Merge branch 'ds/ahead-behind'

"git for-each-ref" learns '%(ahead-behind:<base>)' that computes the
distances from a single reference point in the history with bunch
of commits in bulk.

* ds/ahead-behind:
  commit-reach: add tips_reachable_from_bases()
  for-each-ref: add ahead-behind format atom
  commit-reach: implement ahead_behind() logic
  commit-graph: introduce `ensure_generations_valid()`
  commit-graph: return generation from memory
  commit-graph: simplify compute_generation_numbers()
  commit-graph: refactor compute_topological_levels()
  for-each-ref: explicitly test no matches
  for-each-ref: add --stdin option

Documentation/git-for-each-ref.txt

@@ -9,7 +9,8 @@ SYNOPSIS
 --------
 [verse]
 'git for-each-ref' [--count=<count>] [--shell|--perl|--python|--tcl]
-		   [(--sort=<key>)...] [--format=<format>] [<pattern>...]
+		   [(--sort=<key>)...] [--format=<format>]
+		   [ --stdin | <pattern>... ]
 		   [--points-at=<object>]
 		   [--merged[=<object>]] [--no-merged[=<object>]]
 		   [--contains[=<object>]] [--no-contains[=<object>]]
@@ -32,6 +33,10 @@ OPTIONS
 	literally, in the latter case matching completely or from the
 	beginning up to a slash.
 
+--stdin::
+	If `--stdin` is supplied, then the list of patterns is read from
+	standard input instead of from the argument list.
+
 --count=<count>::
 	By default the command shows all refs that match
 	`<pattern>`.  This option makes it stop after showing
@@ -217,6 +222,11 @@ worktreepath::
 	out, if it is checked out in any linked worktree. Empty string
 	otherwise.
 
+ahead-behind:<committish>::
+	Two integers, separated by a space, demonstrating the number of
+	commits ahead and behind, respectively, when comparing the output
+	ref to the `<committish>` specified in the format.
+
 In addition to the above, for commit and tag objects, the header
 field names (`tree`, `parent`, `object`, `type`, and `tag`) can
 be used to specify the value in the header field.

builtin/branch.c

@@ -448,6 +448,7 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 	if (verify_ref_format(format))
 		die(_("unable to parse format string"));
 
+	filter_ahead_behind(the_repository, format, &array);
 	ref_array_sort(sorting, &array);
 
 	for (i = 0; i < array.nr; i++) {

builtin/for-each-ref.c

@@ -5,6 +5,8 @@
 #include "object.h"
 #include "parse-options.h"
 #include "ref-filter.h"
+#include "strvec.h"
+#include "commit-reach.h"
 
 static char const * const for_each_ref_usage[] = {
 	N_("git for-each-ref [<options>] [<pattern>]"),
@@ -25,6 +27,8 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	struct ref_format format = REF_FORMAT_INIT;
 	struct strbuf output = STRBUF_INIT;
 	struct strbuf err = STRBUF_INIT;
+	int from_stdin = 0;
+	struct strvec vec = STRVEC_INIT;
 
 	struct option opts[] = {
 		OPT_BIT('s', "shell", &format.quote_style,
@@ -49,6 +53,7 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 		OPT_CONTAINS(&filter.with_commit, N_("print only refs which contain the commit")),
 		OPT_NO_CONTAINS(&filter.no_commit, N_("print only refs which don't contain the commit")),
 		OPT_BOOL(0, "ignore-case", &icase, N_("sorting and filtering are case insensitive")),
+		OPT_BOOL(0, "stdin", &from_stdin, N_("read reference patterns from stdin")),
 		OPT_END(),
 	};
 
@@ -75,9 +80,27 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	ref_sorting_set_sort_flags_all(sorting, REF_SORTING_ICASE, icase);
 	filter.ignore_case = icase;
 
-	filter.name_patterns = argv;
+	if (from_stdin) {
+		struct strbuf line = STRBUF_INIT;
+
+		if (argv[0])
+			die(_("unknown arguments supplied with --stdin"));
+
+		while (strbuf_getline(&line, stdin) != EOF)
+			strvec_push(&vec, line.buf);
+
+		strbuf_release(&line);
+
+		/* vec.v is NULL-terminated, just like 'argv'. */
+		filter.name_patterns = vec.v;
+	} else {
+		filter.name_patterns = argv;
+	}
+
 	filter.match_as_path = 1;
 	filter_refs(&array, &filter, FILTER_REFS_ALL);
+	filter_ahead_behind(the_repository, &format, &array);
+
 	ref_array_sort(sorting, &array);
 
 	if (!maxcount || array.nr < maxcount)
@@ -97,5 +120,6 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	free_commit_list(filter.with_commit);
 	free_commit_list(filter.no_commit);
 	ref_sorting_release(sorting);
+	strvec_clear(&vec);
 	return 0;
 }

builtin/tag.c

@@ -67,6 +67,7 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting,
 		die(_("unable to parse format string"));
 	filter->with_commit_tag_algo = 1;
 	filter_refs(&array, filter, FILTER_REFS_TAGS);
+	filter_ahead_behind(the_repository, format, &array);
 	ref_array_sort(sorting, &array);
 
 	for (i = 0; i < array.nr; i++) {

commit-graph.c

@@ -117,12 +117,10 @@ timestamp_t commit_graph_generation(const struct commit *c)
 	struct commit_graph_data *data =
 		commit_graph_data_slab_peek(&commit_graph_data_slab, c);
 
