->pool, new_path);
        free((char*)new_path);
        new_path = (char *)cur_path;

        while (1) {
                /* Find the parent directory of cur_path */
                char *last_slash = strrchr(cur_path, '/');
                if (last_slash) {
                        parent_name = mem_pool_strndup(&opt->priv->pool,
                                                       cur_path,
                                                       last_slash - cur_path);
                } else {
                        parent_name = opt->priv->toplevel_dir;
                        break;
                }

                /* Look it up in opt->priv->paths */
                entry = strmap_get_entry(&opt->priv->paths, parent_name);
                if (entry) {
                        parent_name = entry->key; /* reuse known pointer */
                        break;
                }

                /* Record this is one of the directories we need to insert */
                string_list_append(&dirs_to_insert, parent_name);
                cur_path = parent_name;
        }

        /* Traverse dirs_to_insert and insert them into opt->priv->paths */
        for (i = dirs_to_insert.nr-1; i >= 0; --i) {
                struct conflict_info *dir_ci;
                char *cur_dir = dirs_to_insert.items[i].string;

                CALLOC_ARRAY(dir_ci, 1);

                dir_ci->merged.directory_name = parent_name;
                len = strlen(parent_name);
                /* len+1 because of trailing '/' character */
                dir_ci->merged.basename_offset = (len > 0 ? len+1 : len);
                dir_ci->dirmask = ci->filemask;
                strmap_put(&opt->priv->paths, cur_dir, dir_ci);

                parent_name = cur_dir;
        }

        assert(ci->filemask == 2 || ci->filemask == 4);
        assert(ci->dirmask == 0);
        strmap_remove(&opt->priv->paths, old_path, 0);

        branch_with_new_path   = (ci->filemask == 2) ? opt->branch1 : opt->branch2;
        branch_with_dir_rename = (ci->filemask == 2) ? opt->branch2 : opt->branch1;

        /* Now, finally update ci and stick it into opt->priv->paths */
        ci->merged.directory_name = parent_name;
        len = strlen(parent_name);
        ci->merged.basename_offset = (len > 0 ? len+1 : len);
        new_ci = strmap_get(&opt->priv->paths, new_path);
        if (!new_ci) {
                /* Place ci back into opt->priv->paths, but at new_path */
                strmap_put(&opt->priv->paths, new_path, ci);
        } else {
                int index;

                /* A few sanity checks */
                VERIFY_CI(new_ci);
                assert(ci->filemask == 2 || ci->filemask == 4);
                assert((new_ci->filemask & ci->filemask) == 0);
                assert(!new_ci->merged.clean);

                /* Copy stuff from ci into new_ci */
                new_ci->filemask |= ci->filemask;
                if (new_ci->dirmask)
                        new_ci->df_conflict = 1;
                index = (ci->filemask >> 1);
                new_ci->pathnames[index] = ci->pathnames[index];
                new_ci->stages[index].mode = ci->stages[index].mode;
                oidcpy(&new_ci->stages[index].oid, &ci->stages[index].oid);

                ci = new_ci;
        }

        if (opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_TRUE) {
                /* Notify user of updated path */
                if (pair->status == 'A')
                        path_msg(opt, new_path, 1,
                                 _("Path updated: %s added in %s inside a "
                                   "directory that was renamed in %s; moving "
                                   "it to %s."),
                                 old_path, branch_with_new_path,
                                 branch_with_dir_rename, new_path);
                else
                        path_msg(opt, new_path, 1,
                                 _("Path updated: %s renamed to %s in %s, "
                                   "inside a directory that was renamed in %s; "
                                   "moving it to %s."),
                                 pair->one->path, old_path, branch_with_new_path,
                                 branch_with_dir_rename, new_path);
        } else {
                /*
                 * opt->detect_directory_renames has the value
                 * MERGE_DIRECTORY_RENAMES_CONFLICT, so mark these as conflicts.
                 */
                ci->path_conflict = 1;
                if (pair->status == 'A')
                        path_msg(opt, new_path, 0,
                                 _("CONFLICT (file location): %s added in %s "
                                   "inside a directory that was renamed in %s, "
                                   "suggesting it should perhaps be moved to "
                                   "%s."),
                                 old_path, branch_with_new_path,
                                 branch_with_dir_rename, new_path);
                else
                        path_msg(opt, new_path, 0,
                                 _("CONFLICT (file location): %s renamed to %s "
                                   "in %s, inside a directory that was renamed "
                                   "in %s, suggesting it should perhaps be "
                                   "moved to %s."),
                                 pair->one->path, old_path, branch_with_new_path,
                                 branch_with_dir_rename, new_path);
        }

        /*
         * Finally, record the new location.
         */
        pair->two->path = new_path;
}

/*** Function Grouping: functions related to regular rename detection ***/

static int process_renames(struct merge_options *opt,
                           struct diff_queue_struct *renames)
{
        int clean_merge = 1, i;

        for (i = 0; i < renames->nr; ++i) {
                const char *oldpath = NULL, *newpath;
                struct diff_filepair *pair = renames->queue[i];
                struct conflict_info *oldinfo = NULL, *newinfo = NULL;
                struct strmap_entry *old_ent, *new_ent;
                unsigned int old_sidemask;
                int target_index, other_source_index;
                int source_deleted, collision, type_changed;
                const char *rename_branch = NULL, *delete_branch = NULL;

                old_ent = strmap_get_entry(&opt->priv->paths, pair->one->path);
                new_ent = strmap_get_entry(&opt->priv->paths, pair->two->path);
                if (old_ent) {
                        oldpath = old_ent->key;
                        oldinfo = old_ent->value;
                }
                newpath = pair->two->path;
                if (new_ent) {
                        newpath = new_ent->key;
                        newinfo = new_ent->value;
                }

                /*
                 * If pair->one->path isn't in opt->priv->paths, that means
                 * that either directory rename detection removed that
                 * path, or a parent directory of oldpath was resolved and
                 * we don't even need the rename; in either case, we can
                 * skip it.  If oldinfo->merged.clean, then the other side
                 * of history had no changes to oldpath and we don't need
                 * the rename and can skip it.
                 */
                if (!oldinfo || oldinfo->merged.clean)
                        continue;

                /*
                 * diff_filepairs have copies of pathnames, thus we have to
                 * use standard 'strcmp()' (negated) instead of '=='.
                 */
                if (i + 1 < renames->nr &&
                    !strcmp(oldpath, renames->queue[i+1]->one->path)) {
                        /* Handle rename/rename(1to2) or rename/rename(1to1) */
                        const char *pathnames[3];
                        struct version_info merged;
                        struct conflict_info *base, *side1, *side2;
                        unsigned was_binary_blob = 0;

                        pathnames[0] = oldpath;
                        pathnames[1] = newpath;
                        pathnames[2] = renames->queue[i+1]->two->path;

                        base = strmap_get(&opt->priv->paths, pathnames[0]);
                        side1 = strmap_get(&opt->priv->paths, pathnames[1]);
                        side2 = strmap_get(&opt->priv->paths, pathnames[2]);

                        VERIFY_CI(base);
                        VERIFY_CI(side1);
                        VERIFY_CI(side2);

                        if (!strcmp(pathnames[1], pathnames[2])) {
                                struct rename_info *ri = &opt->priv->renames;
                                int j;

                                /* Both sides renamed the same way */
                                assert(side1 == side2);
                                memcpy(&side1->stages[0], &base->stages[0],
                                       sizeof(merged));
                                side1->filemask |= (1 << MERGE_BASE);
                                /* Mark base as resolved by removal */
                                base->merged.is_null = 1;
                                base->merged.clean = 1;

                                /*
                                 * Disable remembering renames optimization;
                                 * rename/rename(1to1) is incredibly rare, and
                                 * just disabling the optimization is easier
                                 * than purging cached_pairs,
                                 * cached_target_names, and dir_rename_counts.
                                 */
                                for (j = 0; j < 3; j++)
                                        ri->merge_trees[j] = NULL;

                                /* We handled both renames, i.e. i+1 handled */
                                i++;
                                /* Move to next rename */
                                continue;
                        }

                        /* This is a rename/rename(1to2) */
                        clean_merge = handle_content_merge(opt,
                                                           pair->one->path,
                                                           &base->stages[0],
                                                           &side1->stages[1],
                                                           &side2->stages[2],
                                                           pathnames,
                                                           1 + 2 * opt->priv->call_depth,
                                                           &merged);
                        if (!clean_merge &&
                            merged.mode == side1->stages[1].mode &&
                            oideq(&merged.oid, &side1->stages[1].oid))
                                was_binary_blob = 1;
                        memcpy(&side1->stages[1], &merged, sizeof(merged));
                        if (was_binary_blob) {
                                /*
                                 * Getting here means we were attempting to
                                 * merge a binary blob.
                                 *
                                 * Since we can't merge binaries,
                                 * handle_content_merge() just takes one
                                 * side.  But we don't want to copy the
                                 * contents of one side to both paths.  We
                                 * used the contents of side1 above for
                                 * side1->stages, let's use the contents of
                                 * side2 for side2->stages below.
                                 */
                                oidcpy(&merged.oid, &side2->stages[2].oid);
                                merged.mode = side2->stages[2].mode;
                        }
                        memcpy(&side2->stages[2], &merged, sizeof(merged));

                        side1->path_conflict = 1;
                        side2->path_conflict = 1;
                        /*
                         * TODO: For renames we normally remove the path at the
                         * old name.  It would thus seem consistent to do the
                         * same for rename/rename(1to2) cases, but we haven't
                         * done so traditionally and a number of the regression
                         * tests now encode an expectation that the file is
                         * left there at stage 1.  If we ever decide to change
                         * this, add the following two lines here:
                         *    base->merged.is_null = 1;
                         *    base->merged.clean = 1;
                         * and remove the setting of base->path_conflict to 1.
                         */
                        base->path_conflict = 1;
                        path_msg(opt, oldpath, 0,
                                 _("CONFLICT (rename/rename): %s renamed to "
                                   "%s in %s and to %s in %s."),
                                 pathnames[0],
                                 pathnames[1], opt->branch1,
                                 pathnames[2], opt->branch2);

                        i++; /* We handled both renames, i.e. i+1 handled */
                        continue;
                }

