]> git.ipfire.org Git - thirdparty/git.git/blobdiff - graph.c
git-commit-graph.txt: fix list rendering
[thirdparty/git.git] / graph.c
diff --git a/graph.c b/graph.c
index f53135485f565baf7a0de1a5e4cffe2a4b2b55bd..4cd9915075ff23d8a5218609b06239b9d9f08435 100644 (file)
--- a/graph.c
+++ b/graph.c
@@ -34,6 +34,7 @@ static void graph_padding_line(struct git_graph *graph, struct strbuf *sb);
  * handle directly. It is assumed that this is the same file handle as the
  * file specified by the graph diff options. This is necessary so that
  * graph_show_strbuf can be called even with a NULL graph.
+ * If a NULL graph is supplied, the strbuf is printed as-is.
  */
 static void graph_show_strbuf(struct git_graph *graph,
                              FILE *file,
@@ -112,14 +113,42 @@ static const char *column_get_color_code(unsigned short color)
        return column_colors[color];
 }
 
-static void strbuf_write_column(struct strbuf *sb, const struct column *c,
-                               char col_char)
+struct graph_line {
+       struct strbuf *buf;
+       size_t width;
+};
+
+static inline void graph_line_addch(struct graph_line *line, int c)
+{
+       strbuf_addch(line->buf, c);
+       line->width++;
+}
+
+static inline void graph_line_addchars(struct graph_line *line, int c, size_t n)
+{
+       strbuf_addchars(line->buf, c, n);
+       line->width += n;
+}
+
+static inline void graph_line_addstr(struct graph_line *line, const char *s)
+{
+       strbuf_addstr(line->buf, s);
+       line->width += strlen(s);
+}
+
+static inline void graph_line_addcolor(struct graph_line *line, unsigned short color)
+{
+       strbuf_addstr(line->buf, column_get_color_code(color));
+}
+
+static void graph_line_write_column(struct graph_line *line, const struct column *c,
+                                   char col_char)
 {
        if (c->color < column_colors_max)
-               strbuf_addstr(sb, column_get_color_code(c->color));
-       strbuf_addch(sb, col_char);
+               graph_line_addcolor(line, c->color);
+       graph_line_addch(line, col_char);
        if (c->color < column_colors_max)
-               strbuf_addstr(sb, column_get_color_code(column_colors_max));
+               graph_line_addcolor(line, column_colors_max);
 }
 
 struct git_graph {
@@ -174,10 +203,64 @@ struct git_graph {
         * previous commit.
         */
        int prev_commit_index;
+       /*
+        * Which layout variant to use to display merge commits. If the
+        * commit's first parent is known to be in a column to the left of the
+        * merge, then this value is 0 and we use the layout on the left.
+        * Otherwise, the value is 1 and the layout on the right is used. This
+        * field tells us how many columns the first parent occupies.
+        *
+        *              0)                      1)
+        *
+        *              | | | *-.               | | *---.
+        *              | |_|/|\ \              | | |\ \ \
+        *              |/| | | | |             | | | | | *
+        */
+       int merge_layout;
+       /*
+        * The number of columns added to the graph by the current commit. For
+        * 2-way and octopus merges, this is usually one less than the
+        * number of parents:
+        *
+        *              | | |                   | |    \
+        *              | * |                   | *---. \
+        *              | |\ \                  | |\ \ \ \
+        *              | | | |                 | | | | | |
+        *
+        *              num_parents: 2          num_parents: 4
+        *              edges_added: 1          edges_added: 3
+        *
+        * For left-skewed merges, the first parent fuses with its neighbor and
+        * so one less column is added:
+        *
+        *              | | |                   | |  \
+        *              | * |                   | *-. \
+        *              |/| |                   |/|\ \ \
+        *              | | |                   | | | | |
+        *
+        *              num_parents: 2          num_parents: 4
+        *              edges_added: 0          edges_added: 2
+        *
+        * This number determines how edges to the right of the merge are
+        * displayed in commit and post-merge lines; if no columns have been
+        * added then a vertical line should be used where a right-tracking
+        * line would otherwise be used.
+        *
+        *              | * \                   | * |
+        *              | |\ \                  |/| |
+        *              | | * \                 | * |
+        */
+       int edges_added;
+       /*
+        * The number of columns added by the previous commit, which is used to
+        * smooth edges appearing to the right of a commit in a commit line
+        * following a post-merge line.
+        */
+       int prev_edges_added;
        /*
         * The maximum number of columns that can be stored in the columns
         * and new_columns arrays.  This is also half the number of entries
-        * that can be stored in the mapping and new_mapping arrays.
+        * that can be stored in the mapping and old_mapping arrays.
         */
        int column_capacity;
        /*
@@ -215,12 +298,12 @@ struct git_graph {
         */
        int *mapping;
        /*
-        * A temporary array for computing the next mapping state
-        * while we are outputting a mapping line.  This is stored as part
-        * of the git_graph simply so we don't have to allocate a new
-        * temporary array each time we have to output a collapsing line.
+        * A copy of the contents of the mapping array from the last commit,
+        * which we use to improve the display of columns that are tracking
+        * from right to left through a commit line.  We also use this to
+        * avoid allocating a fresh array when we compute the next mapping.
         */
-       int *new_mapping;
+       int *old_mapping;
        /*
         * The current default column color being used.  This is
         * stored as an index into the array column_colors.
@@ -285,6 +368,9 @@ struct git_graph *graph_init(struct rev_info *opt)
        graph->prev_state = GRAPH_PADDING;
        graph->commit_index = 0;
        graph->prev_commit_index = 0;
+       graph->merge_layout = 0;
+       graph->edges_added = 0;
+       graph->prev_edges_added = 0;
        graph->num_columns = 0;
        graph->num_new_columns = 0;
        graph->mapping_size = 0;
@@ -303,7 +389,7 @@ struct git_graph *graph_init(struct rev_info *opt)
        ALLOC_ARRAY(graph->columns, graph->column_capacity);
        ALLOC_ARRAY(graph->new_columns, graph->column_capacity);
        ALLOC_ARRAY(graph->mapping, 2 * graph->column_capacity);
-       ALLOC_ARRAY(graph->new_mapping, 2 * graph->column_capacity);
+       ALLOC_ARRAY(graph->old_mapping, 2 * graph->column_capacity);
 
