describe: use prio_queue

Replace the use a list-based priority queue whose order is maintained by
commit_list_insert_by_date() with a prio_queue.  This avoids quadratic
worst-case complexity.  And in the somewhat contrived example of
describing the 4751 commits from v2.41.0 to v2.47.0 in one go (to get a
sizable chunk of describe work with minimal ref loading overhead) it's
significantly faster:

Benchmark 1: ./git_2.50.1 describe $(git rev-list v2.41.0..v2.47.0)
  Time (mean ± σ):      1.558 s ±  0.002 s    [User: 1.492 s, System: 0.051 s]
  Range (min … max):    1.557 s …  1.562 s    10 runs

Benchmark 2: ./git describe $(git rev-list v2.41.0..v2.47.0)
  Time (mean ± σ):      1.209 s ±  0.006 s    [User: 1.143 s, System: 0.051 s]
  Range (min … max):    1.201 s …  1.219 s    10 runs

Summary
  ./git describe $(git rev-list v2.41.0..v2.47.0) ran
    1.29 ± 0.01 times faster than ./git_2.50.1 describe $(git rev-list v2.41.0..v2.47.0)

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/describe.c b/builtin/describe.c
index fbf305d76244874129dcd4d130506971ff57d213..80722ae0c0421ba5963bca912d3c246443a1e795 100644 (file)
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -23,6 +23,7 @@
 #include "list-objects.h"
 #include "commit-slab.h"
 #include "wildmatch.h"
+#include "prio-queue.h"
 
 #define MAX_TAGS       (FLAG_BITS - 1)
 #define DEFAULT_CANDIDATES 10
@@ -249,24 +250,26 @@ static int compare_pt(const void *a_, const void *b_)
        return 0;
 }
 
-static unsigned long finish_depth_computation(
-       struct commit_list **list,
-       struct possible_tag *best)
+static bool all_have_flag(const struct prio_queue *queue, unsigned flag)
+{
+       for (size_t i = 0; i < queue->nr; i++) {
+               struct commit *commit = queue->array[i].data;
+               if (!(commit->object.flags & flag))
+                       return false;
+       }
+       return true;
+}
+
+static unsigned long finish_depth_computation(struct prio_queue *queue,
+                                             struct possible_tag *best)
 {
        unsigned long seen_commits = 0;
-       while (*list) {
-               struct commit *c = pop_commit(list);
+       while (queue->nr) {
+               struct commit *c = prio_queue_get(queue);
                struct commit_list *parents = c->parents;
                seen_commits++;
                if (c->object.flags & best->flag_within) {
-                       struct commit_list *a = *list;
-                       while (a) {
-                               struct commit *i = a->item;
-                               if (!(i->object.flags & best->flag_within))
-                                       break;
-                               a = a->next;
-                       }
-                       if (!a)
+                       if (all_have_flag(queue, best->flag_within))
                                break;
                } else
                        best->depth++;
@@ -274,7 +277,7 @@ static unsigned long finish_depth_computation(
                        struct commit *p = parents->item;
                        repo_parse_commit(the_repository, p);
                        if (!(p->object.flags & SEEN))
-                               commit_list_insert_by_date(p, list);
+                               prio_queue_put(queue, p);
                        p->object.flags |= c->object.flags;
                        parents = parents->next;
                }
@@ -316,7 +319,7 @@ static void append_suffix(int depth, const struct object_id *oid, struct strbuf
 static void describe_commit(struct object_id *oid, struct strbuf *dst)
 {
        struct commit *cmit, *gave_up_on = NULL;
-       struct commit_list *list;
+       struct prio_queue queue = { compare_commits_by_commit_date };
        struct commit_name *n;
        struct possible_tag all_matches[MAX_TAGS];
        unsigned int match_cnt = 0, annotated_cnt = 0, cur_match;
@@ -359,11 +362,10 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
                have_util = 1;
        }
 
-       list = NULL;
        cmit->object.flags = SEEN;
-       commit_list_insert(cmit, &list);
-       while (list) {
-               struct commit *c = pop_commit(&list);
+       prio_queue_put(&queue, cmit);
+       while (queue.nr) {
+               struct commit *c = prio_queue_get(&queue);
                struct commit_list *parents = c->parents;
                struct commit_name **slot;
 
@@ -397,7 +399,7 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
                                t->depth++;
                }
                /* Stop if last remaining path already covered by best candidate(s) */
-               if (annotated_cnt && !list) {
+               if (annotated_cnt && !queue.nr) {
                        int best_depth = INT_MAX;
                        unsigned best_within = 0;
                        for (cur_match = 0; cur_match < match_cnt; cur_match++) {
@@ -420,7 +422,7 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
                        struct commit *p = parents->item;
                        repo_parse_commit(the_repository, p);
                        if (!(p->object.flags & SEEN))
-                               commit_list_insert_by_date(p, &list);
+                               prio_queue_put(&queue, p);
                        p->object.flags |= c->object.flags;
                        parents = parents->next;
 
@@ -435,6 +437,7 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
                        strbuf_add_unique_abbrev(dst, cmit_oid, abbrev);
                        if (suffix)
                                strbuf_addstr(dst, suffix);
+                       clear_prio_queue(&queue);
                        return;
                }
                if (unannotated_cnt)
@@ -450,11 +453,11 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
        QSORT(all_matches, match_cnt, compare_pt);
 
        if (gave_up_on) {
-               commit_list_insert_by_date(gave_up_on, &list);
+               prio_queue_put(&queue, gave_up_on);
                seen_commits--;
        }
-       seen_commits += finish_depth_computation(&list, &all_matches[0]);
-       free_commit_list(list);
+       seen_commits += finish_depth_computation(&queue, &all_matches[0]);
+       clear_prio_queue(&queue);
 
        if (debug) {
                static int label_width = -1;