                VERIFY_CI(oldinfo);
                VERIFY_CI(newinfo);
                target_index = pair->score; /* from collect_renames() */
                assert(target_index == 1 || target_index == 2);
                other_source_index = 3 - target_index;
                old_sidemask = (1 << other_source_index); /* 2 or 4 */
                source_deleted = (oldinfo->filemask == 1);
                collision = ((newinfo->filemask & old_sidemask) != 0);
                type_changed = !source_deleted &&
                        (S_ISREG(oldinfo->stages[other_source_index].mode) !=
                         S_ISREG(newinfo->stages[target_index].mode));
                if (type_changed && collision) {
                        /*
                         * special handling so later blocks can handle this...
                         *
                         * if type_changed && collision are both true, then this
                         * was really a double rename, but one side wasn't
                         * detected due to lack of break detection.  I.e.
                         * something like
                         *    orig: has normal file 'foo'
                         *    side1: renames 'foo' to 'bar', adds 'foo' symlink
                         *    side2: renames 'foo' to 'bar'
                         * In this case, the foo->bar rename on side1 won't be
                         * detected because the new symlink named 'foo' is
                         * there and we don't do break detection.  But we detect
                         * this here because we don't want to merge the content
                         * of the foo symlink with the foo->bar file, so we
                         * have some logic to handle this special case.  The
                         * easiest way to do that is make 'bar' on side1 not
                         * be considered a colliding file but the other part
                         * of a normal rename.  If the file is very different,
                         * well we're going to get content merge conflicts
                         * anyway so it doesn't hurt.  And if the colliding
                         * file also has a different type, that'll be handled
                         * by the content merge logic in process_entry() too.
                         *
                         * See also t6430, 'rename vs. rename/symlink'
                         */
                        collision = 0;
                }
                if (source_deleted) {
                        if (target_index == 1) {
                                rename_branch = opt->branch1;
                                delete_branch = opt->branch2;
                        } else {
                                rename_branch = opt->branch2;
                                delete_branch = opt->branch1;
                        }
                }

                assert(source_deleted || oldinfo->filemask & old_sidemask);

                /* Need to check for special types of rename conflicts... */
                if (collision && !source_deleted) {
                        /* collision: rename/add or rename/rename(2to1) */
                        const char *pathnames[3];
                        struct version_info merged;

                        struct conflict_info *base, *side1, *side2;
                        unsigned clean;

                        pathnames[0] = oldpath;
                        pathnames[other_source_index] = oldpath;
                        pathnames[target_index] = newpath;

                        base = strmap_get(&opt->priv->paths, pathnames[0]);
                        side1 = strmap_get(&opt->priv->paths, pathnames[1]);
                        side2 = strmap_get(&opt->priv->paths, pathnames[2]);

                        VERIFY_CI(base);
                        VERIFY_CI(side1);
                        VERIFY_CI(side2);

                        clean = handle_content_merge(opt, pair->one->path,
                                                     &base->stages[0],
                                                     &side1->stages[1],
                                                     &side2->stages[2],
                                                     pathnames,
                                                     1 + 2 * opt->priv->call_depth,
                                                     &merged);

                        memcpy(&newinfo->stages[target_index], &merged,
                               sizeof(merged));
                        if (!clean) {
                                path_msg(opt, newpath, 0,
                                         _("CONFLICT (rename involved in "
                                           "collision): rename of %s -> %s has "
                                           "content conflicts AND collides "
                                           "with another path; this may result "
                                           "in nested conflict markers."),
                                         oldpath, newpath);
                        }
                } else if (collision && source_deleted) {
                        /*
                         * rename/add/delete or rename/rename(2to1)/delete:
                         * since oldpath was deleted on the side that didn't
                         * do the rename, there's not much of a content merge
                         * we can do for the rename.  oldinfo->merged.is_null
                         * was already set, so we just leave things as-is so
                         * they look like an add/add conflict.
                         */

                        newinfo->path_conflict = 1;
                        path_msg(opt, newpath, 0,
                                 _("CONFLICT (rename/delete): %s renamed "
                                   "to %s in %s, but deleted in %s."),
                                 oldpath, newpath, rename_branch, delete_branch);
                } else {
                        /*
                         * a few different cases...start by copying the
                         * existing stage(s) from oldinfo over the newinfo
                         * and update the pathname(s).
                         */
                        memcpy(&newinfo->stages[0], &oldinfo->stages[0],
                               sizeof(newinfo->stages[0]));
                        newinfo->filemask |= (1 << MERGE_BASE);
                        newinfo->pathnames[0] = oldpath;
                        if (type_changed) {
                                /* rename vs. typechange */
                                /* Mark the original as resolved by removal */
                                memcpy(&oldinfo->stages[0].oid, null_oid(),
                                       sizeof(oldinfo->stages[0].oid));
                                oldinfo->stages[0].mode = 0;
                                oldinfo->filemask &= 0x06;
                        } else if (source_deleted) {
                                /* rename/delete */
                                newinfo->path_conflict = 1;
                                path_msg(opt, newpath, 0,
                                         _("CONFLICT (rename/delete): %s renamed"
                                           " to %s in %s, but deleted in %s."),
                                         oldpath, newpath,
                                         rename_branch, delete_branch);
                        } else {
                                /* normal rename */
                                memcpy(&newinfo->stages[other_source_index],
                                       &oldinfo->stages[other_source_index],
                                       sizeof(newinfo->stages[0]));
                                newinfo->filemask |= (1 << other_source_index);
                                newinfo->pathnames[other_source_index] = oldpath;
                        }
                }

                if (!type_changed) {
                        /* Mark the original as resolved by removal */
                        oldinfo->merged.is_null = 1;
                        oldinfo->merged.clean = 1;
                }

        }

        return clean_merge;
}

static inline int possible_side_renames(struct rename_info *renames,
                                        unsigned side_index)
{
        return renames->pairs[side_index].nr > 0 &&
               !strintmap_empty(&renames->relevant_sources[side_index]);
}

static inline int possible_renames(struct rename_info *renames)
{
        return possible_side_renames(renames, 1) ||
               possible_side_renames(renames, 2) ||
               !strmap_empty(&renames->cached_pairs[1]) ||
               !strmap_empty(&renames->cached_pairs[2]);
}

static void resolve_diffpair_statuses(struct diff_queue_struct *q)
{
        /*
         * A simplified version of diff_resolve_rename_copy(); would probably
         * just use that function but it's static...
         */
        int i;
        struct diff_filepair *p;

        for (i = 0; i < q->nr; ++i) {
                p = q->queue[i];
                p->status = 0; /* undecided */
                if (!DIFF_FILE_VALID(p->one))
                        p->status = DIFF_STATUS_ADDED;
                else if (!DIFF_FILE_VALID(p->two))
                        p->status = DIFF_STATUS_DELETED;
                else if (DIFF_PAIR_RENAME(p))
                        p->status = DIFF_STATUS_RENAMED;
        }
}

static void prune_cached_from_relevant(struct rename_info *renames,
                                       unsigned side)
{
        /* Reason for this function described in add_pair() */
        struct hashmap_iter iter;
        struct strmap_entry *entry;

        /* Remove from relevant_sources all entries in cached_pairs[side] */
        strmap_for_each_entry(&renames->cached_pairs[side], &iter, entry) {
                strintmap_remove(&renames->relevant_sources[side],
                                 entry->key);
        }
        /* Remove from relevant_sources all entries in cached_irrelevant[side] */
        strset_for_each_entry(&renames->cached_irrelevant[side], &iter, entry) {
                strintmap_remove(&renames->relevant_sources[side],
                                 entry->key);
        }
}

static void use_cached_pairs(struct merge_options *opt,
                             struct strmap *cached_pairs,
                             struct diff_queue_struct *pairs)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;

        /*
         * Add to side_pairs all entries from renames->cached_pairs[side_index].
         * (Info in cached_irrelevant[side_index] is not relevant here.)
         */
        strmap_for_each_entry(cached_pairs, &iter, entry) {
                struct diff_filespec *one, *two;
                const char *old_name = entry->key;
                const char *new_name = entry->value;
                if (!new_name)
                        new_name = old_name;

                /*
                 * cached_pairs has *copies* of old_name and new_name,
                 * because it has to persist across merges.  Since
                 * pool_alloc_filespec() will just re-use the existing
                 * filenames, which will also get re-used by
                 * opt->priv->paths if they become renames, and then
                 * get freed at the end of the merge, that would leave
                 * the copy in cached_pairs dangling.  Avoid this by
                 * making a copy here.
                 */
                old_name = mem_pool_strdup(&opt->priv->pool, old_name);
                new_name = mem_pool_strdup(&opt->priv->pool, new_name);

                /* We don't care about oid/mode, only filenames and status */
                one = pool_alloc_filespec(&opt->priv->pool, old_name);
                two = pool_alloc_filespec(&opt->priv->pool, new_name);
                pool_diff_queue(&opt->priv->pool, pairs, one, two);
                pairs->queue[pairs->nr-1]->status = entry->value ? 'R' : 'D';
        }
}

static void cache_new_pair(struct rename_info *renames,
                           int side,
                           char *old_path,
                           char *new_path,
                           int free_old_value)
{
        char *old_value;
        new_path = xstrdup(new_path);
        old_value = strmap_put(&renames->cached_pairs[side],
                               old_path, new_path);
        strset_add(&renames->cached_target_names[side], new_path);
        if (free_old_value)
                free(old_value);
        else
                assert(!old_value);
}

static void possibly_cache_new_pair(struct rename_info *renames,
                                    struct diff_filepair *p,
                                    unsigned side,
                                    char *new_path)
{
        int dir_renamed_side = 0;

        if (new_path) {
                /*
                 * Directory renames happen on the other side of history from
                 * the side that adds new files to the old directory.
                 */
                dir_renamed_side = 3 - side;
        } else {
                int val = strintmap_get(&renames->relevant_sources[side],
                                        p->one->path);
                if (val == RELEVANT_NO_MORE) {
                        assert(p->status == 'D');
                        strset_add(&renames->cached_irrelevant[side],
                                   p->one->path);
                }
                if (val <= 0)
                        return;
        }

        if (p->status == 'D') {
                /*
                 * If we already had this delete, we'll just set it's value
                 * to NULL again, so no harm.
                 */
                strmap_put(&renames->cached_pairs[side], p->one->path, NULL);
        } else if (p->status == 'R') {
                if (!new_path)
                        new_path = p->two->path;
                else
                        cache_new_pair(renames, dir_renamed_side,
                                       p->two->path, new_path, 0);
                cache_new_pair(renames, side, p->one->path, new_path, 1);
        } else if (p->status == 'A' && new_path) {
                cache_new_pair(renames, dir_renamed_side,
                               p->two->path, new_path, 0);
        }
}

static int compare_pairs(const void *a_, const void *b_)
{
        const struct diff_filepair *a = *((const struct diff_filepair **)a_);
        const struct diff_filepair *b = *((const struct diff_filepair **)b_);