        /*
         * The diff output prefix callback, with this we can make
@@ -333,7 +419,7 @@ static void graph_ensure_capacity(struct git_graph *graph, int num_columns)
        REALLOC_ARRAY(graph->columns, graph->column_capacity);
        REALLOC_ARRAY(graph->new_columns, graph->column_capacity);
        REALLOC_ARRAY(graph->mapping, graph->column_capacity * 2);
-       REALLOC_ARRAY(graph->new_mapping, graph->column_capacity * 2);
+       REALLOC_ARRAY(graph->old_mapping, graph->column_capacity * 2);
 }
 
 /*
@@ -432,74 +518,76 @@ static unsigned short graph_find_commit_color(const struct git_graph *graph,
        return graph_get_current_column_color(graph);
 }
 
-static void graph_insert_into_new_columns(struct git_graph *graph,
-                                         struct commit *commit,
-                                         int *mapping_index)
+static int graph_find_new_column_by_commit(struct git_graph *graph,
+                                          struct commit *commit)
 {
        int i;
-
-       /*
-        * If the commit is already in the new_columns list, we don't need to
-        * add it.  Just update the mapping correctly.
-        */
        for (i = 0; i < graph->num_new_columns; i++) {
-               if (graph->new_columns[i].commit == commit) {
-                       graph->mapping[*mapping_index] = i;
-                       *mapping_index += 2;
-                       return;
-               }
+               if (graph->new_columns[i].commit == commit)
+                       return i;
        }
-
-       /*
-        * This commit isn't already in new_columns.  Add it.
-        */
-       graph->new_columns[graph->num_new_columns].commit = commit;
-       graph->new_columns[graph->num_new_columns].color = graph_find_commit_color(graph, commit);
-       graph->mapping[*mapping_index] = graph->num_new_columns;
-       *mapping_index += 2;
-       graph->num_new_columns++;
+       return -1;
 }
 
-static void graph_update_width(struct git_graph *graph,
-                              int is_commit_in_existing_columns)
+static void graph_insert_into_new_columns(struct git_graph *graph,
+                                         struct commit *commit,
+                                         int idx)
 {
-       /*
-        * Compute the width needed to display the graph for this commit.
-        * This is the maximum width needed for any row.  All other rows
-        * will be padded to this width.
-        *
-        * Compute the number of columns in the widest row:
-        * Count each existing column (graph->num_columns), and each new
-        * column added by this commit.
-        */
-       int max_cols = graph->num_columns + graph->num_parents;
+       int i = graph_find_new_column_by_commit(graph, commit);
+       int mapping_idx;
 
        /*
-        * Even if the current commit has no parents to be printed, it
-        * still takes up a column for itself.
+        * If the commit is not already in the new_columns array, then add it
+        * and record it as being in the final column.
         */
-       if (graph->num_parents < 1)
-               max_cols++;
+       if (i < 0) {
+               i = graph->num_new_columns++;
+               graph->new_columns[i].commit = commit;
+               graph->new_columns[i].color = graph_find_commit_color(graph, commit);
+       }
 
-       /*
-        * We added a column for the current commit as part of
-        * graph->num_parents.  If the current commit was already in
-        * graph->columns, then we have double counted it.
-        */
-       if (is_commit_in_existing_columns)
-               max_cols--;
+       if (graph->num_parents > 1 && idx > -1 && graph->merge_layout == -1) {
+               /*
+                * If this is the first parent of a merge, choose a layout for
+                * the merge line based on whether the parent appears in a
+                * column to the left of the merge
+                */
+               int dist, shift;
 
