Merge branch 'kb/maint-rebase-autosquash'

[git/kirr.git] / graph.c

#include "cache.h"
#include "commit.h"
#include "color.h"
#include "graph.h"
#include "diff.h"
#include "revision.h"

/* Internal API */

/*
 * Output a padding line in the graph.
 * This is similar to graph_next_line().  However, it is guaranteed to
 * never print the current commit line.  Instead, if the commit line is
 * next, it will simply output a line of vertical padding, extending the
 * branch lines downwards, but leaving them otherwise unchanged.
 */
static void graph_padding_line(struct git_graph *graph, struct strbuf *sb);

/*
 * Print a strbuf to stdout.  If the graph is non-NULL, all lines but the
 * first will be prefixed with the graph output.
 *
 * If the strbuf ends with a newline, the output will end after this
 * newline.  A new graph line will not be printed after the final newline.
 * If the strbuf is empty, no output will be printed.
 *
 * Since the first line will not include the graph output, the caller is
 * responsible for printing this line's graph (perhaps via
 * graph_show_commit() or graph_show_oneline()) before calling
 * graph_show_strbuf().
 */
static void graph_show_strbuf(struct git_graph *graph, struct strbuf const *sb);

/*
 * TODO:
 * - Limit the number of columns, similar to the way gitk does.
 *   If we reach more than a specified number of columns, omit
 *   sections of some columns.
 */

struct column {
        /*
         * The parent commit of this column.
         */
        struct commit *commit;
        /*
         * The color to (optionally) print this column in.  This is an
         * index into column_colors.
         */
        unsigned short color;
};

enum graph_state {
        GRAPH_PADDING,
        GRAPH_SKIP,
        GRAPH_PRE_COMMIT,
        GRAPH_COMMIT,
        GRAPH_POST_MERGE,
        GRAPH_COLLAPSING
};

/*
 * The list of available column colors.
 */
static const char *column_colors_ansi[] = {
        GIT_COLOR_RED,
        GIT_COLOR_GREEN,
        GIT_COLOR_YELLOW,
        GIT_COLOR_BLUE,
        GIT_COLOR_MAGENTA,
        GIT_COLOR_CYAN,
        GIT_COLOR_BOLD_RED,
        GIT_COLOR_BOLD_GREEN,
        GIT_COLOR_BOLD_YELLOW,
        GIT_COLOR_BOLD_BLUE,
        GIT_COLOR_BOLD_MAGENTA,
        GIT_COLOR_BOLD_CYAN,
        GIT_COLOR_RESET,
};

#define COLUMN_COLORS_ANSI_MAX (ARRAY_SIZE(column_colors_ansi) - 1)

static const char **column_colors;
static unsigned short column_colors_max;

void graph_set_column_colors(const char **colors, unsigned short colors_max)
{
        column_colors = colors;
        column_colors_max = colors_max;
}

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)
{
        if (c->color < column_colors_max)
                strbuf_addstr(sb, column_get_color_code(c->color));
        strbuf_addch(sb, col_char);
        if (c->color < column_colors_max)
                strbuf_addstr(sb, column_get_color_code(column_colors_max));
}