-	if (!data)
-		return GENERATION_NUMBER_INFINITY;
-	else if (data->graph_pos == COMMIT_NOT_FROM_GRAPH)
-		return GENERATION_NUMBER_INFINITY;
+	if (data && data->generation)
+		return data->generation;
 
-	return data->generation;
+	return GENERATION_NUMBER_INFINITY;
 }
 
 static struct commit_graph_data *commit_graph_data_at(const struct commit *c)
@@ -1447,24 +1445,52 @@ static void close_reachable(struct write_commit_graph_context *ctx)
 	stop_progress(&ctx->progress);
 }
 
-static void compute_topological_levels(struct write_commit_graph_context *ctx)
+struct compute_generation_info {
+	struct repository *r;
+	struct packed_commit_list *commits;
+	struct progress *progress;
+	int progress_cnt;
+
+	timestamp_t (*get_generation)(struct commit *c, void *data);
+	void (*set_generation)(struct commit *c, timestamp_t gen, void *data);
+	void *data;
+};
+
+static timestamp_t compute_generation_from_max(struct commit *c,
+					       timestamp_t max_gen,
+					       int generation_version)
+{
+	switch (generation_version) {
+	case 1: /* topological levels */
+		if (max_gen > GENERATION_NUMBER_V1_MAX - 1)
+			max_gen = GENERATION_NUMBER_V1_MAX - 1;
+		return max_gen + 1;
+
+	case 2: /* corrected commit date */
+		if (c->date && c->date > max_gen)
+			max_gen = c->date - 1;
+		return max_gen + 1;
+
+	default:
+		BUG("attempting unimplemented version");
+	}
+}
+
+static void compute_reachable_generation_numbers(
+			struct compute_generation_info *info,
+			int generation_version)
 {
 	int i;
 	struct commit_list *list = NULL;
 
-	if (ctx->report_progress)
-		ctx->progress = start_delayed_progress(
-					_("Computing commit graph topological levels"),
-					ctx->commits.nr);
-	for (i = 0; i < ctx->commits.nr; i++) {
-		struct commit *c = ctx->commits.list[i];
-		uint32_t level;
-
-		repo_parse_commit(ctx->r, c);
-		level = *topo_level_slab_at(ctx->topo_levels, c);
+	for (i = 0; i < info->commits->nr; i++) {
+		struct commit *c = info->commits->list[i];
+		timestamp_t gen;
+		repo_parse_commit(info->r, c);
+		gen = info->get_generation(c, info->data);
+		display_progress(info->progress, info->progress_cnt + 1);
 
-		display_progress(ctx->progress, i + 1);
-		if (level != GENERATION_NUMBER_ZERO)
+		if (gen != GENERATION_NUMBER_ZERO && gen != GENERATION_NUMBER_INFINITY)
 			continue;
 
 		commit_list_insert(c, &list);
@@ -1472,41 +1498,91 @@ static void compute_topological_levels(struct write_commit_graph_context *ctx)
 			struct commit *current = list->item;
 			struct commit_list *parent;
 			int all_parents_computed = 1;
-			uint32_t max_level = 0;
+			uint32_t max_gen = 0;
 
 			for (parent = current->parents; parent; parent = parent->next) {
-				repo_parse_commit(ctx->r, parent->item);
-				level = *topo_level_slab_at(ctx->topo_levels, parent->item);
+				repo_parse_commit(info->r, parent->item);
+				gen = info->get_generation(parent->item, info->data);
 
-				if (level == GENERATION_NUMBER_ZERO) {
+				if (gen == GENERATION_NUMBER_ZERO) {
 					all_parents_computed = 0;
 					commit_list_insert(parent->item, &list);
 					break;
 				}
 
-				if (level > max_level)
-					max_level = level;
+				if (gen > max_gen)
+					max_gen = gen;
 			}
 
 			if (all_parents_computed) {
 				pop_commit(&list);
-
-				if (max_level > GENERATION_NUMBER_V1_MAX - 1)
-					max_level = GENERATION_NUMBER_V1_MAX - 1;
-				*topo_level_slab_at(ctx->topo_levels, current) = max_level + 1;
+				gen = compute_generation_from_max(
+						current, max_gen,
+						generation_version);
+				info->set_generation(current, gen, info->data);
 			}
 		}
 	}
+}
+
+static timestamp_t get_topo_level(struct commit *c, void *data)
+{
+	struct write_commit_graph_context *ctx = data;
+	return *topo_level_slab_at(ctx->topo_levels, c);
+}
+
+static void set_topo_level(struct commit *c, timestamp_t t, void *data)
+{
+	struct write_commit_graph_context *ctx = data;
+	*topo_level_slab_at(ctx->topo_levels, c) = (uint32_t)t;
+}
+
+static void compute_topological_levels(struct write_commit_graph_context *ctx)
+{
+	struct compute_generation_info info = {
+		.r = ctx->r,
+		.commits = &ctx->commits,
+		.get_generation = get_topo_level,
+		.set_generation = set_topo_level,
+		.data = ctx,
+	};
+
+	if (ctx->report_progress)
+		info.progress = ctx->progress
+			      = start_delayed_progress(
+					_("Computing commit graph topological levels"),
+					ctx->commits.nr);
+
+	compute_reachable_generation_numbers(&info, 1);
+
 	stop_progress(&ctx->progress);
 }
 