        return strcmp(a->one->path, b->one->path);
}

/* Call diffcore_rename() to update deleted/added pairs into rename pairs */
static int detect_regular_renames(struct merge_options *opt,
                                  unsigned side_index)
{
        struct diff_options diff_opts;
        struct rename_info *renames = &opt->priv->renames;

        prune_cached_from_relevant(renames, side_index);
        if (!possible_side_renames(renames, side_index)) {
                /*
                 * No rename detection needed for this side, but we still need
                 * to make sure 'adds' are marked correctly in case the other
                 * side had directory renames.
                 */
                resolve_diffpair_statuses(&renames->pairs[side_index]);
                return 0;
        }

        partial_clear_dir_rename_count(&renames->dir_rename_count[side_index]);
        repo_diff_setup(opt->repo, &diff_opts);
        diff_opts.flags.recursive = 1;
        diff_opts.flags.rename_empty = 0;
        diff_opts.detect_rename = DIFF_DETECT_RENAME;
        diff_opts.rename_limit = opt->rename_limit;
        if (opt->rename_limit <= 0)
                diff_opts.rename_limit = 7000;
        diff_opts.rename_score = opt->rename_score;
        diff_opts.show_rename_progress = opt->show_rename_progress;
        diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_setup_done(&diff_opts);

        diff_queued_diff = renames->pairs[side_index];
        trace2_region_enter("diff", "diffcore_rename", opt->repo);
        diffcore_rename_extended(&diff_opts,
                                 &opt->priv->pool,
                                 &renames->relevant_sources[side_index],
                                 &renames->dirs_removed[side_index],
                                 &renames->dir_rename_count[side_index],
                                 &renames->cached_pairs[side_index]);
        trace2_region_leave("diff", "diffcore_rename", opt->repo);
        resolve_diffpair_statuses(&diff_queued_diff);

        if (diff_opts.needed_rename_limit > 0)
                renames->redo_after_renames = 0;
        if (diff_opts.needed_rename_limit > renames->needed_limit)
                renames->needed_limit = diff_opts.needed_rename_limit;

        renames->pairs[side_index] = diff_queued_diff;

        diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_queued_diff.nr = 0;
        diff_queued_diff.queue = NULL;
        diff_flush(&diff_opts);

        return 1;
}

/*
 * Get information of all renames which occurred in 'side_pairs', making use
 * of any implicit directory renames in side_dir_renames (also making use of
 * implicit directory renames rename_exclusions as needed by
 * check_for_directory_rename()).  Add all (updated) renames into result.
 */
static int collect_renames(struct merge_options *opt,
                           struct diff_queue_struct *result,
                           unsigned side_index,
                           struct strmap *dir_renames_for_side,
                           struct strmap *rename_exclusions)
{
        int i, clean = 1;
        struct strmap collisions;
        struct diff_queue_struct *side_pairs;
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        struct rename_info *renames = &opt->priv->renames;

        side_pairs = &renames->pairs[side_index];
        compute_collisions(&collisions, dir_renames_for_side, side_pairs);

        for (i = 0; i < side_pairs->nr; ++i) {
                struct diff_filepair *p = side_pairs->queue[i];
                char *new_path; /* non-NULL only with directory renames */

                if (p->status != 'A' && p->status != 'R') {
                        possibly_cache_new_pair(renames, p, side_index, NULL);
                        pool_diff_free_filepair(&opt->priv->pool, p);
                        continue;
                }

                new_path = check_for_directory_rename(opt, p->two->path,
                                                      side_index,
                                                      dir_renames_for_side,
                                                      rename_exclusions,
                                                      &collisions,
                                                      &clean);

                possibly_cache_new_pair(renames, p, side_index, new_path);
                if (p->status != 'R' && !new_path) {
                        pool_diff_free_filepair(&opt->priv->pool, p);
                        continue;
                }

                if (new_path)
                        apply_directory_rename_modifications(opt, p, new_path);

                /*
                 * p->score comes back from diffcore_rename_extended() with
                 * the similarity of the renamed file.  The similarity is
                 * was used to determine that the two files were related
                 * and are a rename, which we have already used, but beyond
                 * that we have no use for the similarity.  So p->score is
                 * now irrelevant.  However, process_renames() will need to
                 * know which side of the merge this rename was associated
                 * with, so overwrite p->score with that value.
                 */
                p->score = side_index;
                result->queue[result->nr++] = p;
        }

        /* Free each value in the collisions map */
        strmap_for_each_entry(&collisions, &iter, entry) {
                struct collision_info *info = entry->value;
                string_list_clear(&info->source_files, 0);
        }
        /*
         * In compute_collisions(), we set collisions.strdup_strings to 0
         * so that we wouldn't have to make another copy of the new_path
         * allocated by apply_dir_rename().  But now that we've used them
         * and have no other references to these strings, it is time to
         * deallocate them.
         */
        free_strmap_strings(&collisions);
        strmap_clear(&collisions, 1);
        return clean;
}

static int detect_and_process_renames(struct merge_options *opt,
                                      struct tree *merge_base,
                                      struct tree *side1,
                                      struct tree *side2)
{
        struct diff_queue_struct combined;
        struct rename_info *renames = &opt->priv->renames;
        int need_dir_renames, s, clean = 1;
        unsigned detection_run = 0;

        memset(&combined, 0, sizeof(combined));
        if (!possible_renames(renames))
                goto cleanup;

        trace2_region_enter("merge", "regular renames", opt->repo);
        detection_run |= detect_regular_renames(opt, MERGE_SIDE1);
        detection_run |= detect_regular_renames(opt, MERGE_SIDE2);
        if (renames->redo_after_renames && detection_run) {
                int i, side;
                struct diff_filepair *p;

                /* Cache the renames, we found */
                for (side = MERGE_SIDE1; side <= MERGE_SIDE2; side++) {
                        for (i = 0; i < renames->pairs[side].nr; ++i) {
                                p = renames->pairs[side].queue[i];
                                possibly_cache_new_pair(renames, p, side, NULL);
                        }
                }

                /* Restart the merge with the cached renames */
                renames->redo_after_renames = 2;
                trace2_region_leave("merge", "regular renames", opt->repo);
                goto cleanup;
        }
        use_cached_pairs(opt, &renames->cached_pairs[1], &renames->pairs[1]);
        use_cached_pairs(opt, &renames->cached_pairs[2], &renames->pairs[2]);
        trace2_region_leave("merge", "regular renames", opt->repo);

        trace2_region_enter("merge", "directory renames", opt->repo);
        need_dir_renames =
          !opt->priv->call_depth &&
          (opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_TRUE ||
           opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_CONFLICT);

        if (need_dir_renames) {
                get_provisional_directory_renames(opt, MERGE_SIDE1, &clean);
                get_provisional_directory_renames(opt, MERGE_SIDE2, &clean);
                handle_directory_level_conflicts(opt);
        }

        ALLOC_GROW(combined.queue,
                   renames->pairs[1].nr + renames->pairs[2].nr,
                   combined.alloc);
        clean &= collect_renames(opt, &combined, MERGE_SIDE1,
                                 &renames->dir_renames[2],
                                 &renames->dir_renames[1]);
        clean &= collect_renames(opt, &combined, MERGE_SIDE2,
                                 &renames->dir_renames[1],
                                 &renames->dir_renames[2]);
        STABLE_QSORT(combined.queue, combined.nr, compare_pairs);
        trace2_region_leave("merge", "directory renames", opt->repo);

        trace2_region_enter("merge", "process renames", opt->repo);
        clean &= process_renames(opt, &combined);
        trace2_region_leave("merge", "process renames", opt->repo);

        goto simple_cleanup; /* collect_renames() handles some of cleanup */

cleanup:
        /*
         * Free now unneeded filepairs, which would have been handled
         * in collect_renames() normally but we skipped that code.
         */
        for (s = MERGE_SIDE1; s <= MERGE_SIDE2; s++) {
                struct diff_queue_struct *side_pairs;
                int i;

                side_pairs = &renames->pairs[s];
                for (i = 0; i < side_pairs->nr; ++i) {
                        struct diff_filepair *p = side_pairs->queue[i];
                        pool_diff_free_filepair(&opt->priv->pool, p);
                }
        }

simple_cleanup:
        /* Free memory for renames->pairs[] and combined */
        for (s = MERGE_SIDE1; s <= MERGE_SIDE2; s++) {
                free(renames->pairs[s].queue);
                DIFF_QUEUE_CLEAR(&renames->pairs[s]);
        }
        if (combined.nr) {
                int i;
                for (i = 0; i < combined.nr; i++)
                        pool_diff_free_filepair(&opt->priv->pool,
                                                combined.queue[i]);
                free(combined.queue);
        }

        return clean;
}

/*** Function Grouping: functions related to process_entries() ***/

static int sort_dirs_next_to_their_children(const char *one, const char *two)
{
        unsigned char c1, c2;

        /*
         * Here we only care that entries for directories appear adjacent
         * to and before files underneath the directory.  We can achieve
         * that by pretending to add a trailing slash to every file and
         * then sorting.  In other words, we do not want the natural
         * sorting of
         *     foo
         *     foo.txt
         *     foo/bar
         * Instead, we want "foo" to sort as though it were "foo/", so that
         * we instead get
         *     foo.txt
         *     foo
         *     foo/bar
         * To achieve this, we basically implement our own strcmp, except that
         * if we get to the end of either string instead of comparing NUL to
         * another character, we compare '/' to it.
         *
         * If this unusual "sort as though '/' were appended" perplexes
         * you, perhaps it will help to note that this is not the final
         * sort.  write_tree() will sort again without the trailing slash
         * magic, but just on paths immediately under a given tree.
         *
         * The reason to not use df_name_compare directly was that it was
         * just too expensive (we don't have the string lengths handy), so
         * it was reimplemented.
         */