struct git_graph {
        /*
         * The commit currently being processed
         */
        struct commit *commit;
        /* The rev-info used for the current traversal */
        struct rev_info *revs;
        /*
         * The number of interesting parents that this commit has.
         *
         * Note that this is not the same as the actual number of parents.
         * This count excludes parents that won't be printed in the graph
         * output, as determined by graph_is_interesting().
         */
        int num_parents;
        /*
         * The width of the graph output for this commit.
         * All rows for this commit are padded to this width, so that
         * messages printed after the graph output are aligned.
         */
        int width;
        /*
         * The next expansion row to print
         * when state is GRAPH_PRE_COMMIT
         */
        int expansion_row;
        /*
         * The current output state.
         * This tells us what kind of line graph_next_line() should output.
         */
        enum graph_state state;
        /*
         * The output state for the previous line of output.
         * This is primarily used to determine how the first merge line
         * should appear, based on the last line of the previous commit.
         */
        enum graph_state prev_state;
        /*
         * The index of the column that refers to this commit.
         *
         * If none of the incoming columns refer to this commit,
         * this will be equal to num_columns.
         */
        int commit_index;
        /*
         * The commit_index for the previously displayed commit.
         *
         * This is used to determine how the first line of a merge
         * graph output should appear, based on the last line of the
         * previous commit.
         */
        int prev_commit_index;
        /*
         * 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.
         */
        int column_capacity;
        /*
         * The number of columns (also called "branch lines" in some places)
         */
        int num_columns;
        /*
         * The number of columns in the new_columns array
         */
        int num_new_columns;
        /*
         * The number of entries in the mapping array
         */
        int mapping_size;
        /*
         * The column state before we output the current commit.
         */
        struct column *columns;
        /*
         * The new column state after we output the current commit.
         * Only valid when state is GRAPH_COLLAPSING.
         */
        struct column *new_columns;
        /*
         * An array that tracks the current state of each
         * character in the output line during state GRAPH_COLLAPSING.
         * Each entry is -1 if this character is empty, or a non-negative
         * integer if the character contains a branch line.  The value of
         * the integer indicates the target position for this branch line.
         * (I.e., this array maps the current column positions to their
         * desired positions.)
         *
         * The maximum capacity of this array is always
         * sizeof(int) * 2 * column_capacity.
         */
        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.
         */
        int *new_mapping;
        /*
         * The current default column color being used.  This is
         * stored as an index into the array column_colors.
         */
        unsigned short default_column_color;
};

static struct strbuf *diff_output_prefix_callback(struct diff_options *opt, void *data)
{
        struct git_graph *graph = data;
        static struct strbuf msgbuf = STRBUF_INIT;

        assert(graph);

        strbuf_reset(&msgbuf);
        graph_padding_line(graph, &msgbuf);
        return &msgbuf;
}

struct git_graph *graph_init(struct rev_info *opt)
{
        struct git_graph *graph = xmalloc(sizeof(struct git_graph));

        if (!column_colors)
                graph_set_column_colors(column_colors_ansi,
                                        COLUMN_COLORS_ANSI_MAX);

        graph->commit = NULL;
        graph->revs = opt;
        graph->num_parents = 0;
        graph->expansion_row = 0;
        graph->state = GRAPH_PADDING;
        graph->prev_state = GRAPH_PADDING;
        graph->commit_index = 0;
        graph->prev_commit_index = 0;
        graph->num_columns = 0;
        graph->num_new_columns = 0;
        graph->mapping_size = 0;
        /*
         * Start the column color at the maximum value, since we'll
         * always increment it for the first commit we output.
         * This way we start at 0 for the first commit.
         */
        graph->default_column_color = column_colors_max - 1;

        /*
         * Allocate a reasonably large default number of columns
         * We'll automatically grow columns later if we need more room.
         */
        graph->column_capacity = 30;
        graph->columns = xmalloc(sizeof(struct column) *
                                 graph->column_capacity);
        graph->new_columns = xmalloc(sizeof(struct column) *
                                     graph->column_capacity);
        graph->mapping = xmalloc(sizeof(int) * 2 * graph->column_capacity);
        graph->new_mapping = xmalloc(sizeof(int) * 2 * graph->column_capacity);

        /*
         * The diff output prefix callback, with this we can make
         * all the diff output to align with the graph lines.
         */
        opt->diffopt.output_prefix = diff_output_prefix_callback;
        opt->diffopt.output_prefix_data = graph;

        return graph;
}

static void graph_update_state(struct git_graph *graph, enum graph_state s)
{
        graph->prev_state = graph->state;
        graph->state = s;
}

static void graph_ensure_capacity(struct git_graph *graph, int num_columns)
{
        if (graph->column_capacity >= num_columns)
                return;

        do {
                graph->column_capacity *= 2;
        } while (graph->column_capacity < num_columns);

        graph->columns = xrealloc(graph->columns,
                                  sizeof(struct column) *
                                  graph->column_capacity);
        graph->new_columns = xrealloc(graph->new_columns,
                                      sizeof(struct column) *
                                      graph->column_capacity);
        graph->mapping = xrealloc(graph->mapping,
                                  sizeof(int) * 2 * graph->column_capacity);
        graph->new_mapping = xrealloc(graph->new_mapping,
                                      sizeof(int) * 2 * graph->column_capacity);
}

