[PATCH] revision: add --maximal option

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Jan 18, 2026, 02:34 UTC
Message-ID: <pull.2032.git.1768703645125.gitgitgadget@gmail.com>

From: Derrick Stolee <stolee@gmail.com>

When inspecting a range of commits from some set of starting references, it is sometimes useful to learn which commits are not reachable from any other commits in the selected range.

One such application is in the creation of a sequence of bundles for the bundle URI feature. Creating a stack of bundles representing different slices of time includes defining which references to include. If all references are used, then this may be overwhelming or redundant. Instead, selecting commits that are maximal to the range could help defining a smaller reference set to use in the bundle header.

Add a new '--maximal' option to restrict the output of a revision range to be only the commits that are not reachable from any other commit in the range, based on the reachability definition of the walk.

This is accomplished by adding a new 28th bit flag, CHILD_VISITED, that is set as we walk. This does extend the bit range in object.h, but using an earlier bit may collide with another feature.

The tests demonstrate the behavior of the feature with a positive-only range, ranges with negative references, and walk-modifying flags like --first-parent and --exclude-first-parent-only.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
    revision: add --maximal option
    
    My motivation for this feature is very similar to the bundle URI
    application. I can get around it by creating a tool that uses git
    rev-list --parents and then uses a hashset to collect the parent list
    and filter out any commits that ever appear as parents. It would be more
    efficient to use Git's native revision-walking feature.
    
    This does bring the object struct up to a 32-bit boundary with 28 flag
    bits, 3 type bits, and a parsed bit. That's the biggest concern I have
    about this update adding a new flag bit. I would understand if this
    feature is not worth running out of room for extensions there.
    
    I considered looking through the earlier bit positions to see the impact
    of an overlap, but they certainly looked potentially risky to reuse.
    
    I wonder if anyone else has thought about this as a useful technique.
    For instance, it could be part of a strategy for choosing commits for
    reachability bitmaps.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2032%2Fderrickstolee%2Fmaximal-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2032/derrickstolee/maximal-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2032

 Documentation/rev-list-options.adoc |  4 ++
 object.h                            |  4 +-
 revision.c                          |  9 +++-
 revision.h                          |  5 +-
 t/t6600-test-reach.sh               | 75 +++++++++++++++++++++++++++++
 5 files changed, 92 insertions(+), 5 deletions(-)

diff --git a/Documentation/rev-list-options.adoc b/Documentation/rev-list-options.adoc
index 453ec59057..f0d2ab32a9 100644
--- a/Documentation/rev-list-options.adoc
+++ b/Documentation/rev-list-options.adoc
@@ -444,6 +444,10 @@ The following options affect the way the simplification is performed:
 	times; if so, a commit is included if it is any of the commits
 	given or if it is an ancestor or descendant of one of them.
 
+`--maximal`::
+	Restrict the output commits to be those that are not reachable
+	from any other commits in the revision range.
+
 A more detailed explanation follows.
 
 Suppose you specified `foo` as the _<paths>_.  We shall call commits
diff --git a/object.h b/object.h
index 4bca957b8d..dfe7a1f0ea 100644
--- a/object.h
+++ b/object.h
@@ -64,7 +64,7 @@ void object_array_init(struct object_array *array);
 
 /*
  * object flag allocation:
- * revision.h:               0---------10         15               23------27
+ * revision.h:               0---------10         15               23--------28
  * fetch-pack.c:             01    67
  * negotiator/default.c:       2--5
  * walker.c:                 0-2
@@ -86,7 +86,7 @@ void object_array_init(struct object_array *array);
  * builtin/unpack-objects.c:                                 2021
  * pack-bitmap.h:                                              2122
  */
-#define FLAG_BITS  28
+#define FLAG_BITS  29
 
