From 1878421b60112cdb6dd0c07fa995d3e368003e5a Mon Sep 17 00:00:00 2001
From: Phillip Wood <phillip.wood@dunelm.org.uk>
Date: Sat, 20 Jul 2024 14:49:25 +0100
Subject: [PATCH] add-patch: update hunk splitability after editing

When the users edits a hunk if they change deletion lines to context
lines or vice versa then the number of hunks that the edited hunk can be
split into may differ from the unedited hunk and so we need to update
hunk->splittable_into. 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 <phillip.wood@dunelm.org.uk>
---
 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 95c67d8c80c4f3..93d6f1b95f9af1 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -1181,19 +1181,29 @@ 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 b8a05d95f3f130..6a9eef776c92f1 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -1230,4 +1230,25 @@ test_expect_success 'hunk splitting works with diff.suppressBlankEmpty' '
 	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 n K 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