        /*
         * NOTE: This function will never be called with two equal strings,
         * because it is used to sort the keys of a strmap, and strmaps have
         * unique keys by construction.  That simplifies our c1==c2 handling
         * below.
         */

        while (*one && (*one == *two)) {
                one++;
                two++;
        }

        c1 = *one ? *one : '/';
        c2 = *two ? *two : '/';

        if (c1 == c2) {
                /* Getting here means one is a leading directory of the other */
                return (*one) ? 1 : -1;
        } else
                return c1 - c2;
}

static int read_oid_strbuf(struct merge_options *opt,
                           const struct object_id *oid,
                           struct strbuf *dst)
{
        void *buf;
        enum object_type type;
        unsigned long size;
        buf = read_object_file(oid, &type, &size);
        if (!buf)
                return err(opt, _("cannot read object %s"), oid_to_hex(oid));
        if (type != OBJ_BLOB) {
                free(buf);
                return err(opt, _("object %s is not a blob"), oid_to_hex(oid));
        }
        strbuf_attach(dst, buf, size, size + 1);
        return 0;
}

static int blob_unchanged(struct merge_options *opt,
                          const struct version_info *base,
                          const struct version_info *side,
                          const char *path)
{
        struct strbuf basebuf = STRBUF_INIT;
        struct strbuf sidebuf = STRBUF_INIT;
        int ret = 0; /* assume changed for safety */
        struct index_state *idx = &opt->priv->attr_index;

        if (!idx->initialized)
                initialize_attr_index(opt);

        if (base->mode != side->mode)
                return 0;
        if (oideq(&base->oid, &side->oid))
                return 1;

        if (read_oid_strbuf(opt, &base->oid, &basebuf) ||
            read_oid_strbuf(opt, &side->oid, &sidebuf))
                goto error_return;
        /*
         * Note: binary | is used so that both renormalizations are
         * performed.  Comparison can be skipped if both files are
         * unchanged since their sha1s have already been compared.
         */
        if (renormalize_buffer(idx, path, basebuf.buf, basebuf.len, &basebuf) |
            renormalize_buffer(idx, path, sidebuf.buf, sidebuf.len, &sidebuf))
                ret = (basebuf.len == sidebuf.len &&
                       !memcmp(basebuf.buf, sidebuf.buf, basebuf.len));

error_return:
        strbuf_release(&basebuf);
        strbuf_release(&sidebuf);
        return ret;
}

struct directory_versions {
        /*
         * versions: list of (basename -> version_info)
         *
         * The basenames are in reverse lexicographic order of full pathnames,
         * as processed in process_entries().  This puts all entries within
         * a directory together, and covers the directory itself after
         * everything within it, allowing us to write subtrees before needing
         * to record information for the tree itself.
         */
        struct string_list versions;

        /*
         * offsets: list of (full relative path directories -> integer offsets)
         *
         * Since versions contains basenames from files in multiple different
         * directories, we need to know which entries in versions correspond
         * to which directories.  Values of e.g.
         *     ""             0
         *     src            2
         *     src/moduleA    5
         * Would mean that entries 0-1 of versions are files in the toplevel
         * directory, entries 2-4 are files under src/, and the remaining
         * entries starting at index 5 are files under src/moduleA/.
         */
        struct string_list offsets;

        /*
         * last_directory: directory that previously processed file found in
         *
         * last_directory starts NULL, but records the directory in which the
         * previous file was found within.  As soon as
         *    directory(current_file) != last_directory
         * then we need to start updating accounting in versions & offsets.
         * Note that last_directory is always the last path in "offsets" (or
         * NULL if "offsets" is empty) so this exists just for quick access.
         */
        const char *last_directory;

        /* last_directory_len: cached computation of strlen(last_directory) */
        unsigned last_directory_len;
};

static int tree_entry_order(const void *a_, const void *b_)
{
        const struct string_list_item *a = a_;
        const struct string_list_item *b = b_;

        const struct merged_info *ami = a->util;
        const struct merged_info *bmi = b->util;
        return base_name_compare(a->string, strlen(a->string), ami->result.mode,
                                 b->string, strlen(b->string), bmi->result.mode);
}

static void write_tree(struct object_id *result_oid,
                       struct string_list *versions,
                       unsigned int offset,
                       size_t hash_size)
{
        size_t maxlen = 0, extra;
        unsigned int nr;
        struct strbuf buf = STRBUF_INIT;
        int i;

        assert(offset <= versions->nr);
        nr = versions->nr - offset;
        if (versions->nr)
                /* No need for STABLE_QSORT -- filenames must be unique */
                QSORT(versions->items + offset, nr, tree_entry_order);

        /* Pre-allocate some space in buf */
        extra = hash_size + 8; /* 8: 6 for mode, 1 for space, 1 for NUL char */
        for (i = 0; i < nr; i++) {
                maxlen += strlen(versions->items[offset+i].string) + extra;
        }
        strbuf_grow(&buf, maxlen);

        /* Write each entry out to buf */
        for (i = 0; i < nr; i++) {
                struct merged_info *mi = versions->items[offset+i].util;
                struct version_info *ri = &mi->result;
                strbuf_addf(&buf, "%o %s%c",
                            ri->mode,
                            versions->items[offset+i].string, '\0');
                strbuf_add(&buf, ri->oid.hash, hash_size);
        }

        /* Write this object file out, and record in result_oid */
        write_object_file(buf.buf, buf.len, tree_type, result_oid);
        strbuf_release(&buf);
}

static void record_entry_for_tree(struct directory_versions *dir_metadata,
                                  const char *path,
                                  struct merged_info *mi)
{
        const char *basename;

        if (mi->is_null)
                /* nothing to record */
                return;

        basename = path + mi->basename_offset;
        assert(strchr(basename, '/') == NULL);
        string_list_append(&dir_metadata->versions,
                           basename)->util = &mi->result;
}

static void write_completed_directory(struct merge_options *opt,
                                      const char *new_directory_name,
                                      struct directory_versions *info)
{
        const char *prev_dir;
        struct merged_info *dir_info = NULL;
        unsigned int offset;

        /*
         * Some explanation of info->versions and info->offsets...
         *
         * process_entries() iterates over all relevant files AND
         * directories in reverse lexicographic order, and calls this
         * function.  Thus, an example of the paths that process_entries()
         * could operate on (along with the directories for those paths
         * being shown) is:
         *
         *     xtract.c             ""
         *     tokens.txt           ""
         *     src/moduleB/umm.c    src/moduleB
         *     src/moduleB/stuff.h  src/moduleB
         *     src/moduleB/baz.c    src/moduleB
         *     src/moduleB          src
         *     src/moduleA/foo.c    src/moduleA
         *     src/moduleA/bar.c    src/moduleA
         *     src/moduleA          src
         *     src                  ""
         *     Makefile             ""
         *
         * info->versions:
         *
         *     always contains the unprocessed entries and their
         *     version_info information.  For example, after the first five
         *     entries above, info->versions would be:
         *
         *         xtract.c     <xtract.c's version_info>
         *         token.txt    <token.txt's version_info>
         *         umm.c        <src/moduleB/umm.c's version_info>
         *         stuff.h      <src/moduleB/stuff.h's version_info>
         *         baz.c        <src/moduleB/baz.c's version_info>
         *
         *     Once a subdirectory is completed we remove the entries in
         *     that subdirectory from info->versions, writing it as a tree
         *     (write_tree()).  Thus, as soon as we get to src/moduleB,
         *     info->versions would be updated to
         *
         *         xtract.c     <xtract.c's version_info>
         *         token.txt    <token.txt's version_info>
         *         moduleB      <src/moduleB's version_info>
         *
         * info->offsets:
         *
         *     helps us track which entries in info->versions correspond to
         *     which directories.  When we are N directories deep (e.g. 4
         *     for src/modA/submod/subdir/), we have up to N+1 unprocessed
         *     directories (+1 because of toplevel dir).  Corresponding to
         *     the info->versions example above, after processing five entries
         *     info->offsets will be:
         *
         *         ""           0
         *         src/moduleB  2
         *
         *     which is used to know that xtract.c & token.txt are from the
         *     toplevel dirctory, while umm.c & stuff.h & baz.c are from the
         *     src/moduleB directory.  Again, following the example above,
         *     once we need to process src/moduleB, then info->offsets is
         *     updated to
         *
         *         ""           0
         *         src          2
         *
         *     which says that moduleB (and only moduleB so far) is in the
         *     src directory.
         *
         *     One unique thing to note about info->offsets here is that
         *     "src" was not added to info->offsets until there was a path
         *     (a file OR directory) immediately below src/ that got
         *     processed.
         *
         * Since process_entry() just appends new entries to info->versions,
         * write_completed_directory() only needs to do work if the next path
         * is in a directory that is different than the last directory found
         * in info->offsets.
         */

        /*
         * If we are working with the same directory as the last entry, there
         * is no work to do.  (See comments above the directory_name member of
         * struct merged_info for why we can use pointer comparison instead of
         * strcmp here.)
         */
        if (new_directory_name == info->last_directory)
                return;

        /*
         * If we are just starting (last_directory is NULL), or last_directory
         * is a prefix of the current directory, then we can just update
         * info->offsets to record the offset where we started this directory
         * and update last_directory to have quick access to it.
         */
        if (info->last_directory == NULL ||
            !strncmp(new_directory_name, info->last_directory,
                     info->last_directory_len)) {
                uintptr_t offset = info->versions.nr;

                info->last_directory = new_directory_name;
                info->last_directory_len = strlen(info->last_directory);
                /*
                 * Record the offset into info->versions where we will
                 * start recording basenames of paths found within
                 * new_directory_name.
                 */
                string_list_append(&info->offsets,
                                   info->last_directory)->util = (void*)offset;
                return;
        }

        /*
         * The next entry that will be processed will be within
         * new_directory_name.  Since at this point we know that
         * new_directory_name is within a different directory than
         * info->last_directory, we have all entries for info->last_directory
         * in info->versions and we need to create a tree object for them.
         */
        dir_info = strmap_get(&opt->priv->paths, info->last_directory);
        assert(dir_info);
        offset = (uintptr_t)info->offsets.items[info->offsets.nr-1].util;
        if (offset == info->versions.nr) {
                /*
                 * Actually, we don't need to create a tree object in this
                 * case.  Whenever all files within a directory disappear
                 * during the merge (e.g. unmodified on one side and
                 * deleted on the other, or files were renamed elsewhere),
                 * then we get here and the directory itself needs to be
                 * omitted from its parent tree as well.
                 */
                dir_info->is_null = 1;
        } else {
                /*
                 * Write out the tree to the git object directory, and also
                 * record the mode and oid in dir_info->result.
                 */
                dir_info->is_null = 0;
                dir_info->result.mode = S_IFDIR;
                write_tree(&dir_info->result.oid, &info->versions, offset,
                           opt->repo->hash_algo->rawsz);
        }

        /*
         * We've now used several entries from info->versions and one entry
         * from info->offsets, so we get rid of those values.
         */
        info->offsets.nr--;
        info->versions.nr = offset;

        /*
         * Now we've taken care of the completed directory, but we need to
         * prepare things since future entries will be in
         * new_directory_name.  (In particular, process_entry() will be
         * appending new entries to info->versions.)  So, we need to make
         * sure new_directory_name is the last entry in info->offsets.
         */
        prev_dir = info->offsets.nr == 0 ? NULL :
                   info->offsets.items[info->offsets.nr-1].string;
        if (new_directory_name != prev_dir) {
                uintptr_t c = info->versions.nr;
                string_list_append(&info->offsets,
                                   new_directory_name)->util = (void*)c;
        }

