t4053: avoid writing to unopened pipe

[git.git] / graph.c

#include "git-compat-util.h"
#include "gettext.h"
#include "config.h"
#include "commit.h"
#include "color.h"
#include "graph.h"
#include "revision.h"
#include "strvec.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.  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().
 *
 * Note that unlike some other graph display functions, you must pass the file
 * 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,
                              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
};

static void graph_show_line_prefix(const struct diff_options *diffopt)
{
        if (!diffopt || !diffopt->line_prefix)
                return;

        fwrite(diffopt->line_prefix,
               sizeof(char),
               diffopt->line_prefix_length,
               diffopt->file);
}

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

static void parse_graph_colors_config(struct strvec *colors, const char *string)
{
        const char *end, *start;

        start = string;
        end = string + strlen(string);
        while (start < end) {
                const char *comma = strchrnul(start, ',');
                char color[COLOR_MAXLEN];

                if (!color_parse_mem(start, comma - start, color))
                        strvec_push(colors, color);
                else
                        warning(_("ignored invalid color '%.*s' in log.graphColors"),
                                (int)(comma - start), start);
                start = comma + 1;
        }
        strvec_push(colors, GIT_COLOR_RESET);
}

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];
}

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)
                graph_line_addcolor(line, c->color);
        graph_line_addch(line, col_char);
        if (c->color < column_colors_max)
                graph_line_addcolor(line, 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;
        /*
         * 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 old_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 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 *old_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(opt);

        strbuf_reset(&msgbuf);
        if (opt->line_prefix)
                strbuf_add(&msgbuf, opt->line_prefix,
                           opt->line_prefix_length);
        if (graph)
                graph_padding_line(graph, &msgbuf);
        return &msgbuf;
}

static const struct diff_options *default_diffopt;

void graph_setup_line_prefix(struct diff_options *diffopt)
{
        default_diffopt = diffopt;

        /* setup an output prefix callback if necessary */
        if (diffopt && !diffopt->output_prefix)
                diffopt->output_prefix = diff_output_prefix_callback;
}


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

        if (!column_colors) {
                char *string;
                if (git_config_get_string("log.graphcolors", &string)) {
                        /* not configured -- use default */
                        graph_set_column_colors(column_colors_ansi,
                                                column_colors_ansi_max);
                } else {
                        static struct strvec custom_colors = STRVEC_INIT;
                        strvec_clear(&custom_colors);
                        parse_graph_colors_config(&custom_colors, string);
                        free(string);
                        /* graph_set_column_colors takes a max-index, not a count */
                        graph_set_column_colors(custom_colors.v,
                                                custom_colors.nr - 1);
                }
        }

        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->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;
        /*
         * 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;
        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->old_mapping, 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;
}

void graph_clear(struct git_graph *graph)
{
        if (!graph)
                return;

        free(graph->columns);
        free(graph->new_columns);
        free(graph->mapping);
        free(graph->old_mapping);
        free(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);

        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->old_mapping, graph->column_capacity * 2);
}

/*
 * 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 (!want_color(graph->revs->diffopt.use_color))
                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 int graph_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 i;
        }
        return -1;
}

static void graph_insert_into_new_columns(struct git_graph *graph,
                                          struct commit *commit,
                                          int idx)
{
        int i = graph_find_new_column_by_commit(graph, commit);
        int mapping_idx;

        /*
         * If the commit is not already in the new_columns array, then add it
         * and record it as being in the final column.
         */
        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);
        }

        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;

                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 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.
         */
        SWAP(graph->columns, graph->new_columns);
        graph->num_columns = graph->num_new_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;

        graph->width = 0;
        graph->prev_edges_added = graph->edges_added;
        graph->edges_added = 0;

        /*
         * 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;
        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) {
                        seen_this = 1;
                        graph->commit_index = i;
                        graph->merge_layout = -1;
                        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, i);
                        }
                        /*
                         * 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 (graph->num_parents == 0)
                                graph->width += 2;
                } else {
                        graph_insert_into_new_columns(graph, col_commit, -1);
                }
        }