+static timestamp_t get_generation_from_graph_data(struct commit *c, void *data)
+{
+	return commit_graph_data_at(c)->generation;
+}
+
+static void set_generation_v2(struct commit *c, timestamp_t t, void *data)
+{
+	struct commit_graph_data *g = commit_graph_data_at(c);
+	g->generation = (uint32_t)t;
+}
+
 static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 {
 	int i;
-	struct commit_list *list = NULL;
+	struct compute_generation_info info = {
+		.r = ctx->r,
+		.commits = &ctx->commits,
+		.get_generation = get_generation_from_graph_data,
+		.set_generation = set_generation_v2,
+		.data = ctx,
+	};
 
 	if (ctx->report_progress)
-		ctx->progress = start_delayed_progress(
+		info.progress = ctx->progress
+			      = start_delayed_progress(
 					_("Computing commit graph generation numbers"),
 					ctx->commits.nr);
 
@@ -1518,47 +1594,7 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 		}
 	}
 
-	for (i = 0; i < ctx->commits.nr; i++) {
-		struct commit *c = ctx->commits.list[i];
-		timestamp_t corrected_commit_date;
-
-		repo_parse_commit(ctx->r, c);
-		corrected_commit_date = commit_graph_data_at(c)->generation;
-
-		display_progress(ctx->progress, i + 1);
-		if (corrected_commit_date != GENERATION_NUMBER_ZERO)
-			continue;
-
-		commit_list_insert(c, &list);
-		while (list) {
-			struct commit *current = list->item;
-			struct commit_list *parent;
-			int all_parents_computed = 1;
-			timestamp_t max_corrected_commit_date = 0;
-
-			for (parent = current->parents; parent; parent = parent->next) {
-				repo_parse_commit(ctx->r, parent->item);
-				corrected_commit_date = commit_graph_data_at(parent->item)->generation;
-
-				if (corrected_commit_date == GENERATION_NUMBER_ZERO) {
-					all_parents_computed = 0;
-					commit_list_insert(parent->item, &list);
-					break;
-				}
-
-				if (corrected_commit_date > max_corrected_commit_date)
-					max_corrected_commit_date = corrected_commit_date;
-			}
-
-			if (all_parents_computed) {
-				pop_commit(&list);
-
-				if (current->date && current->date > max_corrected_commit_date)
-					max_corrected_commit_date = current->date - 1;
-				commit_graph_data_at(current)->generation = max_corrected_commit_date + 1;
-			}
-		}
-	}
+	compute_reachable_generation_numbers(&info, 2);
 
 	for (i = 0; i < ctx->commits.nr; i++) {
 		struct commit *c = ctx->commits.list[i];
@@ -1569,6 +1605,35 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 	stop_progress(&ctx->progress);
 }
 
+static void set_generation_in_graph_data(struct commit *c, timestamp_t t,
+					 void *data)
+{
+	commit_graph_data_at(c)->generation = t;
+}
+
+/*
+ * After this method, all commits reachable from those in the given
+ * list will have non-zero, non-infinite generation numbers.
+ */
+void ensure_generations_valid(struct repository *r,
+			      struct commit **commits, size_t nr)
+{
+	int generation_version = get_configured_generation_version(r);
+	struct packed_commit_list list = {
+		.list = commits,
+		.alloc = nr,
+		.nr = nr,
+	};
+	struct compute_generation_info info = {
+		.r = r,
+		.commits = &list,
+		.get_generation = get_generation_from_graph_data,
+		.set_generation = set_generation_in_graph_data,
+	};
+
+	compute_reachable_generation_numbers(&info, generation_version);
+}
+
 static void trace2_bloom_filter_write_statistics(struct write_commit_graph_context *ctx)
 {
 	trace2_data_intmax("commit-graph", ctx->r, "filter-computed",

commit-graph.h

@@ -189,4 +189,12 @@ struct commit_graph_data {
  */
 timestamp_t commit_graph_generation(const struct commit *);
 uint32_t commit_graph_position(const struct commit *);
+
+/*
+ * After this method, all commits reachable from those in the given
+ * list will have non-zero, non-infinite generation numbers.
+ */
+void ensure_generations_valid(struct repository *r,
+			      struct commit **commits, size_t nr);
+
 #endif

commit-reach.c

@@ -10,6 +10,7 @@
 #include "revision.h"
 #include "tag.h"
 #include "commit-reach.h"
+#include "ewah/ewok.h"
 
 /* Remember to update object flag allocation in object.h */
 #define PARENT1		(1u<<16)
@@ -947,3 +948,218 @@ struct commit_list *get_reachable_subset(struct commit **from, int nr_from,
 