/*
 * Returns 1 if the commit will be printed in the graph output,
 * and 0 otherwise.
 */
static int graph_is_interesting(struct git_graph *graph, struct commit *commit)
{
        /*
         * If revs->boundary is set, commits whose children have
         * been shown are always interesting, even if they have the
         * UNINTERESTING or TREESAME flags set.
         */
        if (graph->revs && graph->revs->boundary) {
                if (commit->object.flags & CHILD_SHOWN)
                        return 1;
        }

        /*
         * Otherwise, use get_commit_action() to see if this commit is
         * interesting
         */
        return get_commit_action(graph->revs, commit) == commit_show;
}

static struct commit_list *next_interesting_parent(struct git_graph *graph,
                                                   struct commit_list *orig)
{
        struct commit_list *list;

        /*
         * If revs->first_parent_only is set, only the first
         * parent is interesting.  None of the others are.
         */
        if (graph->revs->first_parent_only)
                return NULL;

        /*
         * Return the next interesting commit after orig
         */
        for (list = orig->next; list; list = list->next) {
                if (graph_is_interesting(graph, list->item))
                        return list;
        }

        return NULL;
}

static struct commit_list *first_interesting_parent(struct git_graph *graph)
{
        struct commit_list *parents = graph->commit->parents;

        /*
         * If this commit has no parents, ignore it
         */
        if (!parents)
                return NULL;

        /*
         * If the first parent is interesting, return it
         */
        if (graph_is_interesting(graph, parents->item))
                return parents;

        /*
         * Otherwise, call next_interesting_parent() to get
         * the next interesting parent
         */
        return next_interesting_parent(graph, parents);
}

static unsigned short graph_get_current_column_color(const struct git_graph *graph)
{
        if (!DIFF_OPT_TST(&graph->revs->diffopt, COLOR_DIFF))
                return column_colors_max;
        return graph->default_column_color;
}

/*
 * Update the graph's default column color.
 */
static void graph_increment_column_color(struct git_graph *graph)
{
        graph->default_column_color = (graph->default_column_color + 1) %
                column_colors_max;
}

static unsigned short graph_find_commit_color(const struct git_graph *graph,
                                              const struct commit *commit)
{
        int i;
        for (i = 0; i < graph->num_columns; i++) {
                if (graph->columns[i].commit == commit)
                        return graph->columns[i].color;
        }
        return graph_get_current_column_color(graph);
}

static void graph_insert_into_new_columns(struct git_graph *graph,
                                          struct commit *commit,
                                          int *mapping_index)
{
        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;
                }
        }

        /*
         * 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++;
}

static void graph_update_width(struct git_graph *graph,
                               int is_commit_in_existing_columns)
{
        /*
         * 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;

        /*
         * Even if the current commit has no parents to be printed, it
         * still takes up a column for itself.
         */
        if (graph->num_parents < 1)
                max_cols++;

        /*
         * 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--;

        /*
         * Each column takes up 2 spaces
         */
        graph->width = max_cols * 2;
}

static void graph_update_columns(struct git_graph *graph)
{
        struct commit_list *parent;
        struct column *tmp_columns;
        int max_new_columns;
        int mapping_idx;
        int i, seen_this, is_commit_in_columns;

        /*
         * Swap graph->columns with graph->new_columns
         * graph->columns contains the state for the previous commit,
         * and new_columns now contains the state for our commit.
         *
         * We'll re-use the old columns array as storage to compute the new
         * columns list for the commit after this one.
         */
        tmp_columns = graph->columns;
        graph->columns = graph->new_columns;
        graph->num_columns = graph->num_new_columns;

        graph->new_columns = tmp_columns;
        graph->num_new_columns = 0;