        /* And, of course, we need to update last_directory to match. */
        info->last_directory = new_directory_name;
        info->last_directory_len = strlen(info->last_directory);
}

/* Per entry merge function */
static void process_entry(struct merge_options *opt,
                          const char *path,
                          struct conflict_info *ci,
                          struct directory_versions *dir_metadata)
{
        int df_file_index = 0;

        VERIFY_CI(ci);
        assert(ci->filemask >= 0 && ci->filemask <= 7);
        /* ci->match_mask == 7 was handled in collect_merge_info_callback() */
        assert(ci->match_mask == 0 || ci->match_mask == 3 ||
               ci->match_mask == 5 || ci->match_mask == 6);

        if (ci->dirmask) {
                record_entry_for_tree(dir_metadata, path, &ci->merged);
                if (ci->filemask == 0)
                        /* nothing else to handle */
                        return;
                assert(ci->df_conflict);
        }

        if (ci->df_conflict && ci->merged.result.mode == 0) {
                int i;

                /*
                 * directory no longer in the way, but we do have a file we
                 * need to place here so we need to clean away the "directory
                 * merges to nothing" result.
                 */
                ci->df_conflict = 0;
                assert(ci->filemask != 0);
                ci->merged.clean = 0;
                ci->merged.is_null = 0;
                /* and we want to zero out any directory-related entries */
                ci->match_mask = (ci->match_mask & ~ci->dirmask);
                ci->dirmask = 0;
                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        if (ci->filemask & (1 << i))
                                continue;
                        ci->stages[i].mode = 0;
                        oidcpy(&ci->stages[i].oid, null_oid());
                }
        } else if (ci->df_conflict && ci->merged.result.mode != 0) {
                /*
                 * This started out as a D/F conflict, and the entries in
                 * the competing directory were not removed by the merge as
                 * evidenced by write_completed_directory() writing a value
                 * to ci->merged.result.mode.
                 */
                struct conflict_info *new_ci;
                const char *branch;
                const char *old_path = path;
                int i;

                assert(ci->merged.result.mode == S_IFDIR);

                /*
                 * If filemask is 1, we can just ignore the file as having
                 * been deleted on both sides.  We do not want to overwrite
                 * ci->merged.result, since it stores the tree for all the
                 * files under it.
                 */
                if (ci->filemask == 1) {
                        ci->filemask = 0;
                        return;
                }

                /*
                 * This file still exists on at least one side, and we want
                 * the directory to remain here, so we need to move this
                 * path to some new location.
                 */
                new_ci = mem_pool_calloc(&opt->priv->pool, 1, sizeof(*new_ci));

                /* We don't really want new_ci->merged.result copied, but it'll
                 * be overwritten below so it doesn't matter.  We also don't
                 * want any directory mode/oid values copied, but we'll zero
                 * those out immediately.  We do want the rest of ci copied.
                 */
                memcpy(new_ci, ci, sizeof(*ci));
                new_ci->match_mask = (new_ci->match_mask & ~new_ci->dirmask);
                new_ci->dirmask = 0;
                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        if (new_ci->filemask & (1 << i))
                                continue;
                        /* zero out any entries related to directories */
                        new_ci->stages[i].mode = 0;
                        oidcpy(&new_ci->stages[i].oid, null_oid());
                }

                /*
                 * Find out which side this file came from; note that we
                 * cannot just use ci->filemask, because renames could cause
                 * the filemask to go back to 7.  So we use dirmask, then
                 * pick the opposite side's index.
                 */
                df_file_index = (ci->dirmask & (1 << 1)) ? 2 : 1;
                branch = (df_file_index == 1) ? opt->branch1 : opt->branch2;
                path = unique_path(&opt->priv->paths, path, branch);
                strmap_put(&opt->priv->paths, path, new_ci);

                path_msg(opt, path, 0,
                         _("CONFLICT (file/directory): directory in the way "
                           "of %s from %s; moving it to %s instead."),
                         old_path, branch, path);

                /*
                 * Zero out the filemask for the old ci.  At this point, ci
                 * was just an entry for a directory, so we don't need to
                 * do anything more with it.
                 */
                ci->filemask = 0;

                /*
                 * Now note that we're working on the new entry (path was
                 * updated above.
                 */
                ci = new_ci;
        }

        /*
         * NOTE: Below there is a long switch-like if-elseif-elseif... block
         *       which the code goes through even for the df_conflict cases
         *       above.
         */
        if (ci->match_mask) {
                ci->merged.clean = !ci->df_conflict && !ci->path_conflict;
                if (ci->match_mask == 6) {
                        /* stages[1] == stages[2] */
                        ci->merged.result.mode = ci->stages[1].mode;
                        oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
                } else {
                        /* determine the mask of the side that didn't match */
                        unsigned int othermask = 7 & ~ci->match_mask;
                        int side = (othermask == 4) ? 2 : 1;

                        ci->merged.result.mode = ci->stages[side].mode;
                        ci->merged.is_null = !ci->merged.result.mode;
                        if (ci->merged.is_null)
                                ci->merged.clean = 1;
                        oidcpy(&ci->merged.result.oid, &ci->stages[side].oid);

                        assert(othermask == 2 || othermask == 4);
                        assert(ci->merged.is_null ==
                               (ci->filemask == ci->match_mask));
                }
        } else if (ci->filemask >= 6 &&
                   (S_IFMT & ci->stages[1].mode) !=
                   (S_IFMT & ci->stages[2].mode)) {
                /* Two different items from (file/submodule/symlink) */
                if (opt->priv->call_depth) {
                        /* Just use the version from the merge base */
                        ci->merged.clean = 0;
                        oidcpy(&ci->merged.result.oid, &ci->stages[0].oid);
                        ci->merged.result.mode = ci->stages[0].mode;
                        ci->merged.is_null = (ci->merged.result.mode == 0);
                } else {
                        /* Handle by renaming one or both to separate paths. */
                        unsigned o_mode = ci->stages[0].mode;
                        unsigned a_mode = ci->stages[1].mode;
                        unsigned b_mode = ci->stages[2].mode;
                        struct conflict_info *new_ci;
                        const char *a_path = NULL, *b_path = NULL;
                        int rename_a = 0, rename_b = 0;

                        new_ci = mem_pool_alloc(&opt->priv->pool,
                                                sizeof(*new_ci));

                        if (S_ISREG(a_mode))
                                rename_a = 1;
                        else if (S_ISREG(b_mode))
                                rename_b = 1;
                        else {
                                rename_a = 1;
                                rename_b = 1;
                        }

                        if (rename_a && rename_b) {
                                path_msg(opt, path, 0,
                                         _("CONFLICT (distinct types): %s had "
                                           "different types on each side; "
                                           "renamed both of them so each can "
                                           "be recorded somewhere."),
                                         path);
                        } else {
                                path_msg(opt, path, 0,
                                         _("CONFLICT (distinct types): %s had "
                                           "different types on each side; "
                                           "renamed one of them so each can be "
                                           "recorded somewhere."),
                                         path);
                        }

                        ci->merged.clean = 0;
                        memcpy(new_ci, ci, sizeof(*new_ci));

                        /* Put b into new_ci, removing a from stages */
                        new_ci->merged.result.mode = ci->stages[2].mode;
                        oidcpy(&new_ci->merged.result.oid, &ci->stages[2].oid);
                        new_ci->stages[1].mode = 0;
                        oidcpy(&new_ci->stages[1].oid, null_oid());
                        new_ci->filemask = 5;
                        if ((S_IFMT & b_mode) != (S_IFMT & o_mode)) {
                                new_ci->stages[0].mode = 0;
                                oidcpy(&new_ci->stages[0].oid, null_oid());
                                new_ci->filemask = 4;
                        }

                        /* Leave only a in ci, fixing stages. */
                        ci->merged.result.mode = ci->stages[1].mode;
                        oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
                        ci->stages[2].mode = 0;
                        oidcpy(&ci->stages[2].oid, null_oid());
                        ci->filemask = 3;
                        if ((S_IFMT & a_mode) != (S_IFMT & o_mode)) {
                                ci->stages[0].mode = 0;
                                oidcpy(&ci->stages[0].oid, null_oid());
                                ci->filemask = 2;
                        }

                        /* Insert entries into opt->priv_paths */
                        assert(rename_a || rename_b);
                        if (rename_a) {
                                a_path = unique_path(&opt->priv->paths,
                                                     path, opt->branch1);
                                strmap_put(&opt->priv->paths, a_path, ci);
                        }

                        if (rename_b)
                                b_path = unique_path(&opt->priv->paths,
                                                     path, opt->branch2);
                        else
                                b_path = path;
                        strmap_put(&opt->priv->paths, b_path, new_ci);

                        if (rename_a && rename_b)
                                strmap_remove(&opt->priv->paths, path, 0);

                        /*
                         * Do special handling for b_path since process_entry()
                         * won't be called on it specially.
                         */
                        strmap_put(&opt->priv->conflicted, b_path, new_ci);
                        record_entry_for_tree(dir_metadata, b_path,
                                              &new_ci->merged);