 	return found_commits;
 }
+
+define_commit_slab(bit_arrays, struct bitmap *);
+static struct bit_arrays bit_arrays;
+
+static void insert_no_dup(struct prio_queue *queue, struct commit *c)
+{
+	if (c->object.flags & PARENT2)
+		return;
+	prio_queue_put(queue, c);
+	c->object.flags |= PARENT2;
+}
+
+static struct bitmap *get_bit_array(struct commit *c, int width)
+{
+	struct bitmap **bitmap = bit_arrays_at(&bit_arrays, c);
+	if (!*bitmap)
+		*bitmap = bitmap_word_alloc(width);
+	return *bitmap;
+}
+
+static void free_bit_array(struct commit *c)
+{
+	struct bitmap **bitmap = bit_arrays_at(&bit_arrays, c);
+	if (!*bitmap)
+		return;
+	bitmap_free(*bitmap);
+	*bitmap = NULL;
+}
+
+void ahead_behind(struct repository *r,
+		  struct commit **commits, size_t commits_nr,
+		  struct ahead_behind_count *counts, size_t counts_nr)
+{
+	struct prio_queue queue = { .compare = compare_commits_by_gen_then_commit_date };
+	size_t width = DIV_ROUND_UP(commits_nr, BITS_IN_EWORD);
+
+	if (!commits_nr || !counts_nr)
+		return;
+
+	for (size_t i = 0; i < counts_nr; i++) {
+		counts[i].ahead = 0;
+		counts[i].behind = 0;
+	}
+
+	ensure_generations_valid(r, commits, commits_nr);
+
+	init_bit_arrays(&bit_arrays);
+
+	for (size_t i = 0; i < commits_nr; i++) {
+		struct commit *c = commits[i];
+		struct bitmap *bitmap = get_bit_array(c, width);
+
+		bitmap_set(bitmap, i);
+		insert_no_dup(&queue, c);
+	}
+
+	while (queue_has_nonstale(&queue)) {
+		struct commit *c = prio_queue_get(&queue);
+		struct commit_list *p;
+		struct bitmap *bitmap_c = get_bit_array(c, width);
+
+		for (size_t i = 0; i < counts_nr; i++) {
+			int reach_from_tip = !!bitmap_get(bitmap_c, counts[i].tip_index);
+			int reach_from_base = !!bitmap_get(bitmap_c, counts[i].base_index);
+
+			if (reach_from_tip ^ reach_from_base) {
+				if (reach_from_base)
+					counts[i].behind++;
+				else
+					counts[i].ahead++;
+			}
+		}
+
+		for (p = c->parents; p; p = p->next) {
+			struct bitmap *bitmap_p;
+
+			repo_parse_commit(r, p->item);
+
+			bitmap_p = get_bit_array(p->item, width);
+			bitmap_or(bitmap_p, bitmap_c);
+
+			/*
+			 * If this parent is reachable from every starting
+			 * commit, then none of its ancestors can contribute
+			 * to the ahead/behind count. Mark it as STALE, so
+			 * we can stop the walk when every commit in the
+			 * queue is STALE.
+			 */
+			if (bitmap_popcount(bitmap_p) == commits_nr)
+				p->item->object.flags |= STALE;
+
+			insert_no_dup(&queue, p->item);
+		}
+
+		free_bit_array(c);
+	}
+
+	/* STALE is used here, PARENT2 is used by insert_no_dup(). */
+	repo_clear_commit_marks(r, PARENT2 | STALE);
+	clear_bit_arrays(&bit_arrays);
+	clear_prio_queue(&queue);
+}
+
+struct commit_and_index {
+	struct commit *commit;
+	unsigned int index;
+	timestamp_t generation;
+};
+
+static int compare_commit_and_index_by_generation(const void *va, const void *vb)
+{
+	const struct commit_and_index *a = (const struct commit_and_index *)va;
+	const struct commit_and_index *b = (const struct commit_and_index *)vb;
+
+	if (a->generation > b->generation)
+		return 1;
+	if (a->generation < b->generation)
+		return -1;
+	return 0;
+}
+
+void tips_reachable_from_bases(struct repository *r,
+			       struct commit_list *bases,
+			       struct commit **tips, size_t tips_nr,
+			       int mark)
+{
+	struct commit_and_index *commits;
+	size_t min_generation_index = 0;
+	timestamp_t min_generation;
+	struct commit_list *stack = NULL;
+
+	if (!bases || !tips || !tips_nr)
+		return;
+
+	/*
+	 * Do a depth-first search starting at 'bases' to search for the
+	 * tips. Stop at the lowest (un-found) generation number. When
+	 * finding the lowest commit, increase the minimum generation
+	 * number to the next lowest (un-found) generation number.
+	 */
+
+	CALLOC_ARRAY(commits, tips_nr);
+
+	for (size_t i = 0; i < tips_nr; i++) {
+		commits[i].commit = tips[i];
+		commits[i].index = i;
+		commits[i].generation = commit_graph_generation(tips[i]);
+	}
+
+	/* Sort with generation number ascending. */
+	QSORT(commits, tips_nr, compare_commit_and_index_by_generation);
+	min_generation = commits[0].generation;
+
+	while (bases) {
+		repo_parse_commit(r, bases->item);
+		commit_list_insert(bases->item, &stack);
+		bases = bases->next;
+	}
+
+	while (stack) {
+		int explored_all_parents = 1;
+		struct commit_list *p;
+		struct commit *c = stack->item;
+		timestamp_t c_gen = commit_graph_generation(c);
+
+		/* Does it match any of our tips? */
+		for (size_t j = min_generation_index; j < tips_nr; j++) {
+			if (c_gen < commits[j].generation)
+				break;
+
+			if (commits[j].commit == c) {
+				tips[commits[j].index]->object.flags |= mark;
+
+				if (j == min_generation_index) {
+					unsigned int k = j + 1;
+					while (k < tips_nr &&
+					       (tips[commits[k].index]->object.flags & mark))
+						k++;
+
+					/* Terminate early if all found. */
+					if (k >= tips_nr)
+						goto done;
+
+					min_generation_index = k;
+					min_generation = commits[k].generation;
+				}
+			}
+		}
+
+		for (p = c->parents; p; p = p->next) {
+			repo_parse_commit(r, p->item);
+
+			/* Have we already explored this parent? */
+			if (p->item->object.flags & SEEN)
+				continue;
+
+			/* Is it below the current minimum generation? */
+			if (commit_graph_generation(p->item) < min_generation)
+				continue;
+
+			/* Ok, we will explore from here on. */
+			p->item->object.flags |= SEEN;
+			explored_all_parents = 0;
+			commit_list_insert(p->item, &stack);
+			break;
+		}
+
+		if (explored_all_parents)
+			pop_commit(&stack);
+	}
+
+done:
+	free(commits);
+	repo_clear_commit_marks(r, SEEN);
+}