-       /*
-        * Each column takes up 2 spaces
-        */
-       graph->width = max_cols * 2;
+               dist = idx - i;
+               shift = (dist > 1) ? 2 * dist - 3 : 1;
+
+               graph->merge_layout = (dist > 0) ? 0 : 1;
+               graph->edges_added = graph->num_parents + graph->merge_layout  - 2;
+
+               mapping_idx = graph->width + (graph->merge_layout - 1) * shift;
+               graph->width += 2 * graph->merge_layout;
+
+       } else if (graph->edges_added > 0 && i == graph->mapping[graph->width - 2]) {
+               /*
+                * If some columns have been added by a merge, but this commit
+                * was found in the last existing column, then adjust the
+                * numbers so that the two edges immediately join, i.e.:
+                *
+                *              * |             * |
+                *              |\ \    =>      |\|
+                *              | |/            | *
+                *              | *
+                */
+               mapping_idx = graph->width - 2;
+               graph->edges_added = -1;
+       } else {
+               mapping_idx = graph->width;
+               graph->width += 2;
+       }
+
+       graph->mapping[mapping_idx] = i;
 }
 
 static void graph_update_columns(struct git_graph *graph)
 {
        struct commit_list *parent;
        int max_new_columns;
-       int mapping_idx;
        int i, seen_this, is_commit_in_columns;
 
        /*
@@ -532,6 +620,10 @@ static void graph_update_columns(struct git_graph *graph)
        for (i = 0; i < graph->mapping_size; i++)
                graph->mapping[i] = -1;
 
+       graph->width = 0;
+       graph->prev_edges_added = graph->edges_added;
+       graph->edges_added = 0;
+
        /*
         * Populate graph->new_columns and graph->mapping
         *
@@ -542,7 +634,6 @@ static void graph_update_columns(struct git_graph *graph)
         * supposed to end up after the collapsing is performed.
         */
        seen_this = 0;
-       mapping_idx = 0;
        is_commit_in_columns = 1;
        for (i = 0; i <= graph->num_columns; i++) {
                struct commit *col_commit;
@@ -556,9 +647,9 @@ static void graph_update_columns(struct git_graph *graph)
                }
 
                if (col_commit == graph->commit) {
-                       int old_mapping_idx = mapping_idx;
                        seen_this = 1;
                        graph->commit_index = i;
+                       graph->merge_layout = -1;
                        for (parent = first_interesting_parent(graph);
                             parent;
                             parent = next_interesting_parent(graph, parent)) {
@@ -571,21 +662,18 @@ static void graph_update_columns(struct git_graph *graph)
                                    !is_commit_in_columns) {
                                        graph_increment_column_color(graph);
                                }
-                               graph_insert_into_new_columns(graph,
-                                                             parent->item,
-                                                             &mapping_idx);
+                               graph_insert_into_new_columns(graph, parent->item, i);
                        }
                        /*
-                        * We always need to increment mapping_idx by at
+                        * We always need to increment graph->width by at
                         * least 2, even if it has no interesting parents.
                         * The current commit always takes up at least 2
                         * spaces.
                         */
-                       if (mapping_idx == old_mapping_idx)
-                               mapping_idx += 2;
+                       if (graph->num_parents == 0)
+                               graph->width += 2;
                } else {
-                       graph_insert_into_new_columns(graph, col_commit,
-                                                     &mapping_idx);
+                       graph_insert_into_new_columns(graph, col_commit, -1);
                }
        }
 
@@ -595,11 +683,43 @@ static void graph_update_columns(struct git_graph *graph)
        while (graph->mapping_size > 1 &&
               graph->mapping[graph->mapping_size - 1] < 0)
                graph->mapping_size--;
+}
+
+static int graph_num_dashed_parents(struct git_graph *graph)
+{
+       return graph->num_parents + graph->merge_layout - 3;
+}
 
+static int graph_num_expansion_rows(struct git_graph *graph)
+{
        /*
-        * Compute graph->width for this commit
+        * Normally, we need two expansion rows for each dashed parent line from
+        * an octopus merge:
+        *
+        *              | *
+        *              | |\
+        *              | | \
+        *              | |  \
+        *              | *-. \
+        *              | |\ \ \
+        *
+        * If the merge is skewed to the left, then its parents occupy one less
+        * column, and we don't need as many expansion rows to route around it;
+        * in some cases that means we don't need any expansion rows at all:
+        *
+        *              | *
+        *              | |\
+        *              | * \
+        *              |/|\ \
         */
-       graph_update_width(graph, is_commit_in_columns);
+       return graph_num_dashed_parents(graph) * 2;
+}
+
+static int graph_needs_pre_commit_line(struct git_graph *graph)
+{
+       return graph->num_parents >= 3 &&
+              graph->commit_index < (graph->num_columns - 1) &&
+              graph->expansion_row < graph_num_expansion_rows(graph);
 }
 
 void graph_update(struct git_graph *graph, struct commit *commit)
@@ -657,8 +777,7 @@ void graph_update(struct git_graph *graph, struct commit *commit)
         */
        if (graph->state != GRAPH_PADDING)
                graph->state = GRAPH_SKIP;
-       else if (graph->num_parents >= 3 &&
-                graph->commit_index < (graph->num_columns - 1))
+       else if (graph_needs_pre_commit_line(graph))
                graph->state = GRAPH_PRE_COMMIT;
        else
                graph->state = GRAPH_COMMIT;
@@ -686,8 +805,7 @@ static int graph_is_mapping_correct(struct git_graph *graph)
        return 1;
 }
 