                        /*
                         * Remaining code for processing this entry should
                         * think in terms of processing a_path.
                         */
                        if (a_path)
                                path = a_path;
                }
        } else if (ci->filemask >= 6) {
                /* Need a two-way or three-way content merge */
                struct version_info merged_file;
                unsigned clean_merge;
                struct version_info *o = &ci->stages[0];
                struct version_info *a = &ci->stages[1];
                struct version_info *b = &ci->stages[2];

                clean_merge = handle_content_merge(opt, path, o, a, b,
                                                   ci->pathnames,
                                                   opt->priv->call_depth * 2,
                                                   &merged_file);
                ci->merged.clean = clean_merge &&
                                   !ci->df_conflict && !ci->path_conflict;
                ci->merged.result.mode = merged_file.mode;
                ci->merged.is_null = (merged_file.mode == 0);
                oidcpy(&ci->merged.result.oid, &merged_file.oid);
                if (clean_merge && ci->df_conflict) {
                        assert(df_file_index == 1 || df_file_index == 2);
                        ci->filemask = 1 << df_file_index;
                        ci->stages[df_file_index].mode = merged_file.mode;
                        oidcpy(&ci->stages[df_file_index].oid, &merged_file.oid);
                }
                if (!clean_merge) {
                        const char *reason = _("content");
                        if (ci->filemask == 6)
                                reason = _("add/add");
                        if (S_ISGITLINK(merged_file.mode))
                                reason = _("submodule");
                        path_msg(opt, path, 0,
                                 _("CONFLICT (%s): Merge conflict in %s"),
                                 reason, path);
                }
        } else if (ci->filemask == 3 || ci->filemask == 5) {
                /* Modify/delete */
                const char *modify_branch, *delete_branch;
                int side = (ci->filemask == 5) ? 2 : 1;
                int index = opt->priv->call_depth ? 0 : side;

                ci->merged.result.mode = ci->stages[index].mode;
                oidcpy(&ci->merged.result.oid, &ci->stages[index].oid);
                ci->merged.clean = 0;

                modify_branch = (side == 1) ? opt->branch1 : opt->branch2;
                delete_branch = (side == 1) ? opt->branch2 : opt->branch1;

                if (opt->renormalize &&
                    blob_unchanged(opt, &ci->stages[0], &ci->stages[side],
                                   path)) {
                        ci->merged.is_null = 1;
                        ci->merged.clean = 1;
                        assert(!ci->df_conflict && !ci->path_conflict);
                } else if (ci->path_conflict &&
                           oideq(&ci->stages[0].oid, &ci->stages[side].oid)) {
                        /*
                         * This came from a rename/delete; no action to take,
                         * but avoid printing "modify/delete" conflict notice
                         * since the contents were not modified.
                         */
                } else {
                        path_msg(opt, path, 0,
                                 _("CONFLICT (modify/delete): %s deleted in %s "
                                   "and modified in %s.  Version %s of %s left "
                                   "in tree."),
                                 path, delete_branch, modify_branch,
                                 modify_branch, path);
                }
        } else if (ci->filemask == 2 || ci->filemask == 4) {
                /* Added on one side */
                int side = (ci->filemask == 4) ? 2 : 1;
                ci->merged.result.mode = ci->stages[side].mode;
                oidcpy(&ci->merged.result.oid, &ci->stages[side].oid);
                ci->merged.clean = !ci->df_conflict && !ci->path_conflict;
        } else if (ci->filemask == 1) {
                /* Deleted on both sides */
                ci->merged.is_null = 1;
                ci->merged.result.mode = 0;
                oidcpy(&ci->merged.result.oid, null_oid());
                assert(!ci->df_conflict);
                ci->merged.clean = !ci->path_conflict;
        }

        /*
         * If still conflicted, record it separately.  This allows us to later
         * iterate over just conflicted entries when updating the index instead
         * of iterating over all entries.
         */
        if (!ci->merged.clean)
                strmap_put(&opt->priv->conflicted, path, ci);

        /* Record metadata for ci->merged in dir_metadata */
        record_entry_for_tree(dir_metadata, path, &ci->merged);
}

static void prefetch_for_content_merges(struct merge_options *opt,
                                        struct string_list *plist)
{
        struct string_list_item *e;
        struct oid_array to_fetch = OID_ARRAY_INIT;

        if (opt->repo != the_repository || !has_promisor_remote())
                return;

        for (e = &plist->items[plist->nr-1]; e >= plist->items; --e) {
                /* char *path = e->string; */
                struct conflict_info *ci = e->util;
                int i;

                /* Ignore clean entries */
                if (ci->merged.clean)
                        continue;

                /* Ignore entries that don't need a content merge */
                if (ci->match_mask || ci->filemask < 6 ||
                    !S_ISREG(ci->stages[1].mode) ||
                    !S_ISREG(ci->stages[2].mode) ||
                    oideq(&ci->stages[1].oid, &ci->stages[2].oid))
                        continue;

                /* Also don't need content merge if base matches either side */
                if (ci->filemask == 7 &&
                    S_ISREG(ci->stages[0].mode) &&
                    (oideq(&ci->stages[0].oid, &ci->stages[1].oid) ||
                     oideq(&ci->stages[0].oid, &ci->stages[2].oid)))
                        continue;

                for (i = 0; i < 3; i++) {
                        unsigned side_mask = (1 << i);
                        struct version_info *vi = &ci->stages[i];

                        if ((ci->filemask & side_mask) &&
                            S_ISREG(vi->mode) &&
                            oid_object_info_extended(opt->repo, &vi->oid, NULL,
                                                     OBJECT_INFO_FOR_PREFETCH))
                                oid_array_append(&to_fetch, &vi->oid);
                }
        }

        promisor_remote_get_direct(opt->repo, to_fetch.oid, to_fetch.nr);
        oid_array_clear(&to_fetch);
}

static void process_entries(struct merge_options *opt,
                            struct object_id *result_oid)
{
        struct hashmap_iter iter;
        struct strmap_entry *e;
        struct string_list plist = STRING_LIST_INIT_NODUP;
        struct string_list_item *entry;
        struct directory_versions dir_metadata = { STRING_LIST_INIT_NODUP,
                                                   STRING_LIST_INIT_NODUP,
                                                   NULL, 0 };

        trace2_region_enter("merge", "process_entries setup", opt->repo);
        if (strmap_empty(&opt->priv->paths)) {
                oidcpy(result_oid, opt->repo->hash_algo->empty_tree);
                return;
        }

        /* Hack to pre-allocate plist to the desired size */
        trace2_region_enter("merge", "plist grow", opt->repo);
        ALLOC_GROW(plist.items, strmap_get_size(&opt->priv->paths), plist.alloc);
        trace2_region_leave("merge", "plist grow", opt->repo);

        /* Put every entry from paths into plist, then sort */
        trace2_region_enter("merge", "plist copy", opt->repo);
        strmap_for_each_entry(&opt->priv->paths, &iter, e) {
                string_list_append(&plist, e->key)->util = e->value;
        }
        trace2_region_leave("merge", "plist copy", opt->repo);

        trace2_region_enter("merge", "plist special sort", opt->repo);
        plist.cmp = sort_dirs_next_to_their_children;
        string_list_sort(&plist);
        trace2_region_leave("merge", "plist special sort", opt->repo);

        trace2_region_leave("merge", "process_entries setup", opt->repo);

        /*
         * Iterate over the items in reverse order, so we can handle paths
         * below a directory before needing to handle the directory itself.
         *
         * This allows us to write subtrees before we need to write trees,
         * and it also enables sane handling of directory/file conflicts
         * (because it allows us to know whether the directory is still in
         * the way when it is time to process the file at the same path).
         */
        trace2_region_enter("merge", "processing", opt->repo);
        prefetch_for_content_merges(opt, &plist);
        for (entry = &plist.items[plist.nr-1]; entry >= plist.items; --entry) {
                char *path = entry->string;
                /*
                 * NOTE: mi may actually be a pointer to a conflict_info, but
                 * we have to check mi->clean first to see if it's safe to
                 * reassign to such a pointer type.
                 */
                struct merged_info *mi = entry->util;

                write_completed_directory(opt, mi->directory_name,
                                          &dir_metadata);
                if (mi->clean)
                        record_entry_for_tree(&dir_metadata, path, mi);
                else {
                        struct conflict_info *ci = (struct conflict_info *)mi;
                        process_entry(opt, path, ci, &dir_metadata);
                }
        }
        trace2_region_leave("merge", "processing", opt->repo);

        trace2_region_enter("merge", "process_entries cleanup", opt->repo);
        if (dir_metadata.offsets.nr != 1 ||
            (uintptr_t)dir_metadata.offsets.items[0].util != 0) {
                printf("dir_metadata.offsets.nr = %d (should be 1)\n",
                       dir_metadata.offsets.nr);
                printf("dir_metadata.offsets.items[0].util = %u (should be 0)\n",
                       (unsigned)(uintptr_t)dir_metadata.offsets.items[0].util);
                fflush(stdout);
                BUG("dir_metadata accounting completely off; shouldn't happen");
        }
        write_tree(result_oid, &dir_metadata.versions, 0,
                   opt->repo->hash_algo->rawsz);
        string_list_clear(&plist, 0);
        string_list_clear(&dir_metadata.versions, 0);
        string_list_clear(&dir_metadata.offsets, 0);
        trace2_region_leave("merge", "process_entries cleanup", opt->repo);
}

/*** Function Grouping: functions related to merge_switch_to_result() ***/

static int checkout(struct merge_options *opt,
                    struct tree *prev,
                    struct tree *next)
{
        /* Switch the index/working copy from old to new */
        int ret;
        struct tree_desc trees[2];
        struct unpack_trees_options unpack_opts;

        memset(&unpack_opts, 0, sizeof(unpack_opts));
        unpack_opts.head_idx = -1;
        unpack_opts.src_index = opt->repo->index;
        unpack_opts.dst_index = opt->repo->index;

        setup_unpack_trees_porcelain(&unpack_opts, "merge");

        /*
         * NOTE: if this were just "git checkout" code, we would probably
         * read or refresh the cache and check for a conflicted index, but
         * builtin/merge.c or sequencer.c really needs to read the index
         * and check for conflicted entries before starting merging for a
         * good user experience (no sense waiting for merges/rebases before
         * erroring out), so there's no reason to duplicate that work here.
         */

        /* 2-way merge to the new branch */
        unpack_opts.update = 1;
        unpack_opts.merge = 1;
        unpack_opts.quiet = 0; /* FIXME: sequencer might want quiet? */
        unpack_opts.verbose_update = (opt->verbosity > 2);
        unpack_opts.fn = twoway_merge;
        if (1/* FIXME: opts->overwrite_ignore*/) {
                CALLOC_ARRAY(unpack_opts.dir, 1);
                unpack_opts.dir->flags |= DIR_SHOW_IGNORED;
                setup_standard_excludes(unpack_opts.dir);
        }
        parse_tree(prev);
        init_tree_desc(&trees[0], prev->buffer, prev->size);
        parse_tree(next);
        init_tree_desc(&trees[1], next->buffer, next->size);

        ret = unpack_trees(2, trees, &unpack_opts);
        clear_unpack_trees_porcelain(&unpack_opts);
        dir_clear(unpack_opts.dir);
        FREE_AND_NULL(unpack_opts.dir);
        return ret;
}

static int record_conflicted_index_entries(struct merge_options *opt)
{
        struct hashmap_iter iter;
        struct strmap_entry *e;
        struct index_state *index = opt->repo->index;
        struct checkout state = CHECKOUT_INIT;
        int errs = 0;
        int original_cache_nr;

        if (strmap_empty(&opt->priv->conflicted))
                return 0;

        /* If any entries have skip_worktree set, we'll have to check 'em out */
        state.force = 1;
        state.quiet = 1;
        state.refresh_cache = 1;
        state.istate = index;
        original_cache_nr = index->cache_nr;