        /*
         * Now update new_columns and mapping with the information for the
         * commit after this one.
         *
         * First, make sure we have enough room.  At most, there will
         * be graph->num_columns + graph->num_parents columns for the next
         * commit.
         */
        max_new_columns = graph->num_columns + graph->num_parents;
        graph_ensure_capacity(graph, max_new_columns);

        /*
         * Clear out graph->mapping
         */
        graph->mapping_size = 2 * max_new_columns;
        for (i = 0; i < graph->mapping_size; i++)
                graph->mapping[i] = -1;

        /*
         * Populate graph->new_columns and graph->mapping
         *
         * Some of the parents of this commit may already be in
         * graph->columns.  If so, graph->new_columns should only contain a
         * single entry for each such commit.  graph->mapping should
         * contain information about where each current branch line is
         * 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;
                if (i == graph->num_columns) {
                        if (seen_this)
                                break;
                        is_commit_in_columns = 0;
                        col_commit = graph->commit;
                } else {
                        col_commit = graph->columns[i].commit;
                }

                if (col_commit == graph->commit) {
                        int old_mapping_idx = mapping_idx;
                        seen_this = 1;
                        graph->commit_index = i;
                        for (parent = first_interesting_parent(graph);
                             parent;
                             parent = next_interesting_parent(graph, parent)) {
                                /*
                                 * If this is a merge, or the start of a new
                                 * childless column, increment the current
                                 * color.
                                 */
                                if (graph->num_parents > 1 ||
                                    !is_commit_in_columns) {
                                        graph_increment_column_color(graph);
                                }
                                graph_insert_into_new_columns(graph,
                                                              parent->item,
                                                              &mapping_idx);
                        }
                        /*
                         * We always need to increment mapping_idx 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;
                } else {
                        graph_insert_into_new_columns(graph, col_commit,
                                                      &mapping_idx);
                }
        }

        /*
         * Shrink mapping_size to be the minimum necessary
         */
        while (graph->mapping_size > 1 &&
               graph->mapping[graph->mapping_size - 1] < 0)
                graph->mapping_size--;

        /*
         * Compute graph->width for this commit
         */
        graph_update_width(graph, is_commit_in_columns);
}

void graph_update(struct git_graph *graph, struct commit *commit)
{
        struct commit_list *parent;

        /*
         * Set the new commit
         */
        graph->commit = commit;

        /*
         * Count how many interesting parents this commit has
         */
        graph->num_parents = 0;
        for (parent = first_interesting_parent(graph);
             parent;
             parent = next_interesting_parent(graph, parent))
        {
                graph->num_parents++;
        }

        /*
         * Store the old commit_index in prev_commit_index.
         * graph_update_columns() will update graph->commit_index for this
         * commit.
         */
        graph->prev_commit_index = graph->commit_index;

        /*
         * Call graph_update_columns() to update
         * columns, new_columns, and mapping.
         */
        graph_update_columns(graph);

        graph->expansion_row = 0;

        /*
         * Update graph->state.
         * Note that we don't call graph_update_state() here, since
         * we don't want to update graph->prev_state.  No line for
         * graph->state was ever printed.
         *
         * If the previous commit didn't get to the GRAPH_PADDING state,
         * it never finished its output.  Goto GRAPH_SKIP, to print out
         * a line to indicate that portion of the graph is missing.
         *
         * If there are 3 or more parents, we may need to print extra rows
         * before the commit, to expand the branch lines around it and make
         * room for it.  We need to do this only if there is a branch row
         * (or more) to the right of this commit.
         *
         * If there are less than 3 parents, we can immediately print the
         * commit line.
         */
        if (graph->state != GRAPH_PADDING)
                graph->state = GRAPH_SKIP;
        else if (graph->num_parents >= 3 &&
                 graph->commit_index < (graph->num_columns - 1))
                graph->state = GRAPH_PRE_COMMIT;
        else
                graph->state = GRAPH_COMMIT;
}

static int graph_is_mapping_correct(struct git_graph *graph)
{
        int i;