commit-reach.h

@@ -104,4 +104,44 @@ struct commit_list *get_reachable_subset(struct commit **from, int nr_from,
 					 struct commit **to, int nr_to,
 					 unsigned int reachable_flag);
 
+struct ahead_behind_count {
+	/**
+	 * As input, the *_index members indicate which positions in
+	 * the 'tips' array correspond to the tip and base of this
+	 * comparison.
+	 */
+	size_t tip_index;
+	size_t base_index;
+
+	/**
+	 * These values store the computed counts for each side of the
+	 * symmetric difference:
+	 *
+	 * 'ahead' stores the number of commits reachable from the tip
+	 * and not reachable from the base.
+	 *
+	 * 'behind' stores the number of commits reachable from the base
+	 * and not reachable from the tip.
+	 */
+	unsigned int ahead;
+	unsigned int behind;
+};
+
+/*
+ * Given an array of commits and an array of ahead_behind_count pairs,
+ * compute the ahead/behind counts for each pair.
+ */
+void ahead_behind(struct repository *r,
+		  struct commit **commits, size_t commits_nr,
+		  struct ahead_behind_count *counts, size_t counts_nr);
+
+/*
+ * For all tip commits, add 'mark' to their flags if and only if they
+ * are reachable from one of the commits in 'bases'.
+ */
+void tips_reachable_from_bases(struct repository *r,
+			       struct commit_list *bases,
+			       struct commit **tips, size_t tips_nr,
+			       int mark);
+
 #endif

ref-filter.c

@@ -158,6 +158,7 @@ enum atom_type {
 	ATOM_THEN,
 	ATOM_ELSE,
 	ATOM_REST,
+	ATOM_AHEADBEHIND,
 };
 
 /*
@@ -600,6 +601,22 @@ static int rest_atom_parser(struct ref_format *format,
 	return 0;
 }
 
+static int ahead_behind_atom_parser(struct ref_format *format, struct used_atom *atom,
+				    const char *arg, struct strbuf *err)
+{
+	struct string_list_item *item;
+
+	if (!arg)
+		return strbuf_addf_ret(err, -1, _("expected format: %%(ahead-behind:<committish>)"));
+
+	item = string_list_append(&format->bases, arg);
+	item->util = lookup_commit_reference_by_name(arg);
+	if (!item->util)
+		die("failed to find '%s'", arg);
+
+	return 0;
+}
+
 static int head_atom_parser(struct ref_format *format UNUSED,
 			    struct used_atom *atom,
 			    const char *arg, struct strbuf *err)
@@ -660,6 +677,7 @@ static struct {
 	[ATOM_THEN] = { "then", SOURCE_NONE },
 	[ATOM_ELSE] = { "else", SOURCE_NONE },
 	[ATOM_REST] = { "rest", SOURCE_NONE, FIELD_STR, rest_atom_parser },
+	[ATOM_AHEADBEHIND] = { "ahead-behind", SOURCE_OTHER, FIELD_STR, ahead_behind_atom_parser },
 	/*
 	 * Please update $__git_ref_fieldlist in git-completion.bash
 	 * when you add new atoms
@@ -1866,6 +1884,7 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 	struct object *obj;
 	int i;
 	struct object_info empty = OBJECT_INFO_INIT;
+	int ahead_behind_atoms = 0;
 
 	CALLOC_ARRAY(ref->value, used_atom_cnt);
 
@@ -1996,6 +2015,16 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			else
 				v->s = xstrdup("");
 			continue;
+		} else if (atom_type == ATOM_AHEADBEHIND) {
+			if (ref->counts) {
+				const struct ahead_behind_count *count;
+				count = ref->counts[ahead_behind_atoms++];
+				v->s = xstrfmt("%d %d", count->ahead, count->behind);
+			} else {
+				/* Not a commit. */
+				v->s = xstrdup("");
+			}
+			continue;
 		} else
 			continue;
 
@@ -2346,6 +2375,7 @@ static void free_array_item(struct ref_array_item *item)
 			free((char *)item->value[i].s);
 		free(item->value);
 	}
+	free(item->counts);
 	free(item);
 }
 
@@ -2374,6 +2404,8 @@ void ref_array_clear(struct ref_array *array)
 		free_worktrees(ref_to_worktree_map.worktrees);
 		ref_to_worktree_map.worktrees = NULL;
 	}
+
+	FREE_AND_NULL(array->counts);
 }
 
