revision.c: implement --max-count-oldest

"--max-count" is a commit limiting option and 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 do
something like

    git log $(git rev-list HEAD | tail -n N | head -n 1)

This is not very user-friendly.

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

Signed-off-by: Mirko Faina <mroik@delayed.space>
[jc: fixed up t4202 <xmqq7boy4o05.fsf@gitster.g>]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.adoc

@@ -16,7 +16,10 @@ ordering and formatting options, such as `--reverse`.
 `-<number>`::
 `-n <number>`::
 `--max-count=<number>`::
-	Limit the output to _<number>_ commits.
+	Limit the output to the first _<number>_ commits that would be shown.
+
+`--max-count-oldest=<number>`::
+	Limit the output to the last _<number>_ commits that would be shown.
 
 `--skip=<number>`::
 	Skip _<number>_ commits before starting to show the commit output.

revision.c

@@ -2339,10 +2339,28 @@ 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_for_incompatible_opt2(1, "--max-count", 1,
+						  "--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_for_incompatible_opt2(1, "--max-count", 1,
+						  "--max-count-oldest");
+		if (revs->skip_count > 0)
+			die_for_incompatible_opt2(1, "--skip", 1,
+						  "--max-count-oldest");
+		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_for_incompatible_opt2(1, "--skip", 1,
+						  "--max-count-oldest");
 		revs->skip_count = parse_count(optarg);
 		return argcount;
 	} else if ((*arg == '-') && isdigit(arg[1])) {
@@ -4521,15 +4539,91 @@ 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;
+	struct commit_list *p;
+
+	revs->max_count = -1;
+	while ((c = get_revision_internal(revs))) {
+		/*
+		 * We need to reset SHOWN status otherwise --graph breaks.
+		 * It is fine to do, get_revision_internal() doesn't consider
+		 * children commits as they have been already processed and the
+		 * traversal happens only child to parent.
+		 *
+		 * We do this because the --graph machinery relies on the status
+		 * of the parents to decide how the printing will happen.
+		 *
+		 * We can't simply replace this instruction with a
+		 * graph_update() as it doesn't do the actualy printing, we'd
+		 * have to remove any commit that goes over the
+		 * --max-count-oldest limit from revs->graph.
+		 */
+		c->object.flags &= ~(SHOWN | CHILD_SHOWN);
+		commit_list_insert(c, &queuei);
+		if (!(c->object.flags & BOUNDARY))
+			queuei_count++;
+		while (queuei_count + queueo_count > max_count) {
+			if (!queueo_count) {
+				while ((c = pop_commit(&queuei))) {
+					commit_list_insert(c, &queueo);
+					queueo_count++;
+				}
+				queuei_count = 0;
+			}
+			c = pop_commit(&queueo);
+			queueo_count--;
+			/* We need to do this otherwise we'll discard the
+			 * commits that go over the --max-count-oldest limit but
+			 * not their respective boundaries. This matters only if
+			 * we're discarding the commit right before the boundary.
+			 */
+			for (p = c->parents; p; p = p->next)
+				p->item->object.flags &= ~CHILD_SHOWN;
+		}
+	}
+
+	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;
+	struct commit_list *p;
+
+	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 +4637,18 @@ 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);
+		if (c) {
+			c->object.flags |= SHOWN;
+			if (!(c->object.flags & BOUNDARY))
+				for (p = c->parents; p; p = p->next)
+					p->item->object.flags |= CHILD_SHOWN;
+		}
+	} else {
+		c = get_revision_internal(revs);
+	}
+
 	if (c && revs->graph)
 		graph_update(revs->graph, c);
 	if (!c) {

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;

t/t4202-log.sh

@@ -1882,6 +1882,46 @@ 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 --reverse | head -n3 >expect &&
+	git log --oneline --max-count-oldest=3 --reverse >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --max-count-oldest with --max-count' '
+	test_when_finished rm stderr &&
+	test_must_fail git log --max-count-oldest=3 --max-count=3 2>stderr &&
+	test_grep "cannot be used together" stderr
+'
+
+test_expect_success 'log --max-count-oldest with --skip' '
+	test_when_finished rm stderr &&
+	test_must_fail git log --max-count-oldest=3 --skip=1 2>stderr &&
+	test_grep "cannot be used together" stderr
+'
+
+test_expect_success 'log --max-count-oldest=1000 --graph --boundary' '
+	test_when_finished rm expect actual &&
+	git log --graph --boundary >expect &&
+	git log --max-count-oldest=1000 --graph --boundary >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --oneline --graph --boundary --max-count-oldest=1' '
+	test_when_finished rm -f actual &&
+	git log --oneline --graph --boundary --max-count-oldest=1 \
+		HEAD~1..HEAD >actual &&
+	test_line_count = 2 actual
+'
+
 cat >expect <<-\EOF
 * reach
 |