From 2bd69b9024c8c7c1b77060e3ed996c74b4775b01 Mon Sep 17 00:00:00 2001
From: Phillip Wood <phillip.wood@dunelm.org.uk>
Date: Wed, 12 Jun 2019 02:25:27 -0700
Subject: add -p: fix checkout -p with pathological context

Commit fecc6f3a68 ("add -p: adjust offsets of subsequent hunks when one is
skipped", 2018-03-01) fixed adding hunks in the correct place when a
previous hunk has been skipped. However it did not address patches that
are applied in reverse. In that case we need to adjust the pre-image
offset so that when apply reverses the patch the post-image offset is
adjusted correctly. We subtract rather than add the delta as the patch
is reversed (the easiest way to think about it is to consider a hunk of
deletions that is skipped - in that case we want to reduce offset so we
need to subtract).

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-add--interactive.perl b/git-add--interactive.perl
index 8361ef4..85d0dc1 100755
--- a/git-add--interactive.perl
+++ b/git-add--interactive.perl
@@ -957,7 +957,11 @@ sub coalesce_overlapping_hunks {
 			next;
 		}
 		if ($ofs_delta) {
-			$n_ofs += $ofs_delta;
+			if ($patch_mode_flavour{IS_REVERSE}) {
+				$o_ofs -= $ofs_delta;
+			} else {
+				$n_ofs += $ofs_delta;
+			}
 			$_->{TEXT}->[0] = format_hunk_header($o_ofs, $o_cnt,
 							     $n_ofs, $n_cnt);
 		}
diff --git a/t/t3701-add-interactive.sh b/t/t3701-add-interactive.sh
index f1bb879..2c54f84 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -571,4 +571,12 @@ test_expect_success 'add -p patch editing works with pathological context lines'
 	test_cmp expected-2 actual
 '
 
+test_expect_success 'checkout -p works with pathological context lines' '
+	test_write_lines a a a a a a >a &&
+	git add a &&
+	test_write_lines a b a b a b a b a b a > a&&
+	test_write_lines s n n y q | git checkout -p &&
+	test_write_lines a b a b a a b a b a >expect &&
+	test_cmp expect a
+'
 test_done
-- 
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