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

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);
+}