-static void graph_pad_horizontally(struct git_graph *graph, struct strbuf *sb,
-                                  int chars_written)
+static void graph_pad_horizontally(struct git_graph *graph, struct graph_line *line)
 {
        /*
         * Add additional spaces to the end of the strbuf, so that all
@@ -696,34 +814,22 @@ static void graph_pad_horizontally(struct git_graph *graph, struct strbuf *sb,
         * This way, fields printed to the right of the graph will remain
         * aligned for the entire commit.
         */
-       if (chars_written < graph->width)
-               strbuf_addchars(sb, ' ', graph->width - chars_written);
+       if (line->width < graph->width)
+               graph_line_addchars(line, ' ', graph->width - line->width);
 }
 
 static void graph_output_padding_line(struct git_graph *graph,
-                                     struct strbuf *sb)
+                                     struct graph_line *line)
 {
        int i;
 
-       /*
-        * We could conceivable be called with a NULL commit
-        * if our caller has a bug, and invokes graph_next_line()
-        * immediately after graph_init(), without first calling
-        * graph_update().  Return without outputting anything in this
-        * case.
-        */
-       if (!graph->commit)
-               return;
-
        /*
         * Output a padding row, that leaves all branch lines unchanged
         */
        for (i = 0; i < graph->num_new_columns; i++) {
-               strbuf_write_column(sb, &graph->new_columns[i], '|');
-               strbuf_addch(sb, ' ');
+               graph_line_write_column(line, &graph->new_columns[i], '|');
+               graph_line_addch(line, ' ');
        }
-
-       graph_pad_horizontally(graph, sb, graph->num_new_columns * 2);
 }
 
 
@@ -733,28 +839,24 @@ int graph_width(struct git_graph *graph)
 }
 
 
-static void graph_output_skip_line(struct git_graph *graph, struct strbuf *sb)
+static void graph_output_skip_line(struct git_graph *graph, struct graph_line *line)
 {
        /*
         * Output an ellipsis to indicate that a portion
         * of the graph is missing.
         */
-       strbuf_addstr(sb, "...");
-       graph_pad_horizontally(graph, sb, 3);
+       graph_line_addstr(line, "...");
 
-       if (graph->num_parents >= 3 &&
-           graph->commit_index < (graph->num_columns - 1))
+       if (graph_needs_pre_commit_line(graph))
                graph_update_state(graph, GRAPH_PRE_COMMIT);
        else
                graph_update_state(graph, GRAPH_COMMIT);
 }
 
 static void graph_output_pre_commit_line(struct git_graph *graph,
-                                        struct strbuf *sb)
+                                        struct graph_line *line)
 {
-       int num_expansion_rows;
        int i, seen_this;
-       int chars_written;
 
        /*
         * This function formats a row that increases the space around a commit
@@ -764,27 +866,24 @@ static void graph_output_pre_commit_line(struct git_graph *graph,
         * We need 2 extra rows for every parent over 2.
         */
        assert(graph->num_parents >= 3);
-       num_expansion_rows = (graph->num_parents - 2) * 2;
 
        /*
         * graph->expansion_row tracks the current expansion row we are on.
         * It should be in the range [0, num_expansion_rows - 1]
         */
        assert(0 <= graph->expansion_row &&
-              graph->expansion_row < num_expansion_rows);
+              graph->expansion_row < graph_num_expansion_rows(graph));
 
        /*
         * Output the row
         */
        seen_this = 0;
-       chars_written = 0;
        for (i = 0; i < graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
                if (col->commit == graph->commit) {
                        seen_this = 1;
-                       strbuf_write_column(sb, col, '|');
-                       strbuf_addchars(sb, ' ', graph->expansion_row);
-                       chars_written += 1 + graph->expansion_row;
+                       graph_line_write_column(line, col, '|');
+                       graph_line_addchars(line, ' ', graph->expansion_row);
                } else if (seen_this && (graph->expansion_row == 0)) {
                        /*
                         * This is the first line of the pre-commit output.
@@ -797,33 +896,27 @@ static void graph_output_pre_commit_line(struct git_graph *graph,
                         */
                        if (graph->prev_state == GRAPH_POST_MERGE &&
                            graph->prev_commit_index < i)
-                               strbuf_write_column(sb, col, '\\');
+                               graph_line_write_column(line, col, '\\');
                        else
-                               strbuf_write_column(sb, col, '|');
-                       chars_written++;
+                               graph_line_write_column(line, col, '|');
                } else if (seen_this && (graph->expansion_row > 0)) {
-                       strbuf_write_column(sb, col, '\\');
-                       chars_written++;
+                       graph_line_write_column(line, col, '\\');
                } else {
-                       strbuf_write_column(sb, col, '|');
-                       chars_written++;
+                       graph_line_write_column(line, col, '|');
                }
-               strbuf_addch(sb, ' ');
-               chars_written++;
+               graph_line_addch(line, ' ');
        }
 