        /*
         * The mapping is up to date if each entry is at its target,
         * or is 1 greater than its target.
         * (If it is 1 greater than the target, '/' will be printed, so it
         * will look correct on the next row.)
         */
        for (i = 0; i < graph->mapping_size; i++) {
                int target = graph->mapping[i];
                if (target < 0)
                        continue;
                if (target == (i / 2))
                        continue;
                return 0;
        }

        return 1;
}

static void graph_pad_horizontally(struct git_graph *graph, struct strbuf *sb,
                                   int chars_written)
{
        /*
         * Add additional spaces to the end of the strbuf, so that all
         * lines for a particular commit have the same width.
         *
         * This way, fields printed to the right of the graph will remain
         * aligned for the entire commit.
         */
        int extra;
        if (chars_written >= graph->width)
                return;

        extra = graph->width - chars_written;
        strbuf_addf(sb, "%*s", (int) extra, "");
}

static void graph_output_padding_line(struct git_graph *graph,
                                      struct strbuf *sb)
{
        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_pad_horizontally(graph, sb, graph->num_new_columns * 2);
}

static void graph_output_skip_line(struct git_graph *graph, struct strbuf *sb)
{
        /*
         * Output an ellipsis to indicate that a portion
         * of the graph is missing.
         */
        strbuf_addstr(sb, "...");
        graph_pad_horizontally(graph, sb, 3);

        if (graph->num_parents >= 3 &&
            graph->commit_index < (graph->num_columns - 1))
                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)
{
        int num_expansion_rows;
        int i, seen_this;
        int chars_written;

        /*
         * This function formats a row that increases the space around a commit
         * with multiple parents, to make room for it.  It should only be
         * called when there are 3 or more parents.
         *
         * 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);

        /*
         * 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_addf(sb, "%*s", graph->expansion_row, "");
                        chars_written += 1 + graph->expansion_row;
                } else if (seen_this && (graph->expansion_row == 0)) {
                        /*
                         * This is the first line of the pre-commit output.
                         * If the previous commit was a merge commit and
                         * ended in the GRAPH_POST_MERGE state, all branch
                         * lines after graph->prev_commit_index were
                         * printed as "\" on the previous line.  Continue
                         * to print them as "\" on this line.  Otherwise,
                         * print the branch lines as "|".
                         */
                        if (graph->prev_state == GRAPH_POST_MERGE &&
                            graph->prev_commit_index < i)
                                strbuf_write_column(sb, col, '\\');
                        else
                                strbuf_write_column(sb, col, '|');
                        chars_written++;
                } else if (seen_this && (graph->expansion_row > 0)) {
                        strbuf_write_column(sb, col, '\\');
                        chars_written++;
                } else {
                        strbuf_write_column(sb, col, '|');
                        chars_written++;
                }
                strbuf_addch(sb, ' ');
                chars_written++;
        }

        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)
                graph_update_state(graph, GRAPH_COMMIT);
}

static void graph_output_commit_char(struct git_graph *graph, struct strbuf *sb)
{
        /*
         * For boundary commits, print 'o'
         * (We should only see boundary commits when revs->boundary is set.)
         */
        if (graph->commit->object.flags & BOUNDARY) {
                assert(graph->revs->boundary);
                strbuf_addch(sb, 'o');
                return;
        }

        /*
         * If revs->left_right is set, print '<' for commits that
         * come from the left side, and '>' for commits from the right
         * side.
         */
        if (graph->revs && graph->revs->left_right) {
                if (graph->commit->object.flags & SYMMETRIC_LEFT)
                        strbuf_addch(sb, '<');
                else
                        strbuf_addch(sb, '>');
                return;
        }

        /*
         * Print '*' in all other cases
         */
        strbuf_addch(sb, '*');
}

/*
 * Draw an octopus merge and return the number of characters written.
 */
static int graph_draw_octopus_merge(struct git_graph *graph,
                                    struct strbuf *sb)
{
        /*
         * Here dashless_commits represents the number of parents
         * which don't need to have dashes (because their edges fit
         * neatly under the commit).
         */
        const int dashless_commits = 2;
        int col_num, i;
        int num_dashes =
                ((graph->num_parents - dashless_commits) * 2) - 1;
        for (i = 0; i < num_dashes; i++) {
                col_num = (i / 2) + dashless_commits;
                strbuf_write_column(sb, &graph->new_columns[col_num], '-');
        }
        col_num = (i / 2) + dashless_commits;
        strbuf_write_column(sb, &graph->new_columns[col_num], '.');
        return num_dashes + 1;
}

static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)
{
        int seen_this = 0;
        int i, chars_written;