        /*
         * Shrink mapping_size to be the minimum necessary
         */
        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)
{
        /*
         * 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:
         *
         *              | *
         *              | |\
         *              | * \
         *              |/|\ \
         */
        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)
{
        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_needs_pre_commit_line(graph))
                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 graph_line *line)
{
        /*
         * 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.
         */
        if (line->width < graph->width)
                graph_line_addchars(line, ' ', graph->width - line->width);
}

static void graph_output_padding_line(struct git_graph *graph,
                                      struct graph_line *line)
{
        int i;

        /*
         * Output a padding row, that leaves all branch lines unchanged
         */
        for (i = 0; i < graph->num_new_columns; i++) {
                graph_line_write_column(line, &graph->new_columns[i], '|');
                graph_line_addch(line, ' ');
        }
}


int graph_width(struct git_graph *graph)
{
        return graph->width;
}


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.
         */
        graph_line_addstr(line, "...");

        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 graph_line *line)
{
        int i, seen_this;

        /*
         * 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);

        /*
         * 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 < graph_num_expansion_rows(graph));

        /*
         * Output the row
         */
        seen_this = 0;
        for (i = 0; i < graph->num_columns; i++) {
                struct column *col = &graph->columns[i];
                if (col->commit == graph->commit) {
                        seen_this = 1;
                        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.
                         * 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)
                                graph_line_write_column(line, col, '\\');
                        else
                                graph_line_write_column(line, col, '|');
                } else if (seen_this && (graph->expansion_row > 0)) {
                        graph_line_write_column(line, col, '\\');
                } else {
                        graph_line_write_column(line, col, '|');
                }
                graph_line_addch(line, ' ');
        }

        /*
         * Increment graph->expansion_row,
         * and move to state GRAPH_COMMIT if necessary
         */
        graph->expansion_row++;
        if (!graph_needs_pre_commit_line(graph))
                graph_update_state(graph, GRAPH_COMMIT);
}

static void graph_output_commit_char(struct git_graph *graph, struct graph_line *line)
{
        /*
         * 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);
                graph_line_addch(line, 'o');
                return;
        }

        /*
         * get_revision_mark() handles all other cases without assert()
         */
        graph_line_addstr(line, get_revision_mark(graph->revs, graph->commit));
}

/*
 * Draw the horizontal dashes of an octopus merge.
 */
static void graph_draw_octopus_merge(struct git_graph *graph, struct graph_line *line)
{
        /*
         * The parents of a merge commit can be arbitrarily reordered as they
         * are mapped onto display columns, for example this is a valid merge:
         *
         *      | | *---.
         *      | | |\ \ \
         *      | | |/ / /
         *      | |/| | /
         *      | |_|_|/
         *      |/| | |
         *      3 1 0 2
         *
         * 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.
         *
         * 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.
         *
         * 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 i, j;
        struct column *col;

        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;
}

static void graph_output_commit_line(struct git_graph *graph, struct graph_line *line)
{
        int seen_this = 0;
        int i;

        /*
         * 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;
        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, line);

                        if (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 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.
                         *
                         * 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_edges_added > 0 &&
                            graph->prev_commit_index < i)
                                graph_line_write_column(line, col, '\\');
                        else
                                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 {
                        graph_line_write_column(line, col, '|');
                }
                graph_line_addch(line, ' ');
        }

        /*
         * 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 const char merge_chars[] = {'/', '|', '\\'};

static void graph_output_post_merge_line(struct git_graph *graph, struct graph_line *line)
{
        int seen_this = 0;
        int i, j;

        struct commit_list *first_parent = first_interesting_parent(graph);
        struct column *parent_col = NULL;

        /*
         * Output the post-merge row
         */
        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 = first_parent;
                        int par_column;
                        int idx = graph->merge_layout;
                        char c;
                        seen_this = 1;

                        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);
                        }
                        if (graph->edges_added == 0)
                                graph_line_addch(line, ' ');

                } else if (seen_this) {
                        if (graph->edges_added > 0)
                                graph_line_write_column(line, col, '\\');
                        else
                                graph_line_write_column(line, col, '|');
                        graph_line_addch(line, ' ');
                } else {
                        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, ' ');
                        }
                }

                if (col_commit == first_parent->item)
                        parent_col = col;
        }

        /*
         * 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 graph_line *line)
{
        int i;
        short used_horizontal = 0;
        int horizontal_edge = -1;
        int horizontal_edge_target = -1;