 #define EXCLUDE_REACHED 0
@@ -2382,33 +2414,22 @@ static void reach_filter(struct ref_array *array,
 			 struct commit_list *check_reachable,
 			 int include_reached)
 {
-	struct rev_info revs;
 	int i, old_nr;
 	struct commit **to_clear;
-	struct commit_list *cr;
 
 	if (!check_reachable)
 		return;
 
 	CALLOC_ARRAY(to_clear, array->nr);
-
-	repo_init_revisions(the_repository, &revs, NULL);
-
 	for (i = 0; i < array->nr; i++) {
 		struct ref_array_item *item = array->items[i];
-		add_pending_object(&revs, &item->commit->object, item->refname);
 		to_clear[i] = item->commit;
 	}
 
-	for (cr = check_reachable; cr; cr = cr->next) {
-		struct commit *merge_commit = cr->item;
-		merge_commit->object.flags |= UNINTERESTING;
-		add_pending_object(&revs, &merge_commit->object, "");
-	}
-
-	revs.limited = 1;
-	if (prepare_revision_walk(&revs))
-		die(_("revision walk setup failed"));
+	tips_reachable_from_bases(the_repository,
+				  check_reachable,
+				  to_clear, array->nr,
+				  UNINTERESTING);
 
 	old_nr = array->nr;
 	array->nr = 0;
@@ -2432,10 +2453,50 @@ static void reach_filter(struct ref_array *array,
 		clear_commit_marks(merge_commit, ALL_REV_FLAGS);
 	}
 
-	release_revisions(&revs);
 	free(to_clear);
 }
 
+void filter_ahead_behind(struct repository *r,
+			 struct ref_format *format,
+			 struct ref_array *array)
+{
+	struct commit **commits;
+	size_t commits_nr = format->bases.nr + array->nr;
+
+	if (!format->bases.nr || !array->nr)
+		return;
+
+	ALLOC_ARRAY(commits, commits_nr);
+	for (size_t i = 0; i < format->bases.nr; i++)
+		commits[i] = format->bases.items[i].util;
+
+	ALLOC_ARRAY(array->counts, st_mult(format->bases.nr, array->nr));
+
+	commits_nr = format->bases.nr;
+	array->counts_nr = 0;
+	for (size_t i = 0; i < array->nr; i++) {
+		const char *name = array->items[i]->refname;
+		commits[commits_nr] = lookup_commit_reference_by_name(name);
+
+		if (!commits[commits_nr])
+			continue;
+
+		CALLOC_ARRAY(array->items[i]->counts, format->bases.nr);
+		for (size_t j = 0; j < format->bases.nr; j++) {
+			struct ahead_behind_count *count;
+			count = &array->counts[array->counts_nr++];
+			count->tip_index = commits_nr;
+			count->base_index = j;
+
+			array->items[i]->counts[j] = count;
+		}
+		commits_nr++;
+	}
+
+	ahead_behind(r, commits, commits_nr, array->counts, array->counts_nr);
+	free(commits);
+}
+
 /*
  * API for filtering a set of refs. Based on the type of refs the user
  * has requested, we iterate through those refs and apply filters

ref-filter.h

@@ -4,6 +4,7 @@
 #include "oid-array.h"
 #include "refs.h"
 #include "commit.h"
+#include "string-list.h"
 
 /* Quoting styles */
 #define QUOTE_NONE 0
@@ -23,6 +24,7 @@
 
 struct atom_value;
 struct ref_sorting;
+struct ahead_behind_count;
 struct option;
 
 enum ref_sorting_order {
@@ -40,6 +42,8 @@ struct ref_array_item {
 	const char *symref;
 	struct commit *commit;
 	struct atom_value *value;
+	struct ahead_behind_count **counts;
+
 	char refname[FLEX_ARRAY];
 };
 
@@ -47,6 +51,9 @@ struct ref_array {
 	int nr, alloc;
 	struct ref_array_item **items;
 	struct rev_info *revs;
+
+	struct ahead_behind_count *counts;
+	size_t counts_nr;
 };
 
 struct ref_filter {
@@ -80,9 +87,15 @@ struct ref_format {
 
 	/* Internal state to ref-filter */
 	int need_color_reset_at_eol;
+
+	/* List of bases for ahead-behind counts. */
+	struct string_list bases;
 };
 
-#define REF_FORMAT_INIT { .use_color = -1 }
+#define REF_FORMAT_INIT {             \
+	.use_color = -1,              \
+	.bases = STRING_LIST_INIT_DUP, \
+}
 
 /*  Macros for checking --merged and --no-merged options */
 #define _OPT_MERGED_NO_MERGED(option, filter, h) \
@@ -143,4 +156,15 @@ struct ref_array_item *ref_array_push(struct ref_array *array,
 				      const char *refname,
 				      const struct object_id *oid);
 