        /* Put every entry from paths into plist, then sort */
        strmap_for_each_entry(&opt->priv->conflicted, &iter, e) {
                const char *path = e->key;
                struct conflict_info *ci = e->value;
                int pos;
                struct cache_entry *ce;
                int i;

                VERIFY_CI(ci);

                /*
                 * The index will already have a stage=0 entry for this path,
                 * because we created an as-merged-as-possible version of the
                 * file and checkout() moved the working copy and index over
                 * to that version.
                 *
                 * However, previous iterations through this loop will have
                 * added unstaged entries to the end of the cache which
                 * ignore the standard alphabetical ordering of cache
                 * entries and break invariants needed for index_name_pos()
                 * to work.  However, we know the entry we want is before
                 * those appended cache entries, so do a temporary swap on
                 * cache_nr to only look through entries of interest.
                 */
                SWAP(index->cache_nr, original_cache_nr);
                pos = index_name_pos(index, path, strlen(path));
                SWAP(index->cache_nr, original_cache_nr);
                if (pos < 0) {
                        if (ci->filemask != 1)
                                BUG("Conflicted %s but nothing in basic working tree or index; this shouldn't happen", path);
                        cache_tree_invalidate_path(index, path);
                } else {
                        ce = index->cache[pos];

                        /*
                         * Clean paths with CE_SKIP_WORKTREE set will not be
                         * written to the working tree by the unpack_trees()
                         * call in checkout().  Our conflicted entries would
                         * have appeared clean to that code since we ignored
                         * the higher order stages.  Thus, we need override
                         * the CE_SKIP_WORKTREE bit and manually write those
                         * files to the working disk here.
                         */
                        if (ce_skip_worktree(ce)) {
                                struct stat st;

                                if (!lstat(path, &st)) {
                                        char *new_name = unique_path(&opt->priv->paths,
                                                                     path,
                                                                     "cruft");

                                        path_msg(opt, path, 1,
                                                 _("Note: %s not up to date and in way of checking out conflicted version; old copy renamed to %s"),
                                                 path, new_name);
                                        errs |= rename(path, new_name);
                                        free(new_name);
                                }
                                errs |= checkout_entry(ce, &state, NULL, NULL);
                        }

                        /*
                         * Mark this cache entry for removal and instead add
                         * new stage>0 entries corresponding to the
                         * conflicts.  If there are many conflicted entries, we
                         * want to avoid memmove'ing O(NM) entries by
                         * inserting the new entries one at a time.  So,
                         * instead, we just add the new cache entries to the
                         * end (ignoring normal index requirements on sort
                         * order) and sort the index once we're all done.
                         */
                        ce->ce_flags |= CE_REMOVE;
                }

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        struct version_info *vi;
                        if (!(ci->filemask & (1ul << i)))
                                continue;
                        vi = &ci->stages[i];
                        ce = make_cache_entry(index, vi->mode, &vi->oid,
                                              path, i+1, 0);
                        add_index_entry(index, ce, ADD_CACHE_JUST_APPEND);
                }
        }

        /*
         * Remove the unused cache entries (and invalidate the relevant
         * cache-trees), then sort the index entries to get the conflicted
         * entries we added to the end into their right locations.
         */
        remove_marked_cache_entries(index, 1);
        /*
         * No need for STABLE_QSORT -- cmp_cache_name_compare sorts primarily
         * on filename and secondarily on stage, and (name, stage #) are a
         * unique tuple.
         */
        QSORT(index->cache, index->cache_nr, cmp_cache_name_compare);

        return errs;
}

void merge_switch_to_result(struct merge_options *opt,
                            struct tree *head,
                            struct merge_result *result,
                            int update_worktree_and_index,
                            int display_update_msgs)
{
        assert(opt->priv == NULL);
        if (result->clean >= 0 && update_worktree_and_index) {
                const char *filename;
                FILE *fp;

                trace2_region_enter("merge", "checkout", opt->repo);
                if (checkout(opt, head, result->tree)) {
                        /* failure to function */
                        result->clean = -1;
                        return;
                }
                trace2_region_leave("merge", "checkout", opt->repo);

                trace2_region_enter("merge", "record_conflicted", opt->repo);
                opt->priv = result->priv;
                if (record_conflicted_index_entries(opt)) {
                        /* failure to function */
                        opt->priv = NULL;
                        result->clean = -1;
                        return;
                }
                opt->priv = NULL;
                trace2_region_leave("merge", "record_conflicted", opt->repo);

                trace2_region_enter("merge", "write_auto_merge", opt->repo);
                filename = git_path_auto_merge(opt->repo);
                fp = xfopen(filename, "w");
                fprintf(fp, "%s\n", oid_to_hex(&result->tree->object.oid));
                fclose(fp);
                trace2_region_leave("merge", "write_auto_merge", opt->repo);
        }

        if (display_update_msgs) {
                struct merge_options_internal *opti = result->priv;
                struct hashmap_iter iter;
                struct strmap_entry *e;
                struct string_list olist = STRING_LIST_INIT_NODUP;
                int i;

                trace2_region_enter("merge", "display messages", opt->repo);

                /* Hack to pre-allocate olist to the desired size */
                ALLOC_GROW(olist.items, strmap_get_size(&opti->output),
                           olist.alloc);

                /* Put every entry from output into olist, then sort */
                strmap_for_each_entry(&opti->output, &iter, e) {
                        string_list_append(&olist, e->key)->util = e->value;
                }
                string_list_sort(&olist);

                /* Iterate over the items, printing them */
                for (i = 0; i < olist.nr; ++i) {
                        struct strbuf *sb = olist.items[i].util;

                        printf("%s", sb->buf);
                }
                string_list_clear(&olist, 0);

                /* Also include needed rename limit adjustment now */
                diff_warn_rename_limit("merge.renamelimit",
                                       opti->renames.needed_limit, 0);

                trace2_region_leave("merge", "display messages", opt->repo);
        }

        merge_finalize(opt, result);
}

void merge_finalize(struct merge_options *opt,
                    struct merge_result *result)
{
        struct merge_options_internal *opti = result->priv;

        if (opt->renormalize)
                git_attr_set_direction(GIT_ATTR_CHECKIN);
        assert(opt->priv == NULL);

        clear_or_reinit_internal_opts(opti, 0);
        FREE_AND_NULL(opti);
}

/*** Function Grouping: helper functions for merge_incore_*() ***/

static struct tree *shift_tree_object(struct repository *repo,
                                      struct tree *one, struct tree *two,
                                      const char *subtree_shift)
{
        struct object_id shifted;

        if (!*subtree_shift) {
                shift_tree(repo, &one->object.oid, &two->object.oid, &shifted, 0);
        } else {
                shift_tree_by(repo, &one->object.oid, &two->object.oid, &shifted,
                              subtree_shift);
        }
        if (oideq(&two->object.oid, &shifted))
                return two;
        return lookup_tree(repo, &shifted);
}

static inline void set_commit_tree(struct commit *c, struct tree *t)
{
        c->maybe_tree = t;
}

static struct commit *make_virtual_commit(struct repository *repo,
                                          struct tree *tree,
                                          const char *comment)
{
        struct commit *commit = alloc_commit_node(repo);

        set_merge_remote_desc(commit, comment, (struct object *)commit);
        set_commit_tree(commit, tree);
        commit->object.parsed = 1;
        return commit;
}

static void merge_start(struct merge_options *opt, struct merge_result *result)
{
        struct rename_info *renames;
        int i;
        struct mem_pool *pool = NULL;

        /* Sanity checks on opt */
        trace2_region_enter("merge", "sanity checks", opt->repo);
        assert(opt->repo);

        assert(opt->branch1 && opt->branch2);

        assert(opt->detect_directory_renames >= MERGE_DIRECTORY_RENAMES_NONE &&
               opt->detect_directory_renames <= MERGE_DIRECTORY_RENAMES_TRUE);
        assert(opt->rename_limit >= -1);
        assert(opt->rename_score >= 0 && opt->rename_score <= MAX_SCORE);
        assert(opt->show_rename_progress >= 0 && opt->show_rename_progress <= 1);

        assert(opt->xdl_opts >= 0);
        assert(opt->recursive_variant >= MERGE_VARIANT_NORMAL &&
               opt->recursive_variant <= MERGE_VARIANT_THEIRS);

        /*
         * detect_renames, verbosity, buffer_output, and obuf are ignored
         * fields that were used by "recursive" rather than "ort" -- but
         * sanity check them anyway.
         */
        assert(opt->detect_renames >= -1 &&
               opt->detect_renames <= DIFF_DETECT_COPY);
        assert(opt->verbosity >= 0 && opt->verbosity <= 5);
        assert(opt->buffer_output <= 2);
        assert(opt->obuf.len == 0);

        assert(opt->priv == NULL);
        if (result->_properly_initialized != 0 &&
            result->_properly_initialized != RESULT_INITIALIZED)
                BUG("struct merge_result passed to merge_incore_*recursive() must be zeroed or filled with values from a previous run");
        assert(!!result->priv == !!result->_properly_initialized);
        if (result->priv) {
                opt->priv = result->priv;
                result->priv = NULL;
                /*
                 * opt->priv non-NULL means we had results from a previous
                 * run; do a few sanity checks that user didn't mess with
                 * it in an obvious fashion.
                 */
                assert(opt->priv->call_depth == 0);
                assert(!opt->priv->toplevel_dir ||
                       0 == strlen(opt->priv->toplevel_dir));
        }
        trace2_region_leave("merge", "sanity checks", opt->repo);

        /* Default to histogram diff.  Actually, just hardcode it...for now. */
        opt->xdl_opts = DIFF_WITH_ALG(opt, HISTOGRAM_DIFF);

        /* Handle attr direction stuff for renormalization */
        if (opt->renormalize)
                git_attr_set_direction(GIT_ATTR_CHECKOUT);

        /* Initialization of opt->priv, our internal merge data */
        trace2_region_enter("merge", "allocate/init", opt->repo);
        if (opt->priv) {
                clear_or_reinit_internal_opts(opt->priv, 1);
                trace2_region_leave("merge", "allocate/init", opt->repo);
                return;
        }
        opt->priv = xcalloc(1, sizeof(*opt->priv));