        /*
         * Output the row containing this commit
         * Iterate up to and including graph->num_columns,
         * since the current commit may not be in any of the existing
         * columns.  (This happens when the current commit doesn't have any
         * 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;
                if (i == graph->num_columns) {
                        if (seen_this)
                                break;
                        col_commit = graph->commit;
                } else {
                        col_commit = graph->columns[i].commit;
                }

                if (col_commit == graph->commit) {
                        seen_this = 1;
                        graph_output_commit_char(graph, sb);
                        chars_written++;

                        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)) {
                        /*
                         * This is a 2-way merge commit.
                         * There is no GRAPH_PRE_COMMIT stage for 2-way
                         * merges, so this is the first line of output
                         * for this commit.  Check to see what the previous
                         * line of output was.
                         *
                         * If it was GRAPH_POST_MERGE, the branch line
                         * coming into this commit may have been '\',
                         * and not '|' or '/'.  If so, output the branch
                         * line as '\' on this line, instead of '|'.  This
                         * makes the output look nicer.
                         */
                        if (graph->prev_state == GRAPH_POST_MERGE &&
                            graph->prev_commit_index < i)
                                strbuf_write_column(sb, col, '\\');
                        else
                                strbuf_write_column(sb, col, '|');
                        chars_written++;
                } else {
                        strbuf_write_column(sb, col, '|');
                        chars_written++;
                }
                strbuf_addch(sb, ' ');
                chars_written++;
        }

        graph_pad_horizontally(graph, sb, chars_written);

        /*
         * Update graph->state
         */
        if (graph->num_parents > 1)
                graph_update_state(graph, GRAPH_POST_MERGE);
        else if (graph_is_mapping_correct(graph))
                graph_update_state(graph, GRAPH_PADDING);
        else
                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 void graph_output_post_merge_line(struct git_graph *graph, struct strbuf *sb)
{
        int seen_this = 0;
        int i, j, chars_written;

        /*
         * 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;
                if (i == graph->num_columns) {
                        if (seen_this)
                                break;
                        col_commit = graph->commit;
                } else {
                        col_commit = col->commit;
                }

                if (col_commit == graph->commit) {
                        /*
                         * Since the current commit is a merge find
                         * the columns for the parent commits in
                         * new_columns and use those to format the
                         * edges.
                         */
                        struct commit_list *parents = NULL;
                        struct column *par_column;
                        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++) {
                                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;
                } else if (seen_this) {
                        strbuf_write_column(sb, col, '\\');
                        strbuf_addch(sb, ' ');
                        chars_written += 2;
                } else {
                        strbuf_write_column(sb, col, '|');
                        strbuf_addch(sb, ' ');
                        chars_written += 2;
                }
        }

        graph_pad_horizontally(graph, sb, chars_written);

        /*
         * Update graph->state
         */
        if (graph_is_mapping_correct(graph))
                graph_update_state(graph, GRAPH_PADDING);
        else
                graph_update_state(graph, GRAPH_COLLAPSING);
}

static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf *sb)
{
        int i;
        int *tmp_mapping;
        short used_horizontal = 0;
        int horizontal_edge = -1;
        int horizontal_edge_target = -1;

        /*
         * Clear out the new_mapping array
         */
        for (i = 0; i < graph->mapping_size; i++)
                graph->new_mapping[i] = -1;

        for (i = 0; i < graph->mapping_size; i++) {
                int target = graph->mapping[i];
                if (target < 0)
                        continue;