 #define TYPE_BITS 3
 
diff --git a/revision.c b/revision.c
index 1858e093ee..29864426d6 100644
--- a/revision.c
+++ b/revision.c
@@ -1150,7 +1150,8 @@ static int process_parents(struct rev_info *revs, struct commit *commit,
 			struct commit *p = parent->item;
 			parent = parent->next;
 			if (p)
-				p->object.flags |= UNINTERESTING;
+				p->object.flags |= UNINTERESTING |
+						   CHILD_VISITED;
 			if (repo_parse_commit_gently(revs->repo, p, 1) < 0)
 				continue;
 			if (p->parents)
@@ -1204,7 +1205,7 @@ static int process_parents(struct rev_info *revs, struct commit *commit,
 			if (!*slot)
 				*slot = *revision_sources_at(revs->sources, commit);
 		}
-		p->object.flags |= pass_flags;
+		p->object.flags |= pass_flags | CHILD_VISITED;
 		if (!(p->object.flags & SEEN)) {
 			p->object.flags |= (SEEN | NOT_USER_GIVEN);
 			if (list)
@@ -2381,6 +2382,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->first_parent_only = 1;
 	} else if (!strcmp(arg, "--exclude-first-parent-only")) {
 		revs->exclude_first_parent_only = 1;
+	} else if (!strcmp(arg, "--maximal")) {
+		revs->maximal = 1;
 	} else if (!strcmp(arg, "--ancestry-path")) {
 		revs->ancestry_path = 1;
 		revs->simplify_history = 0;
@@ -4125,6 +4128,8 @@ enum commit_action get_commit_action(struct rev_info *revs, struct commit *commi
 {
 	if (commit->object.flags & SHOWN)
 		return commit_ignore;
+	if (revs->maximal && (commit->object.flags & CHILD_VISITED))
+		return commit_ignore;
 	if (revs->unpacked && has_object_pack(revs->repo, &commit->object.oid))
 		return commit_ignore;
 	if (revs->no_kept_objects) {
diff --git a/revision.h b/revision.h
index b36acfc2d9..e5c2c82145 100644
--- a/revision.h
+++ b/revision.h
@@ -52,7 +52,9 @@
 #define NOT_USER_GIVEN	(1u<<25)
 #define TRACK_LINEAR	(1u<<26)
 #define ANCESTRY_PATH	(1u<<27)
-#define ALL_REV_FLAGS	(((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR | PULL_MERGE)
+#define CHILD_VISITED	(1u<<28)
+#define ALL_REV_FLAGS	(((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR \
+				      | PULL_MERGE | CHILD_VISITED)
 
 #define DECORATE_SHORT_REFS	1
 #define DECORATE_FULL_REFS	2
@@ -198,6 +200,7 @@ struct rev_info {
 			cherry_mark:1,
 			bisect:1,
 			ancestry_path:1,
+			maximal:1,
 
 			/* True if --ancestry-path was specified without an
 			 * argument. The bottom revisions are implicitly
diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh
index 6638d1aa1d..a759409756 100755
--- a/t/t6600-test-reach.sh
+++ b/t/t6600-test-reach.sh
@@ -762,4 +762,79 @@ test_expect_success 'for-each-ref is-base: --sort' '
 		--sort=refname --sort=-is-base:commit-2-3
 '
 
+test_expect_success 'rev-list --maximal (all positive)' '
+	# Only one maximal.
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-4-2
+	refs/heads/commit-4-4
+	refs/heads/commit-8-4
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-8-4)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# All maximal.
+	cat >input <<-\EOF &&
+	refs/heads/commit-5-2
+	refs/heads/commit-4-3
+	refs/heads/commit-3-4
+	refs/heads/commit-2-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-5-2)
+	$(git rev-parse refs/heads/commit-4-3)
+	$(git rev-parse refs/heads/commit-3-4)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# Mix of both.
+	cat >input <<-\EOF &&
+	refs/heads/commit-5-2
+	refs/heads/commit-3-2
+	refs/heads/commit-2-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-5-2)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin
+'
+
+test_expect_success 'rev-list --maximal (range)' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-5
+	refs/heads/commit-6-4
+	^refs/heads/commit-4-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-6-4)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# first-parent changes reachability: the first parent
+	# reduces the second coordinate to 1 before reducing the
+	# first coordinate.
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-5
+	refs/heads/commit-6-4
+	^refs/heads/commit-4-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-6-4)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin \
+		--first-parent --exclude-first-parent-only
+'
+
 test_done

base-commit: b5c409c40f1595e3e590760c6f14a16b6683e22c

-- 
gitgitgadget