-       graph_pad_horizontally(graph, sb, chars_written);
-
        /*
         * Increment graph->expansion_row,
         * and move to state GRAPH_COMMIT if necessary
         */
        graph->expansion_row++;
-       if (graph->expansion_row >= num_expansion_rows)
+       if (!graph_needs_pre_commit_line(graph))
                graph_update_state(graph, GRAPH_COMMIT);
 }
 
-static void graph_output_commit_char(struct git_graph *graph, struct strbuf *sb)
+static void graph_output_commit_char(struct git_graph *graph, struct graph_line *line)
 {
        /*
         * For boundary commits, print 'o'
@@ -831,72 +924,67 @@ static void graph_output_commit_char(struct git_graph *graph, struct strbuf *sb)
         */
        if (graph->commit->object.flags & BOUNDARY) {
                assert(graph->revs->boundary);
-               strbuf_addch(sb, 'o');
+               graph_line_addch(line, 'o');
                return;
        }
 
        /*
         * get_revision_mark() handles all other cases without assert()
         */
-       strbuf_addstr(sb, get_revision_mark(graph->revs, graph->commit));
+       graph_line_addstr(line, get_revision_mark(graph->revs, graph->commit));
 }
 
 /*
- * Draw the horizontal dashes of an octopus merge and return the number of
- * characters written.
+ * Draw the horizontal dashes of an octopus merge.
  */
-static int graph_draw_octopus_merge(struct git_graph *graph,
-                                   struct strbuf *sb)
+static void graph_draw_octopus_merge(struct git_graph *graph, struct graph_line *line)
 {
        /*
-        * Here dashless_parents represents the number of parents which don't
-        * need to have dashes (the edges labeled "0" and "1").  And
-        * dashful_parents are the remaining ones.
+        * The parents of a merge commit can be arbitrarily reordered as they
+        * are mapped onto display columns, for example this is a valid merge:
         *
-        * | *---.
-        * | |\ \ \
-        * | | | | |
-        * x 0 1 2 3
+        *      | | *---.
+        *      | | |\ \ \
+        *      | | |/ / /
+        *      | |/| | /
+        *      | |_|_|/
+        *      |/| | |
+        *      3 1 0 2
         *
-        */
-       const int dashless_parents = 2;
-       int dashful_parents = graph->num_parents - dashless_parents;
-
-       /*
-        * Usually, we add one new column for each parent (like the diagram
-        * above) but sometimes the first parent goes into an existing column,
-        * like this:
+        * The numbers denote which parent of the merge each visual column
+        * corresponds to; we can't assume that the parents will initially
+        * display in the order given by new_columns.
         *
-        * | *---.
-        * | |\ \ \
-        * |/ / / /
-        * x 0 1 2
+        * To find the right color for each dash, we need to consult the
+        * mapping array, starting from the column 2 places to the right of the
+        * merge commit, and use that to find out which logical column each
+        * edge will collapse to.
         *
-        * In which case the number of parents will be one greater than the
-        * number of added columns.
+        * Commits are rendered once all edges have collapsed to their correct
+        * logcial column, so commit_index gives us the right visual offset for
+        * the merge commit.
         */
-       int added_cols = (graph->num_new_columns - graph->num_columns);
-       int parent_in_old_cols = graph->num_parents - added_cols;
 
-       /*
-        * In both cases, commit_index corresponds to the edge labeled "0".
-        */
-       int first_col = graph->commit_index + dashless_parents
-           - parent_in_old_cols;
+       int i, j;
+       struct column *col;
 
-       int i;
-       for (i = 0; i < dashful_parents; i++) {
-               strbuf_write_column(sb, &graph->new_columns[i+first_col], '-');
-               strbuf_write_column(sb, &graph->new_columns[i+first_col],
-                                   i == dashful_parents-1 ? '.' : '-');
+       int dashed_parents = graph_num_dashed_parents(graph);
+
+       for (i = 0; i < dashed_parents; i++) {
+               j = graph->mapping[(graph->commit_index + i + 2) * 2];
+               col = &graph->new_columns[j];
+
+               graph_line_write_column(line, col, '-');
+               graph_line_write_column(line, col, (i == dashed_parents - 1) ? '.' : '-');
        }
-       return 2 * dashful_parents;
+
+       return;
 }
 
