Subject: [PATCH v3 2/2] add-patch: update hunk splitability after editing

From: "Phillip Wood via GitGitGadget" <[email-masked]>
Date: Thu, 25 Sep 2025 15:10:38 +0000

From: Phillip Wood <[email-masked]>

If, when the user edits a hunk, they change deletion lines into
context lines or vice versa, then the number of hunks that the edited
hunk can be split into may differ from the unedited hunk. This means
that so we should recalculate `hunk->splittable_into` after the hunk
has been edited. In practice users are unlikely to hit this bug as it
is doubtful that a user who has edited a hunk will split it afterwards.

Signed-off-by: Phillip Wood <[email-masked]>
---
 add-patch.c                | 12 +++++++++++-
 t/t3701-add-interactive.sh | 21 +++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/add-patch.c b/add-patch.c
index 61f42de9ea..bcc2d7666f 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -1185,19 +1185,29 @@ static ssize_t recount_edited_hunk(struct add_p_state *s, struct hunk *hunk,
 {
 	struct hunk_header *header = &hunk->header;
 	size_t i;
+	char ch, marker = ' ';
 
+	hunk->splittable_into = 0;
 	header->old_count = header->new_count = 0;
 	for (i = hunk->start; i < hunk->end; ) {
-		switch(normalize_marker(&s->plain.buf[i])) {
+		ch = normalize_marker(&s->plain.buf[i]);
+		switch (ch) {
 		case '-':
 			header->old_count++;
+			if (marker == ' ')
+				hunk->splittable_into++;
+			marker = ch;
 			break;
 		case '+':
 			header->new_count++;
+			if (marker == ' ')
+				hunk->splittable_into++;
+			marker = ch;
 			break;
 		case ' ':
 			header->old_count++;
 			header->new_count++;
+			marker = ch;
 			break;
 		}
 
diff --git a/t/t3701-add-interactive.sh b/t/t3701-add-interactive.sh
index a6829fd085..13739a4582 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -1311,4 +1311,25 @@ test_expect_success 'splitting previous hunk marks split hunks as undecided' '
 	test_cmp expect actual
 '
 
+test_expect_success 'splitting edited hunk' '
+	# Before the first hunk is edited it can be split into two
+	# hunks, after editing it can be split into three hunks.
+
+	write_script fake-editor.sh <<-\EOF &&
+	sed "s/^ c/-c/" "$1" >"$1.tmp" &&
+	mv "$1.tmp" "$1"
+	EOF
+
+	test_write_lines a b c d e f g h i j k l m n >file &&
+	git add file &&
+	test_write_lines A b c d E f g h i j k l M n >file &&
+	(
+		test_set_editor "$(pwd)/fake-editor.sh" &&
+		test_write_lines e K s j y n y q | git add -p file
+	) &&
+	git cat-file blob :file >actual &&
+	test_write_lines a b d e f g h i j k l M n >expect &&
+	test_cmp expect actual
+'
+
 test_done
-- 
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