+/*
+ * If the provided format includes ahead-behind atoms, then compute the
+ * ahead-behind values for the array of filtered references. Must be
+ * called after filter_refs() but before outputting the formatted refs.
+ *
+ * If this is not called, then any ahead-behind atoms will be blank.
+ */
+void filter_ahead_behind(struct repository *r,
+			 struct ref_format *format,
+			 struct ref_array *array);
+
 #endif /*  REF_FILTER_H  */

t/perf/p1500-graph-walks.sh

@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='Commit walk performance tests'
+. ./perf-lib.sh
+
+test_perf_large_repo
+
+test_expect_success 'setup' '
+	git for-each-ref --format="%(refname)" "refs/heads/*" "refs/tags/*" >allrefs &&
+	sort -r allrefs | head -n 50 >refs &&
+	for ref in $(cat refs)
+	do
+		git branch -f ref-$ref $ref &&
+		echo ref-$ref ||
+		return 1
+	done >branches &&
+	for ref in $(cat refs)
+	do
+		git tag -f tag-$ref $ref &&
+		echo tag-$ref ||
+		return 1
+	done >tags &&
+	git commit-graph write --reachable
+'
+
+test_perf 'ahead-behind counts: git for-each-ref' '
+	git for-each-ref --format="%(ahead-behind:HEAD)" --stdin <refs
+'
+
+test_perf 'ahead-behind counts: git branch' '
+	xargs git branch -l --format="%(ahead-behind:HEAD)" <branches
+'
+
+test_perf 'ahead-behind counts: git tag' '
+	xargs git tag -l --format="%(ahead-behind:HEAD)" <tags
+'
+
+test_perf 'contains: git for-each-ref --merged' '
+	git for-each-ref --merged=HEAD --stdin <refs
+'
+
+test_perf 'contains: git branch --merged' '
+	xargs git branch --merged=HEAD <branches
+'
+
+test_perf 'contains: git tag --merged' '
+	xargs git tag --merged=HEAD <tags
+'
+
+test_done

t/t3203-branch-output.sh

@@ -337,6 +337,20 @@ test_expect_success 'git branch --format option' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git branch --format with ahead-behind' '
+	cat >expect <<-\EOF &&
+	(HEAD detached from fromtag) 0 0
+	refs/heads/ambiguous 0 0
+	refs/heads/branch-one 1 0
+	refs/heads/branch-two 0 0
+	refs/heads/main 1 0
+	refs/heads/ref-to-branch 1 0
+	refs/heads/ref-to-remote 1 0
+	EOF
+	git branch --format="%(refname) %(ahead-behind:HEAD)" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git branch with --format=%(rest) must fail' '
 	test_must_fail git branch --format="%(rest)" >actual
 '

t/t5318-commit-graph.sh

@@ -630,7 +630,7 @@ test_expect_success 'detect incorrect generation number' '
 
 test_expect_success 'detect incorrect generation number' '
 	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\01" \
-		"non-zero generation number"
+		"commit-graph generation for commit"
 '
 
 test_expect_success 'detect incorrect commit date' '

t/t6300-for-each-ref.sh

@@ -1464,4 +1464,54 @@ sig_crlf="$(printf "%s" "$sig" | append_cr; echo dummy)"
 sig_crlf=${sig_crlf%dummy}
 test_atom refs/tags/fake-sig-crlf contents:signature "$sig_crlf"
 
+test_expect_success 'git for-each-ref --stdin: empty' '
+	>in &&
+	git for-each-ref --format="%(refname)" --stdin <in >actual &&
+	git for-each-ref --format="%(refname)" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git for-each-ref --stdin: fails if extra args' '
+	>in &&
+	test_must_fail git for-each-ref --format="%(refname)" \
+		--stdin refs/heads/extra <in 2>err &&
+	grep "unknown arguments supplied with --stdin" err
+'
+
+test_expect_success 'git for-each-ref --stdin: matches' '
+	cat >in <<-EOF &&
+	refs/tags/multi*
+	refs/heads/amb*
+	EOF
+
+	cat >expect <<-EOF &&
+	refs/heads/ambiguous
+	refs/tags/multi-ref1-100000-user1
+	refs/tags/multi-ref1-100000-user2
+	refs/tags/multi-ref1-200000-user1
+	refs/tags/multi-ref1-200000-user2
+	refs/tags/multi-ref2-100000-user1
+	refs/tags/multi-ref2-100000-user2
+	refs/tags/multi-ref2-200000-user1
+	refs/tags/multi-ref2-200000-user2
+	refs/tags/multiline
+	EOF
+
+	git for-each-ref --format="%(refname)" --stdin <in >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git for-each-ref with non-existing refs' '
+	cat >in <<-EOF &&
+	refs/heads/this-ref-does-not-exist
+	refs/tags/bogus
+	EOF
+
+	git for-each-ref --format="%(refname)" --stdin <in >actual &&
+	test_must_be_empty actual &&
+
+	xargs git for-each-ref --format="%(refname)" <in >actual &&
+	test_must_be_empty actual
+'
+
 test_done

t/t6301-for-each-ref-errors.sh

@@ -54,4 +54,18 @@ test_expect_success 'Missing objects are reported correctly' '
 	test_must_be_empty brief-err
 '
 
+test_expect_success 'ahead-behind requires an argument' '
+	test_must_fail git for-each-ref \
+		--format="%(ahead-behind)" 2>err &&
+	echo "fatal: expected format: %(ahead-behind:<committish>)" >expect &&
+	test_cmp expect err
+'
+
+test_expect_success 'missing ahead-behind base' '
+	test_must_fail git for-each-ref \
+		--format="%(ahead-behind:refs/heads/missing)" 2>err &&
+	echo "fatal: failed to find '\''refs/heads/missing'\''" >expect &&
+	test_cmp expect err
+'
+
 test_done

t/t6600-test-reach.sh

@@ -443,4 +443,173 @@ test_expect_success 'get_reachable_subset:none' '
 	test_all_modes get_reachable_subset
 '
 