-static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
+static void graph_output_commit_line(struct git_graph *graph, struct graph_line *line)
 {
        int seen_this = 0;
-       int i, chars_written;
+       int i;
 
        /*
         * Output the row containing this commit
@@ -906,7 +994,6 @@ static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
         * children that we have already processed.)
         */
        seen_this = 0;
-       chars_written = 0;
        for (i = 0; i <= graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
                struct commit *col_commit;
@@ -920,19 +1007,17 @@ static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
 
                if (col_commit == graph->commit) {
                        seen_this = 1;
-                       graph_output_commit_char(graph, sb);
-                       chars_written++;
+                       graph_output_commit_char(graph, line);
 
                        if (graph->num_parents > 2)
-                               chars_written += graph_draw_octopus_merge(graph,
-                                                                         sb);
-               } else if (seen_this && (graph->num_parents > 2)) {
-                       strbuf_write_column(sb, col, '\\');
-                       chars_written++;
-               } else if (seen_this && (graph->num_parents == 2)) {
+                               graph_draw_octopus_merge(graph, line);
+               } else if (seen_this && (graph->edges_added > 1)) {
+                       graph_line_write_column(line, col, '\\');
+               } else if (seen_this && (graph->edges_added == 1)) {
                        /*
-                        * This is a 2-way merge commit.
-                        * There is no GRAPH_PRE_COMMIT stage for 2-way
+                        * This is either a right-skewed 2-way merge
+                        * commit, or a left-skewed 3-way merge.
+                        * There is no GRAPH_PRE_COMMIT stage for such
                         * merges, so this is the first line of output
                         * for this commit.  Check to see what the previous
                         * line of output was.
@@ -944,21 +1029,21 @@ static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
                         * makes the output look nicer.
                         */
                        if (graph->prev_state == GRAPH_POST_MERGE &&
+                           graph->prev_edges_added > 0 &&
                            graph->prev_commit_index < i)
-                               strbuf_write_column(sb, col, '\\');
+                               graph_line_write_column(line, col, '\\');
                        else
-                               strbuf_write_column(sb, col, '|');
-                       chars_written++;
+                               graph_line_write_column(line, col, '|');
+               } else if (graph->prev_state == GRAPH_COLLAPSING &&
+                          graph->old_mapping[2 * i + 1] == i &&
+                          graph->mapping[2 * i] < i) {
+                       graph_line_write_column(line, col, '/');
                } else {
-                       strbuf_write_column(sb, col, '|');
-                       chars_written++;
+                       graph_line_write_column(line, col, '|');
                }
-               strbuf_addch(sb, ' ');
-               chars_written++;
+               graph_line_addch(line, ' ');
        }
 
-       graph_pad_horizontally(graph, sb, chars_written);
-
        /*
         * Update graph->state
         */
@@ -970,26 +1055,19 @@ static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
                graph_update_state(graph, GRAPH_COLLAPSING);
 }
 
-static struct column *find_new_column_by_commit(struct git_graph *graph,
-                                               struct commit *commit)
-{
-       int i;
-       for (i = 0; i < graph->num_new_columns; i++) {
-               if (graph->new_columns[i].commit == commit)
-                       return &graph->new_columns[i];
-       }
-       return NULL;
-}
+static const char merge_chars[] = {'/', '|', '\\'};
 
-static void graph_output_post_merge_line(struct git_graph *graph, struct strbuf *sb)
+static void graph_output_post_merge_line(struct git_graph *graph, struct graph_line *line)
 {
        int seen_this = 0;
-       int i, j, chars_written;
+       int i, j;
+
+       struct commit_list *first_parent = first_interesting_parent(graph);
+       struct column *parent_col = NULL;
 
        /*
         * Output the post-merge row
         */
-       chars_written = 0;
        for (i = 0; i <= graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
                struct commit *col_commit;
@@ -1008,37 +1086,49 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct strbuf
                         * new_columns and use those to format the
                         * edges.
                         */
-                       struct commit_list *parents = NULL;
-                       struct column *par_column;
+                       struct commit_list *parents = first_parent;
+                       int par_column;
+                       int idx = graph->merge_layout;
+                       char c;
                        seen_this = 1;
-                       parents = first_interesting_parent(graph);
-                       assert(parents);
-                       par_column = find_new_column_by_commit(graph, parents->item);
-                       assert(par_column);
-
-                       strbuf_write_column(sb, par_column, '|');
-                       chars_written++;
-                       for (j = 0; j < graph->num_parents - 1; j++) {
+
+                       for (j = 0; j < graph->num_parents; j++) {
+                               par_column = graph_find_new_column_by_commit(graph, parents->item);
+                               assert(par_column >= 0);
+
+                               c = merge_chars[idx];
+                               graph_line_write_column(line, &graph->new_columns[par_column], c);
+                               if (idx == 2) {
+                                       if (graph->edges_added > 0 || j < graph->num_parents - 1)
+                                               graph_line_addch(line, ' ');
+                               } else {
+                                       idx++;
+                               }
                                parents = next_interesting_parent(graph, parents);
-                               assert(parents);
-                               par_column = find_new_column_by_commit(graph, parents->item);
-                               assert(par_column);
-                               strbuf_write_column(sb, par_column, '\\');
-                               strbuf_addch(sb, ' ');
                        }
-                       chars_written += j * 2;
+                       if (graph->edges_added == 0)
+                               graph_line_addch(line, ' ');
+
                } else if (seen_this) {
-                       strbuf_write_column(sb, col, '\\');
-                       strbuf_addch(sb, ' ');
-                       chars_written += 2;
+                       if (graph->edges_added > 0)
+                               graph_line_write_column(line, col, '\\');
+                       else
+                               graph_line_write_column(line, col, '|');
+                       graph_line_addch(line, ' ');
                } else {
-                       strbuf_write_column(sb, col, '|');
-                       strbuf_addch(sb, ' ');
-                       chars_written += 2;
+                       graph_line_write_column(line, col, '|');
+                       if (graph->merge_layout != 0 || i != graph->commit_index - 1) {
+                               if (parent_col)
+                                       graph_line_write_column(
+                                               line, parent_col, '_');
+                               else
+                                       graph_line_addch(line, ' ');
+                       }
                }
-       }
 