                /*
                 * Since update_columns() always inserts the leftmost
                 * column first, each branch's target location should
                 * always be either its current location or to the left of
                 * its current location.
                 *
                 * We never have to move branches to the right.  This makes
                 * the graph much more legible, since whenever branches
                 * cross, only one is moving directions.
                 */
                assert(target * 2 <= i);

                if (target * 2 == i) {
                        /*
                         * 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) {
                        /*
                         * Nothing is to the left.
                         * Move to the left by one
                         */
                        graph->new_mapping[i - 1] = target;
                        /*
                         * If there isn't already an edge moving horizontally
                         * select this one.
                         */
                        if (horizontal_edge == -1) {
                                int j;
                                horizontal_edge = i;
                                horizontal_edge_target = target;
                                /*
                                 * The variable target is the index of the graph
                                 * column, and therefore target*2+3 is the
                                 * actual screen column of the first horizontal
                                 * line.
                                 */
                                for (j = (target * 2)+3; j < (i - 2); j += 2)
                                        graph->new_mapping[j] = target;
                        }
                } else if (graph->new_mapping[i - 1] == target) {
                        /*
                         * There is a branch line to our left
                         * already, and it is our target.  We
                         * combine with this line, since we share
                         * the same parent commit.
                         *
                         * We don't have to add anything to the
                         * output or new_mapping, since the
                         * existing branch line has already taken
                         * care of it.
                         */
                } else {
                        /*
                         * There is a branch line to our left,
                         * but it isn't our target.  We need to
                         * cross over it.
                         *
                         * 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;
                        /*
                         * Mark this branch as the horizontal edge to
                         * prevent any other edges from moving
                         * horizontally.
                         */
                        if (horizontal_edge == -1)
                                horizontal_edge = i;
                }
        }

        /*
         * The new mapping may be 1 smaller than the old mapping
         */
        if (graph->new_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];
                if (target < 0)
                        strbuf_addch(sb, ' ');
                else if (target * 2 == i)
                        strbuf_write_column(sb, &graph->new_columns[target], '|');
                else if (target == horizontal_edge_target &&
                         i != horizontal_edge - 1) {
                                /*
                                 * Set the mappings for all but the
                                 * first segment to -1 so that they
                                 * won't continue into the next line.
                                 */
                                if (i != (target * 2)+3)
                                        graph->new_mapping[i] = -1;
                                used_horizontal = 1;
                        strbuf_write_column(sb, &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_pad_horizontally(graph, sb, graph->mapping_size);

        /*
         * Swap mapping and new_mapping
         */
        tmp_mapping = graph->mapping;
        graph->mapping = graph->new_mapping;
        graph->new_mapping = tmp_mapping;

        /*
         * If graph->mapping indicates that all of the branch lines
         * are already in the correct positions, we are done.
         * Otherwise, we need to collapse some branch lines together.
         */
        if (graph_is_mapping_correct(graph))
                graph_update_state(graph, GRAPH_PADDING);
}

int graph_next_line(struct git_graph *graph, struct strbuf *sb)
{
        switch (graph->state) {
        case GRAPH_PADDING:
                graph_output_padding_line(graph, sb);
                return 0;
        case GRAPH_SKIP:
                graph_output_skip_line(graph, sb);
                return 0;
        case GRAPH_PRE_COMMIT:
                graph_output_pre_commit_line(graph, sb);
                return 0;
        case GRAPH_COMMIT:
                graph_output_commit_line(graph, sb);
                return 1;
        case GRAPH_POST_MERGE:
                graph_output_post_merge_line(graph, sb);
                return 0;
        case GRAPH_COLLAPSING:
                graph_output_collapsing_line(graph, sb);
                return 0;
        }

        assert(0);
        return 0;
}

static void graph_padding_line(struct git_graph *graph, struct strbuf *sb)
{
        int i, j;

        if (graph->state != GRAPH_COMMIT) {
                graph_next_line(graph, sb);
                return;
        }

        /*
         * Output the row containing this commit
         * Iterate up to and including graph->num_columns,
         * since the current commit may not be in any of the existing
         * columns.  (This happens when the current commit doesn't have any
         * children that we have already processed.)
         */
        for (i = 0; i < graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
                struct commit *col_commit = col->commit;
                if (col_commit == graph->commit) {
                        strbuf_write_column(sb, col, '|');