+test_expect_success 'for-each-ref ahead-behind:linear' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-1-3
+	refs/heads/commit-1-5
+	refs/heads/commit-1-8
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1 0 8
+	refs/heads/commit-1-3 0 6
+	refs/heads/commit-1-5 0 4
+	refs/heads/commit-1-8 0 1
+	EOF
+	run_all_modes git for-each-ref \
+		--format="%(refname) %(ahead-behind:commit-1-9)" --stdin
+'
+
+test_expect_success 'for-each-ref ahead-behind:all' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-4
+	refs/heads/commit-4-2
+	refs/heads/commit-4-4
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1 0 24
+	refs/heads/commit-2-4 0 17
+	refs/heads/commit-4-2 0 17
+	refs/heads/commit-4-4 0 9
+	EOF
+	run_all_modes git for-each-ref \
+		--format="%(refname) %(ahead-behind:commit-5-5)" --stdin
+'
+
+test_expect_success 'for-each-ref ahead-behind:some' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-5-3
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1 0 53
+	refs/heads/commit-4-8 8 30
+	refs/heads/commit-5-3 0 39
+	refs/heads/commit-9-9 27 0
+	EOF
+	run_all_modes git for-each-ref \
+		--format="%(refname) %(ahead-behind:commit-9-6)" --stdin
+'
+
+test_expect_success 'for-each-ref ahead-behind:some, multibase' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-5-3
+	refs/heads/commit-7-8
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1 0 53 0 53
+	refs/heads/commit-4-8 8 30 0 22
+	refs/heads/commit-5-3 0 39 0 39
+	refs/heads/commit-7-8 14 12 8 6
+	refs/heads/commit-9-9 27 0 27 0
+	EOF
+	run_all_modes git for-each-ref \
+		--format="%(refname) %(ahead-behind:commit-9-6) %(ahead-behind:commit-6-9)" \
+		--stdin
+'
+
+test_expect_success 'for-each-ref ahead-behind:none' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-7-5
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-4-8 16 16
+	refs/heads/commit-7-5 7 4
+	refs/heads/commit-9-9 49 0
+	EOF
+	run_all_modes git for-each-ref \
+		--format="%(refname) %(ahead-behind:commit-8-4)" --stdin
+'
+
+test_expect_success 'for-each-ref merged:linear' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-1-3
+	refs/heads/commit-1-5
+	refs/heads/commit-1-8
+	refs/heads/commit-2-1
+	refs/heads/commit-5-1
+	refs/heads/commit-9-1
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-1-3
+	refs/heads/commit-1-5
+	refs/heads/commit-1-8
+	EOF
+	run_all_modes git for-each-ref --merged=commit-1-9 \
+		--format="%(refname)" --stdin
+'
+
+test_expect_success 'for-each-ref merged:all' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-4
+	refs/heads/commit-4-2
+	refs/heads/commit-4-4
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-4
+	refs/heads/commit-4-2
+	refs/heads/commit-4-4
+	EOF
+	run_all_modes git for-each-ref --merged=commit-5-5 \
+		--format="%(refname)" --stdin
+'
+
+test_expect_success 'for-each-ref ahead-behind:some' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-5-3
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-5-3
+	EOF
+	run_all_modes git for-each-ref --merged=commit-9-6 \
+		--format="%(refname)" --stdin
+'
+
+test_expect_success 'for-each-ref merged:some, multibase' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-5-3
+	refs/heads/commit-7-8
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-4-8
+	refs/heads/commit-5-3
+	EOF
+	run_all_modes git for-each-ref \
+		--merged=commit-5-8 \
+		--merged=commit-8-5 \
+		--format="%(refname)" \
+		--stdin
+'
+
+test_expect_success 'for-each-ref merged:none' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-7-5
+	refs/heads/commit-4-8
+	refs/heads/commit-9-9
+	EOF
+	>expect &&
+	run_all_modes git for-each-ref --merged=commit-8-4 \
+		--format="%(refname)" --stdin
+'
+
 test_done

t/t7004-tag.sh

@@ -792,6 +792,34 @@ test_expect_success 'annotations for blobs are empty' '
 	test_cmp expect actual
 '
 
+# Run this before doing any signing, so the test has the same results
+# regardless of the GPG prereq.
+test_expect_success 'git tag --format with ahead-behind' '
+	test_when_finished git reset --hard tag-one-line &&
+	git commit --allow-empty -m "left" &&
+	git tag -a -m left tag-left &&
+	git reset --hard HEAD~1 &&
+	git commit --allow-empty -m "right" &&
+	git tag -a -m left tag-right &&
+
+	# Use " !" at the end to demonstrate whitespace
+	# around empty ahead-behind token for tag-blob.
+	cat >expect <<-EOF &&
+	refs/tags/tag-blob  !
+	refs/tags/tag-left 1 1 !
+	refs/tags/tag-lines 0 1 !
+	refs/tags/tag-one-line 0 1 !
+	refs/tags/tag-right 0 0 !
+	refs/tags/tag-zero-lines 0 1 !
+	EOF
+	git tag -l --format="%(refname) %(ahead-behind:HEAD) !" >actual 2>err &&
+	grep "refs/tags/tag" actual >actual.focus &&
+	test_cmp expect actual.focus &&
+
+	# Error reported for tags that point to non-commits.
+	grep "error: object [0-9a-f]* is a blob, not a commit" err
+'
+
 # trying to verify annotated non-signed tags:
 
 test_expect_success GPG \