[PATCH v5] revision.c: implement --max-count-oldest

[Mirko Faina <mroik@delayed.space>]

--max-count is a commit limiting option sets a maximum amount of commits
to be shown. If a user wants to see only the first N commits of the
history (the oldest commits) they'd have to combine --max-count with
--skip. This is not very user-friendly.

Teach get_revision() the --max-count-oldest option.

Signed-off-by: Mirko Faina <mroik@delayed.space>
---
 Documentation/rev-list-options.adoc |  3 ++
 revision.c                          | 77 +++++++++++++++++++++++++++--
 revision.h                          |  2 +
 t/t4202-log.sh                      | 14 ++++++
 4 files changed, 93 insertions(+), 3 deletions(-)

diff --git a/Documentation/rev-list-options.adoc b/Documentation/rev-list-options.adoc
index 2d195a1474..736f34efab 100644
--- a/Documentation/rev-list-options.adoc
+++ b/Documentation/rev-list-options.adoc
@@ -18,6 +18,9 @@ ordering and formatting options, such as `--reverse`.
 `--max-count=<number>`::
 	Limit the output to _<number>_ commits.
 
+`--max-count-oldest=<number>`::
+	Limit the output to the _<number>_ oldest commits.
+
 `--skip=<number>`::
 	Skip _<number>_ commits before starting to show the commit output.
 
diff --git a/revision.c b/revision.c
index 599b3a66c3..3aaa77ced5 100644
--- a/revision.c
+++ b/revision.c
@@ -2339,10 +2339,24 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	}
 
 	if ((argcount = parse_long_opt("max-count", argv, &optarg))) {
+		if (revs->max_count_type == 1)
+			die(_("can't use --max-count with --max-count-oldest"));
 		revs->max_count = parse_count(optarg);
 		revs->no_walk = 0;
+		revs->max_count_type = 0;
 		return argcount;
+	} else if ((argcount = parse_long_opt("max-count-oldest", argv, &optarg))) {
+		if (revs->max_count_type == 0 && revs->max_count != -1)
+			die(_("can't use --max-count with --max-count-oldest"));
+		if (revs->skip_count > 0)
+			die(_("con't use --max-count-oldest with --skip"));
+		revs->max_count = parse_count(optarg);
+		revs->no_walk = 0;
+		revs->max_count_type = 1;
+		revs->max_count_stage = 0;
 	} else if ((argcount = parse_long_opt("skip", argv, &optarg))) {
+		if (revs->max_count_type == 1)
+			die(_("con't use --max-count-oldest with --skip"));
 		revs->skip_count = parse_count(optarg);
 		return argcount;
 	} else if ((*arg == '-') && isdigit(arg[1])) {
@@ -4521,15 +4535,68 @@ static struct commit *get_revision_internal(struct rev_info *revs)
 	return c;
 }
 
+static void retrieve_oldest_commits(struct rev_info *revs,
+				    struct commit_list **queue)
+{
+	struct commit *c;
+	int max_count = revs->max_count;
+	int queuei_count = 0;
+	int queueo_count = 0;
+	struct commit_list *queueo = NULL;
+	struct commit_list *queuei = NULL;
+	struct commit_list *reversed_queue = NULL;
+
+	revs->max_count = -1;
+	while ((c = get_revision_internal(revs))) {
+		c->object.flags &= ~SHOWN;
+		commit_list_insert(c, &queuei);
+		queuei_count++;
+		while (queuei_count + queueo_count > max_count) {
+			if (!queueo_count) {
+				while (queuei_count > 0) {
+					c = pop_commit(&queuei);
+					queuei_count--;
+					commit_list_insert(c, &queueo);
+					queueo_count++;
+				}
+			}
+			pop_commit(&queueo);
+			queueo_count--;
+		}
+	}
+
+	while ((c = pop_commit(&queueo)))
+		commit_list_insert(c, &reversed_queue);
+	while ((c = pop_commit(&queuei)))
+		commit_list_insert(c, &queueo);
+	while ((c = pop_commit(&queueo)))
+		commit_list_insert(c, &reversed_queue);
+
+	while ((c = pop_commit(&reversed_queue)))
+		commit_list_insert(c, queue);
+}
+
 struct commit *get_revision(struct rev_info *revs)
 {
 	struct commit *c;
 	struct commit_list *reversed;
+	struct commit_list *queue = NULL;
+
+	if (revs->max_count_type == 1 && !revs->max_count_stage) {
+		retrieve_oldest_commits(revs, &queue);
+		commit_list_free(revs->commits);
+		revs->commits = queue;
+		revs->max_count_stage = 1;
+	}
 
 	if (revs->reverse) {
 		reversed = NULL;
-		while ((c = get_revision_internal(revs)))
-			commit_list_insert(c, &reversed);
+		if (revs->max_count_type == 1)
+			while ((c = pop_commit(&revs->commits)))
+				commit_list_insert(c, &reversed);
+		else
+			while ((c = get_revision_internal(revs)))
+				commit_list_insert(c, &reversed);
 		commit_list_free(revs->commits);
 		revs->commits = reversed;
 		revs->reverse = 0;
@@ -4543,7 +4610,11 @@ struct commit *get_revision(struct rev_info *revs)
 		return c;
 	}
 
-	c = get_revision_internal(revs);
+	if (revs->max_count_stage)
+		c = pop_commit(&revs->commits);
+	else
+		c = get_revision_internal(revs);
+
 	if (c && revs->graph)
 		graph_update(revs->graph, c);
 	if (!c) {
diff --git a/revision.h b/revision.h
index 584f1338b5..e157463cb1 100644
--- a/revision.h
+++ b/revision.h
@@ -309,6 +309,8 @@ struct rev_info {
 	/* special limits */
 	int skip_count;
 	int max_count;
+	unsigned int max_count_type:1;
+	unsigned int max_count_stage:1;
 	timestamp_t max_age;
 	timestamp_t max_age_as_filter;
 	timestamp_t min_age;
diff --git a/t/t4202-log.sh b/t/t4202-log.sh
index 05cee9e41b..668c231cf1 100755
--- a/t/t4202-log.sh
+++ b/t/t4202-log.sh
@@ -1882,6 +1882,20 @@ test_expect_success 'log --graph with --name-status' '
 	test_cmp_graph --name-status tangle..reach
 '
 
+test_expect_success 'log --max-count-oldest=3 --oneline' '
+	test_when_finished rm expect &&
+	git log --oneline | tail -n3 >expect &&
+	git log --oneline --max-count-oldest=3 >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --max-count-oldest=3 --reverse --oneline' '
+	test_when_finished rm expect &&
+	git log --oneline | tail -n3 | tac >expect &&
+	git log --oneline --max-count-oldest=3 --reverse >actual &&
+	test_cmp expect actual
+'
+
 cat >expect <<-\EOF
 * reach
 |
-- 
2.54.0