                        if (graph->num_parents < 3)
                                strbuf_addch(sb, ' ');
                        else {
                                int num_spaces = ((graph->num_parents - 2) * 2);
                                for (j = 0; j < num_spaces; j++)
                                        strbuf_addch(sb, ' ');
                        }
                } else {
                        strbuf_write_column(sb, col, '|');
                        strbuf_addch(sb, ' ');
                }
        }

        graph_pad_horizontally(graph, sb, graph->num_columns);

        /*
         * Update graph->prev_state since we have output a padding line
         */
        graph->prev_state = GRAPH_PADDING;
}

int graph_is_commit_finished(struct git_graph const *graph)
{
        return (graph->state == GRAPH_PADDING);
}

void graph_show_commit(struct git_graph *graph)
{
        struct strbuf msgbuf = STRBUF_INIT;
        int shown_commit_line = 0;

        if (!graph)
                return;

        while (!shown_commit_line) {
                shown_commit_line = graph_next_line(graph, &msgbuf);
                fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);
                if (!shown_commit_line)
                        putchar('\n');
                strbuf_setlen(&msgbuf, 0);
        }

        strbuf_release(&msgbuf);
}

void graph_show_oneline(struct git_graph *graph)
{
        struct strbuf msgbuf = STRBUF_INIT;

        if (!graph)
                return;

        graph_next_line(graph, &msgbuf);
        fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);
        strbuf_release(&msgbuf);
}

void graph_show_padding(struct git_graph *graph)
{
        struct strbuf msgbuf = STRBUF_INIT;

        if (!graph)
                return;

        graph_padding_line(graph, &msgbuf);
        fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);
        strbuf_release(&msgbuf);
}

int graph_show_remainder(struct git_graph *graph)
{
        struct strbuf msgbuf = STRBUF_INIT;
        int shown = 0;

        if (!graph)
                return 0;

        if (graph_is_commit_finished(graph))
                return 0;

        for (;;) {
                graph_next_line(graph, &msgbuf);
                fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);
                strbuf_setlen(&msgbuf, 0);
                shown = 1;

                if (!graph_is_commit_finished(graph))
                        putchar('\n');
                else
                        break;
        }
        strbuf_release(&msgbuf);

        return shown;
}


static void graph_show_strbuf(struct git_graph *graph, struct strbuf const *sb)
{
        char *p;

        if (!graph) {
                fwrite(sb->buf, sizeof(char), sb->len, stdout);
                return;
        }

        /*
         * Print the strbuf line by line,
         * and display the graph info before each line but the first.
         */
        p = sb->buf;
        while (p) {
                size_t len;
                char *next_p = strchr(p, '\n');
                if (next_p) {
                        next_p++;
                        len = next_p - p;
                } else {
                        len = (sb->buf + sb->len) - p;
                }
                fwrite(p, sizeof(char), len, stdout);
                if (next_p && *next_p != '\0')
                        graph_show_oneline(graph);
                p = next_p;
        }
}

void graph_show_commit_msg(struct git_graph *graph,
                           struct strbuf const *sb)
{
        int newline_terminated;

        if (!graph) {
                /*
                 * If there's no graph, just print the message buffer.
                 *
                 * The message buffer for CMIT_FMT_ONELINE and
                 * CMIT_FMT_USERFORMAT are already missing a terminating
                 * newline.  All of the other formats should have it.
                 */
                fwrite(sb->buf, sizeof(char), sb->len, stdout);
                return;
        }

        newline_terminated = (sb->len && sb->buf[sb->len - 1] == '\n');

        /*
         * Show the commit message
         */
        graph_show_strbuf(graph, sb);

        /*
         * If there is more output needed for this commit, show it now
         */
        if (!graph_is_commit_finished(graph)) {
                /*
                 * If sb doesn't have a terminating newline, print one now,
                 * so we can start the remainder of the graph output on a
                 * new line.
                 */
                if (!newline_terminated)
                        putchar('\n');

                graph_show_remainder(graph);

                /*
                 * If sb ends with a newline, our output should too.
                 */
                if (newline_terminated)
                        putchar('\n');
        }
}