show-branch: use prio_queue

Building a list using commit_list_insert_by_date() has quadratic worst
case complexity.  Avoid it by using prio_queue.

Use prio_queue_peek()+prio_queue_replace() instead of prio_queue_get()+
prio_queue_put() if possible, as the former only rebalance the
prio_queue heap once instead of twice.

In sane repositories this won't make much of a difference because the
number of items in the list or queue won't be very high:

Benchmark 1: ./git_v2.52.0 show-branch origin/main origin/next origin/seen origin/todo
  Time (mean ± σ):     538.2 ms ±   0.8 ms    [User: 527.6 ms, System: 9.6 ms]
  Range (min … max):   537.0 ms … 539.2 ms    10 runs

Benchmark 2: ./git show-branch origin/main origin/next origin/seen origin/todo
  Time (mean ± σ):     530.6 ms ±   0.4 ms    [User: 519.8 ms, System: 9.8 ms]
  Range (min … max):   530.1 ms … 531.3 ms    10 runs

Summary
  ./git show-branch origin/main origin/next origin/seen origin/todo ran
    1.01 ± 0.00 times faster than ./git_v2.52.0 show-branch origin/main origin/next origin/seen origin/todo

That number is not limited, though, and in pathological cases like the
one in p6010 we see a sizable improvement:

Test                      v2.52.0           HEAD
------------------------------------------------------------------
6010.4: git show-branch   2.19(2.19+0.00)   0.03(0.02+0.00) -98.6%

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/show-branch.c

@@ -18,6 +18,7 @@
 #include "commit-slab.h"
 #include "date.h"
 #include "wildmatch.h"
+#include "prio-queue.h"
 
 static const char*const show_branch_usage[] = {
     N_("git show-branch [-a | --all] [-r | --remotes] [--topo-order | --date-order]\n"
@@ -59,11 +60,10 @@ static const char *get_color_reset_code(void)
 	return "";
 }
 
-static struct commit *interesting(struct commit_list *list)
+static struct commit *interesting(struct prio_queue *queue)
 {
-	while (list) {
-		struct commit *commit = list->item;
-		list = list->next;
+	for (size_t i = 0; i < queue->nr; i++) {
+		struct commit *commit = queue->array[i].data;
 		if (commit->object.flags & UNINTERESTING)
 			continue;
 		return commit;
@@ -222,17 +222,18 @@ static int mark_seen(struct commit *commit, struct commit_list **seen_p)
 	return 0;
 }
 
-static void join_revs(struct commit_list **list_p,
+static void join_revs(struct prio_queue *queue,
 		      struct commit_list **seen_p,
 		      int num_rev, int extra)
 {
 	int all_mask = ((1u << (REV_SHIFT + num_rev)) - 1);
 	int all_revs = all_mask & ~((1u << REV_SHIFT) - 1);
 
-	while (*list_p) {
+	while (queue->nr) {
 		struct commit_list *parents;
-		int still_interesting = !!interesting(*list_p);
-		struct commit *commit = pop_commit(list_p);
+		int still_interesting = !!interesting(queue);
+		struct commit *commit = prio_queue_peek(queue);
+		bool get_pending = true;
 		int flags = commit->object.flags & all_mask;
 
 		if (!still_interesting && extra <= 0)
@@ -253,8 +254,14 @@ static void join_revs(struct commit_list **list_p,
 			if (mark_seen(p, seen_p) && !still_interesting)
 				extra--;
 			p->object.flags |= flags;
-			commit_list_insert_by_date(p, list_p);
+			if (get_pending)
+				prio_queue_replace(queue, p);
+			else
+				prio_queue_put(queue, p);
+			get_pending = false;
 		}
+		if (get_pending)
+			prio_queue_get(queue);
 	}
 
 	/*
@@ -642,7 +649,8 @@ int cmd_show_branch(int ac,
 {
 	struct commit *rev[MAX_REVS], *commit;
 	char *reflog_msg[MAX_REVS] = {0};
-	struct commit_list *list = NULL, *seen = NULL;
+	struct commit_list *seen = NULL;
+	struct prio_queue queue = { compare_commits_by_commit_date };
 	unsigned int rev_mask[MAX_REVS];
 	int num_rev, i, extra = 0;
 	int all_heads = 0, all_remotes = 0;
@@ -886,14 +894,14 @@ int cmd_show_branch(int ac,
 		 */
 		commit->object.flags |= flag;
 		if (commit->object.flags == flag)
-			commit_list_insert_by_date(commit, &list);
+			prio_queue_put(&queue, commit);
 		rev[num_rev] = commit;
 	}
 	for (i = 0; i < num_rev; i++)
 		rev_mask[i] = rev[i]->object.flags;
 
 	if (0 <= extra)
-		join_revs(&list, &seen, num_rev, extra);
+		join_revs(&queue, &seen, num_rev, extra);
 
 	commit_list_sort_by_date(&seen);
 
@@ -1004,7 +1012,7 @@ out:
 	for (size_t i = 0; i < ARRAY_SIZE(reflog_msg); i++)
 		free(reflog_msg[i]);
 	free_commit_list(seen);
-	free_commit_list(list);
+	clear_prio_queue(&queue);
 	free(args_copy);
 	free(head);
 	return ret;

t/perf/p6010-merge-base.sh

@@ -83,9 +83,9 @@ build_history2 () {
 test_expect_success 'setup' '
 	max_level=15 &&
 	build_history $max_level | git fast-import --export-marks=marks &&
-	git tag one &&
+	git branch one &&
 	build_history2 $max_level | git fast-import --import-marks=marks --force &&
-	git tag two &&
+	git branch two &&
 	git gc &&
 	git log --format=%H --no-merges >expect
 '
@@ -98,4 +98,8 @@ test_expect_success 'verify result' '
 	test_cmp expect actual
 '
 
+test_perf 'git show-branch' '
+	git show-branch one two
+'
+
 test_done