-       graph_pad_horizontally(graph, sb, chars_written);
+               if (col_commit == first_parent->item)
+                       parent_col = col;
+       }
 
        /*
         * Update graph->state
@@ -1049,7 +1139,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct strbuf
                graph_update_state(graph, GRAPH_COLLAPSING);
 }
 
-static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf *sb)
+static void graph_output_collapsing_line(struct git_graph *graph, struct graph_line *line)
 {
        int i;
        short used_horizontal = 0;
@@ -1057,13 +1147,18 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
        int horizontal_edge_target = -1;
 
        /*
-        * Clear out the new_mapping array
+        * Swap the mapping and old_mapping arrays
+        */
+       SWAP(graph->mapping, graph->old_mapping);
+
+       /*
+        * Clear out the mapping array
         */
        for (i = 0; i < graph->mapping_size; i++)
-               graph->new_mapping[i] = -1;
+               graph->mapping[i] = -1;
 
        for (i = 0; i < graph->mapping_size; i++) {
-               int target = graph->mapping[i];
+               int target = graph->old_mapping[i];
                if (target < 0)
                        continue;
 
@@ -1084,14 +1179,14 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
                         * This column is already in the
                         * correct place
                         */
-                       assert(graph->new_mapping[i] == -1);
-                       graph->new_mapping[i] = target;
-               } else if (graph->new_mapping[i - 1] < 0) {
+                       assert(graph->mapping[i] == -1);
+                       graph->mapping[i] = target;
+               } else if (graph->mapping[i - 1] < 0) {
                        /*
                         * Nothing is to the left.
                         * Move to the left by one
                         */
-                       graph->new_mapping[i - 1] = target;
+                       graph->mapping[i - 1] = target;
                        /*
                         * If there isn't already an edge moving horizontally
                         * select this one.
@@ -1107,9 +1202,9 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
                                 * line.
                                 */
                                for (j = (target * 2)+3; j < (i - 2); j += 2)
-                                       graph->new_mapping[j] = target;
+                                       graph->mapping[j] = target;
                        }
-               } else if (graph->new_mapping[i - 1] == target) {
+               } else if (graph->mapping[i - 1] == target) {
                        /*
                         * There is a branch line to our left
                         * already, and it is our target.  We
@@ -1117,7 +1212,7 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
                         * the same parent commit.
                         *
                         * We don't have to add anything to the
-                        * output or new_mapping, since the
+                        * output or mapping, since the
                         * existing branch line has already taken
                         * care of it.
                         */
@@ -1129,39 +1224,46 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
                         *
                         * The space just to the left of this
                         * branch should always be empty.
-                        *
-                        * The branch to the left of that space
-                        * should be our eventual target.
                         */
-                       assert(graph->new_mapping[i - 1] > target);
-                       assert(graph->new_mapping[i - 2] < 0);
-                       assert(graph->new_mapping[i - 3] == target);
-                       graph->new_mapping[i - 2] = target;
+                       assert(graph->mapping[i - 1] > target);
+                       assert(graph->mapping[i - 2] < 0);
+                       graph->mapping[i - 2] = target;
                        /*
                         * Mark this branch as the horizontal edge to
                         * prevent any other edges from moving
                         * horizontally.
                         */
-                       if (horizontal_edge == -1)
-                               horizontal_edge = i;
+                       if (horizontal_edge == -1) {
+                               int j;
+                               horizontal_edge_target = target;
+                               horizontal_edge = i - 1;
+
+                               for (j = (target * 2) + 3; j < (i - 2); j += 2)
+                                       graph->mapping[j] = target;
+                       }
                }
        }
 