        /* Initialization of various renames fields */
        renames = &opt->priv->renames;
        mem_pool_init(&opt->priv->pool, 0);
        pool = &opt->priv->pool;
        for (i = MERGE_SIDE1; i <= MERGE_SIDE2; i++) {
                strintmap_init_with_options(&renames->dirs_removed[i],
                                            NOT_RELEVANT, pool, 0);
                strmap_init_with_options(&renames->dir_rename_count[i],
                                         NULL, 1);
                strmap_init_with_options(&renames->dir_renames[i],
                                         NULL, 0);
                /*
                 * relevant_sources uses -1 for the default, because we need
                 * to be able to distinguish not-in-strintmap from valid
                 * relevant_source values from enum file_rename_relevance.
                 * In particular, possibly_cache_new_pair() expects a negative
                 * value for not-found entries.
                 */
                strintmap_init_with_options(&renames->relevant_sources[i],
                                            -1 /* explicitly invalid */,
                                            pool, 0);
                strmap_init_with_options(&renames->cached_pairs[i],
                                         NULL, 1);
                strset_init_with_options(&renames->cached_irrelevant[i],
                                         NULL, 1);
                strset_init_with_options(&renames->cached_target_names[i],
                                         NULL, 0);
        }
        for (i = MERGE_SIDE1; i <= MERGE_SIDE2; i++) {
                strintmap_init_with_options(&renames->deferred[i].possible_trivial_merges,
                                            0, pool, 0);
                strset_init_with_options(&renames->deferred[i].target_dirs,
                                         pool, 1);
                renames->deferred[i].trivial_merges_okay = 1; /* 1 == maybe */
        }

        /*
         * Although we initialize opt->priv->paths with strdup_strings=0,
         * that's just to avoid making yet another copy of an allocated
         * string.  Putting the entry into paths means we are taking
         * ownership, so we will later free it.
         *
         * In contrast, conflicted just has a subset of keys from paths, so
         * we don't want to free those (it'd be a duplicate free).
         */
        strmap_init_with_options(&opt->priv->paths, pool, 0);
        strmap_init_with_options(&opt->priv->conflicted, pool, 0);

        /*
         * keys & strbufs in output will sometimes need to outlive "paths",
         * so it will have a copy of relevant keys.  It's probably a small
         * subset of the overall paths that have special output.
         */
        strmap_init(&opt->priv->output);

        trace2_region_leave("merge", "allocate/init", opt->repo);
}

static void merge_check_renames_reusable(struct merge_options *opt,
                                         struct merge_result *result,
                                         struct tree *merge_base,
                                         struct tree *side1,
                                         struct tree *side2)
{
        struct rename_info *renames;
        struct tree **merge_trees;
        struct merge_options_internal *opti = result->priv;

        if (!opti)
                return;

        renames = &opti->renames;
        merge_trees = renames->merge_trees;

        /*
         * Handle case where previous merge operation did not want cache to
         * take effect, e.g. because rename/rename(1to1) makes it invalid.
         */
        if (!merge_trees[0]) {
                assert(!merge_trees[0] && !merge_trees[1] && !merge_trees[2]);
                renames->cached_pairs_valid_side = 0; /* neither side valid */
                return;
        }

        /*
         * Handle other cases; note that merge_trees[0..2] will only
         * be NULL if opti is, or if all three were manually set to
         * NULL by e.g. rename/rename(1to1) handling.
         */
        assert(merge_trees[0] && merge_trees[1] && merge_trees[2]);

        /* Check if we meet a condition for re-using cached_pairs */
        if (oideq(&merge_base->object.oid, &merge_trees[2]->object.oid) &&
            oideq(&side1->object.oid, &result->tree->object.oid))
                renames->cached_pairs_valid_side = MERGE_SIDE1;
        else if (oideq(&merge_base->object.oid, &merge_trees[1]->object.oid) &&
                 oideq(&side2->object.oid, &result->tree->object.oid))
                renames->cached_pairs_valid_side = MERGE_SIDE2;
        else
                renames->cached_pairs_valid_side = 0; /* neither side valid */
}

/*** Function Grouping: merge_incore_*() and their internal variants ***/

/*
 * Originally from merge_trees_internal(); heavily adapted, though.
 */
static void merge_ort_nonrecursive_internal(struct merge_options *opt,
                                            struct tree *merge_base,
                                            struct tree *side1,
                                            struct tree *side2,
                                            struct merge_result *result)
{
        struct object_id working_tree_oid;

        if (opt->subtree_shift) {
                side2 = shift_tree_object(opt->repo, side1, side2,
                                          opt->subtree_shift);
                merge_base = shift_tree_object(opt->repo, side1, merge_base,
                                               opt->subtree_shift);
        }

redo:
        trace2_region_enter("merge", "collect_merge_info", opt->repo);
        if (collect_merge_info(opt, merge_base, side1, side2) != 0) {
                /*
                 * TRANSLATORS: The %s arguments are: 1) tree hash of a merge
                 * base, and 2-3) the trees for the two trees we're merging.
                 */
                err(opt, _("collecting merge info failed for trees %s, %s, %s"),
                    oid_to_hex(&merge_base->object.oid),
                    oid_to_hex(&side1->object.oid),
                    oid_to_hex(&side2->object.oid));
                result->clean = -1;
                return;
        }
        trace2_region_leave("merge", "collect_merge_info", opt->repo);

        trace2_region_enter("merge", "renames", opt->repo);
        result->clean = detect_and_process_renames(opt, merge_base,
                                                   side1, side2);
        trace2_region_leave("merge", "renames", opt->repo);
        if (opt->priv->renames.redo_after_renames == 2) {
                trace2_region_enter("merge", "reset_maps", opt->repo);
                clear_or_reinit_internal_opts(opt->priv, 1);
                trace2_region_leave("merge", "reset_maps", opt->repo);
                goto redo;
        }

        trace2_region_enter("merge", "process_entries", opt->repo);
        process_entries(opt, &working_tree_oid);
        trace2_region_leave("merge", "process_entries", opt->repo);

        /* Set return values */
        result->tree = parse_tree_indirect(&working_tree_oid);
        /* existence of conflicted entries implies unclean */
        result->clean &= strmap_empty(&opt->priv->conflicted);
        if (!opt->priv->call_depth) {
                result->priv = opt->priv;
                result->_properly_initialized = RESULT_INITIALIZED;
                opt->priv = NULL;
        }
}

/*
 * Originally from merge_recursive_internal(); somewhat adapted, though.
 */
static void merge_ort_internal(struct merge_options *opt,
                               struct commit_list *merge_bases,
                               struct commit *h1,
                               struct commit *h2,
                               struct merge_result *result)
{
        struct commit_list *iter;
        struct commit *merged_merge_bases;
        const char *ancestor_name;
        struct strbuf merge_base_abbrev = STRBUF_INIT;

        if (!merge_bases) {
                merge_bases = get_merge_bases(h1, h2);
                /* See merge-ort.h:merge_incore_recursive() declaration NOTE */
                merge_bases = reverse_commit_list(merge_bases);
        }

        merged_merge_bases = pop_commit(&merge_bases);
        if (merged_merge_bases == NULL) {
                /* if there is no common ancestor, use an empty tree */
                struct tree *tree;

                tree = lookup_tree(opt->repo, opt->repo->hash_algo->empty_tree);
                merged_merge_bases = make_virtual_commit(opt->repo, tree,
                                                         "ancestor");
                ancestor_name = "empty tree";
        } else if (merge_bases) {
                ancestor_name = "merged common ancestors";
        } else {
                strbuf_add_unique_abbrev(&merge_base_abbrev,
                                         &merged_merge_bases->object.oid,
                                         DEFAULT_ABBREV);
                ancestor_name = merge_base_abbrev.buf;
        }

        for (iter = merge_bases; iter; iter = iter->next) {
                const char *saved_b1, *saved_b2;
                struct commit *prev = merged_merge_bases;

                opt->priv->call_depth++;
                /*
                 * When the merge fails, the result contains files
                 * with conflict markers. The cleanness flag is
                 * ignored (unless indicating an error), it was never
                 * actually used, as result of merge_trees has always
                 * overwritten it: the committed "conflicts" were
                 * already resolved.
                 */
                saved_b1 = opt->branch1;
                saved_b2 = opt->branch2;
                opt->branch1 = "Temporary merge branch 1";
                opt->branch2 = "Temporary merge branch 2";
                merge_ort_internal(opt, NULL, prev, iter->item, result);
                if (result->clean < 0)
                        return;
                opt->branch1 = saved_b1;
                opt->branch2 = saved_b2;
                opt->priv->call_depth--;

                merged_merge_bases = make_virtual_commit(opt->repo,
                                                         result->tree,
                                                         "merged tree");
                commit_list_insert(prev, &merged_merge_bases->parents);
                commit_list_insert(iter->item,
                                   &merged_merge_bases->parents->next);

                clear_or_reinit_internal_opts(opt->priv, 1);
        }

        opt->ancestor = ancestor_name;
        merge_ort_nonrecursive_internal(opt,
                                        repo_get_commit_tree(opt->repo,
                                                             merged_merge_bases),
                                        repo_get_commit_tree(opt->repo, h1),
                                        repo_get_commit_tree(opt->repo, h2),
                                        result);
        strbuf_release(&merge_base_abbrev);
        opt->ancestor = NULL;  /* avoid accidental re-use of opt->ancestor */
}

void merge_incore_nonrecursive(struct merge_options *opt,
                               struct tree *merge_base,
                               struct tree *side1,
                               struct tree *side2,
                               struct merge_result *result)
{
        trace2_region_enter("merge", "incore_nonrecursive", opt->repo);

        trace2_region_enter("merge", "merge_start", opt->repo);
        assert(opt->ancestor != NULL);
        merge_check_renames_reusable(opt, result, merge_base, side1, side2);
        merge_start(opt, result);
        /*
         * Record the trees used in this merge, so if there's a next merge in
         * a cherry-pick or rebase sequence it might be able to take advantage
         * of the cached_pairs in that next merge.
         */
        opt->priv->renames.merge_trees[0] = merge_base;
        opt->priv->renames.merge_trees[1] = side1;
        opt->priv->renames.merge_trees[2] = side2;
        trace2_region_leave("merge", "merge_start", opt->repo);

        merge_ort_nonrecursive_internal(opt, merge_base, side1, side2, result);
        trace2_region_leave("merge", "incore_nonrecursive", opt->repo);
}

void merge_incore_recursive(struct merge_options *opt,
                            struct commit_list *merge_bases,
                            struct commit *side1,
                            struct commit *side2,
                            struct merge_result *result)
{
        trace2_region_enter("merge", "incore_recursive", opt->repo);

        /* We set the ancestor label based on the merge_bases */
        assert(opt->ancestor == NULL);

        trace2_region_enter("merge", "merge_start", opt->repo);
        merge_start(opt, result);
        trace2_region_leave("merge", "merge_start", opt->repo);

        merge_ort_internal(opt, merge_bases, side1, side2, result);
        trace2_region_leave("merge", "incore_recursive", opt->repo);
}