        /*
         * 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->mapping[i] = -1;

        for (i = 0; i < graph->mapping_size; i++) {
                int target = graph->old_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->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->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->mapping[j] = target;
                        }
                } else if (graph->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 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.
                         */
                        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) {
                                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->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->mapping[i];
                if (target < 0)
                        graph_line_addch(line, ' ');
                else if (target * 2 == i)
                        graph_line_write_column(line, &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->mapping[i] = -1;
                                used_horizontal = 1;
                        graph_line_write_column(line, &graph->new_columns[target], '_');
                } else {
                        if (used_horizontal && i < horizontal_edge)
                                graph->mapping[i] = -1;
                        graph_line_write_column(line, &graph->new_columns[target], '/');

                }
        }

        /*
         * 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)
{
        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, &line);
                break;
        case GRAPH_SKIP:
                graph_output_skip_line(graph, &line);
                break;
        case GRAPH_PRE_COMMIT:
                graph_output_pre_commit_line(graph, &line);
                break;
        case GRAPH_COMMIT:
                graph_output_commit_line(graph, &line);
                shown_commit_line = 1;
                break;
        case GRAPH_POST_MERGE:
                graph_output_post_merge_line(graph, &line);
                break;
        case GRAPH_COLLAPSING:
                graph_output_collapsing_line(graph, &line);
                break;
        }

        graph_pad_horizontally(graph, &line);
        return shown_commit_line;
}

static void graph_padding_line(struct git_graph *graph, struct strbuf *sb)
{
        int i;
        struct graph_line line = { .buf = sb, .width = 0 };

        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];

                graph_line_write_column(&line, col, '|');

                if (col->commit == graph->commit && graph->num_parents > 2) {
                        int len = (graph->num_parents - 2) * 2;
                        graph_line_addchars(&line, ' ', len);
                } else {
                        graph_line_addch(&line, ' ');
                }
        }

        graph_pad_horizontally(graph, &line);

        /*
         * 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;

        graph_show_line_prefix(default_diffopt);

        if (!graph)
                return;

        /*
         * When showing a diff of a merge against each of its parents, we
         * are called once for each parent without graph_update having been
         * called.  In this case, simply output a single padding line.
         */
        if (graph_is_commit_finished(graph)) {
                graph_show_padding(graph);
                shown_commit_line = 1;
        }

        while (!shown_commit_line && !graph_is_commit_finished(graph)) {
                shown_commit_line = graph_next_line(graph, &msgbuf);
                fwrite(msgbuf.buf, sizeof(char), msgbuf.len,
                        graph->revs->diffopt.file);
                if (!shown_commit_line) {
                        putc('\n', graph->revs->diffopt.file);
                        graph_show_line_prefix(&graph->revs->diffopt);
                }
                strbuf_setlen(&msgbuf, 0);
        }

        strbuf_release(&msgbuf);
}

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

        graph_show_line_prefix(default_diffopt);

        if (!graph)
                return;

        graph_next_line(graph, &msgbuf);
        fwrite(msgbuf.buf, sizeof(char), msgbuf.len, graph->revs->diffopt.file);
        strbuf_release(&msgbuf);
}

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

        graph_show_line_prefix(default_diffopt);

        if (!graph)
                return;

        graph_padding_line(graph, &msgbuf);
        fwrite(msgbuf.buf, sizeof(char), msgbuf.len, graph->revs->diffopt.file);
        strbuf_release(&msgbuf);
}

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

        graph_show_line_prefix(default_diffopt);

        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,
                        graph->revs->diffopt.file);
                strbuf_setlen(&msgbuf, 0);
                shown = 1;

                if (!graph_is_commit_finished(graph)) {
                        putc('\n', graph->revs->diffopt.file);
                        graph_show_line_prefix(&graph->revs->diffopt);
                } else {
                        break;
                }
        }
        strbuf_release(&msgbuf);

        return shown;
}

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

        /*
         * 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, file);
                if (next_p && *next_p != '\0')
                        graph_show_oneline(graph);
                p = next_p;
        }
}

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

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

        if (!graph)
                return;

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

        /*
         * 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)
                        putc('\n', file);

                graph_show_remainder(graph);

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