+       /*
+        * Copy the current mapping array into old_mapping
+        */
+       COPY_ARRAY(graph->old_mapping, graph->mapping, graph->mapping_size);
+
        /*
         * The new mapping may be 1 smaller than the old mapping
         */
-       if (graph->new_mapping[graph->mapping_size - 1] < 0)
+       if (graph->mapping[graph->mapping_size - 1] < 0)
                graph->mapping_size--;
 
        /*
         * Output out a line based on the new mapping info
         */
        for (i = 0; i < graph->mapping_size; i++) {
-               int target = graph->new_mapping[i];
+               int target = graph->mapping[i];
                if (target < 0)
-                       strbuf_addch(sb, ' ');
+                       graph_line_addch(line, ' ');
                else if (target * 2 == i)
-                       strbuf_write_column(sb, &graph->new_columns[target], '|');
+                       graph_line_write_column(line, &graph->new_columns[target], '|');
                else if (target == horizontal_edge_target &&
                         i != horizontal_edge - 1) {
                                /*
@@ -1170,24 +1272,17 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
                                 * won't continue into the next line.
                                 */
                                if (i != (target * 2)+3)
-                                       graph->new_mapping[i] = -1;
+                                       graph->mapping[i] = -1;
                                used_horizontal = 1;
-                       strbuf_write_column(sb, &graph->new_columns[target], '_');
+                       graph_line_write_column(line, &graph->new_columns[target], '_');
                } else {
                        if (used_horizontal && i < horizontal_edge)
-                               graph->new_mapping[i] = -1;
-                       strbuf_write_column(sb, &graph->new_columns[target], '/');
+                               graph->mapping[i] = -1;
+                       graph_line_write_column(line, &graph->new_columns[target], '/');
 
                }
        }
 
-       graph_pad_horizontally(graph, sb, graph->mapping_size);
-
-       /*
-        * Swap mapping and new_mapping
-        */
-       SWAP(graph->mapping, graph->new_mapping);
-
        /*
         * If graph->mapping indicates that all of the branch lines
         * are already in the correct positions, we are done.
@@ -1199,35 +1294,49 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
 
 int graph_next_line(struct git_graph *graph, struct strbuf *sb)
 {
+       int shown_commit_line = 0;
+       struct graph_line line = { .buf = sb, .width = 0 };
+
+       /*
+        * We could conceivable be called with a NULL commit
+        * if our caller has a bug, and invokes graph_next_line()
+        * immediately after graph_init(), without first calling
+        * graph_update().  Return without outputting anything in this
+        * case.
+        */
+       if (!graph->commit)
+               return -1;
+
        switch (graph->state) {
        case GRAPH_PADDING:
-               graph_output_padding_line(graph, sb);
-               return 0;
+               graph_output_padding_line(graph, &line);
+               break;
        case GRAPH_SKIP:
-               graph_output_skip_line(graph, sb);
-               return 0;
+               graph_output_skip_line(graph, &line);
+               break;
        case GRAPH_PRE_COMMIT:
-               graph_output_pre_commit_line(graph, sb);
-               return 0;
+               graph_output_pre_commit_line(graph, &line);
+               break;
        case GRAPH_COMMIT:
-               graph_output_commit_line(graph, sb);
-               return 1;
+               graph_output_commit_line(graph, &line);
+               shown_commit_line = 1;
+               break;
        case GRAPH_POST_MERGE:
-               graph_output_post_merge_line(graph, sb);
-               return 0;
+               graph_output_post_merge_line(graph, &line);
+               break;
        case GRAPH_COLLAPSING:
-               graph_output_collapsing_line(graph, sb);
-               return 0;
+               graph_output_collapsing_line(graph, &line);
+               break;
        }
 
-       assert(0);
-       return 0;
+       graph_pad_horizontally(graph, &line);
+       return shown_commit_line;
 }
 
 static void graph_padding_line(struct git_graph *graph, struct strbuf *sb)
 {
        int i;
-       int chars_written = 0;
+       struct graph_line line = { .buf = sb, .width = 0 };
 
        if (graph->state != GRAPH_COMMIT) {
                graph_next_line(graph, sb);
@@ -1244,20 +1353,17 @@ static void graph_padding_line(struct git_graph *graph, struct strbuf *sb)
        for (i = 0; i < graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
 
-               strbuf_write_column(sb, col, '|');
-               chars_written++;
+               graph_line_write_column(&line, col, '|');
 
                if (col->commit == graph->commit && graph->num_parents > 2) {
                        int len = (graph->num_parents - 2) * 2;
-                       strbuf_addchars(sb, ' ', len);
-                       chars_written += len;
+                       graph_line_addchars(&line, ' ', len);
                } else {
-                       strbuf_addch(sb, ' ');
-                       chars_written++;
+                       graph_line_addch(&line, ' ');
                }
        }
 
-       graph_pad_horizontally(graph, sb, chars_written);
+       graph_pad_horizontally(graph, &line);
 
        /*
         * Update graph->prev_state since we have output a padding line