From: Phillip Wood <phillip.wood123@gmail.com>
Date: Wed, 15 Sep 2021 11:21:59 GMT
Subject: Re: [PATCH v2 1/2] xdiff: implement a zealous diff3, or "zdiff3"
Message-ID: <2290b657-3c67-0ab8-5c25-cc57feac301c@gmail.com>
In-Reply-To: <b6818661-ac6e-fbde-2cab-429c5550a0da@gmail.com>

On 15/09/2021 11:25, Phillip Wood wrote:
> I do wonder (though a brief try failed to trigger it) if there are cases 
> where the diff algorithm does something "clever" which means it does not 
> treat a common prefix or suffix as unchanged (see d2f82950a9 
> ("Re(-re)*fix trim_common_tail()", 2007-12-20) for a related issue). We 
> could just trim the common prefix and suffix from the two sides 
> ourselves using xdl_recmatch().

Here is an evil test case that shows this problem (diff on top of your patch)


diff --git a/t/t6427-diff3-conflict-markers.sh b/t/t6427-diff3-conflict-markers.sh
index de9c6190b9..836843c6b0 100755
--- a/t/t6427-diff3-conflict-markers.sh
+++ b/t/t6427-diff3-conflict-markers.sh
@@ -219,8 +219,9 @@ test_setup_zdiff3 () {
                 test_write_lines 1 2 3 4 5 6 7 8 9 >basic &&
                 test_write_lines 1 2 3 AA 4 5 BB 6 7 8 >middle-common &&
                 test_write_lines 1 2 3 4 5 6 7 8 9 >interesting &&
+               test_write_lines 1 2 3 4 5 6 7 8 9 >evil &&
  
-               git add basic middle-common &&
+               git add basic middle-common interesting evil &&
                 git commit -m base &&
  
                 git branch left &&
@@ -230,19 +231,21 @@ test_setup_zdiff3 () {
                 test_write_lines 1 2 3 4 A B C D E 7 8 9 >basic &&
                 test_write_lines 1 2 3 CC 4 5 DD 6 7 8 >middle-common &&
                 test_write_lines 1 2 3 4 A B C D E F G H I J 7 8 9 >interesting &&
+               test_write_lines 1 2 3 4 X A B C 7 8 9 >evil &&
                 git add -u &&
                 git commit -m letters &&
  
                 git checkout right &&
                 test_write_lines 1 2 3 4 A X C Y E 7 8 9 >basic &&
                 test_write_lines 1 2 3 EE 4 5 FF 6 7 8 >middle-common &&
                 test_write_lines 1 2 3 4 A B C 5 6 G H I J 7 8 9 >interesting &&
+               test_write_lines 1 2 3 4 Y A B C B C 7 8 9 >evil &&
                 git add -u &&
                 git commit -m permuted
         )
  }
  
-test_expect_failure 'check zdiff3 markers' '
+test_expect_success 'check zdiff3 markers' '
         test_setup_zdiff3 &&
         (
                 cd zdiff3 &&
@@ -251,6 +254,14 @@ test_expect_failure 'check zdiff3 markers' '
  
                 test_must_fail git -c merge.conflictstyle=zdiff3 merge -s recursive right^0 &&
  
+               test_write_lines \
+                       1 2 3 4 \
+                       "<<<<<<< HEAD" X A \
+                       "||||||| $(git rev-parse --short HEAD^1)" 5 6 ======= \
+                       Y A B C ">>>>>>> right^0" \
+                       B C 7 8 9 >expect &&
+               test_cmp expect evil &&
+
                 test_write_lines 1 2 3 4 A "<<<<<<< HEAD" B C D "||||||| $(git rev-parse --short HEAD^1)" 5 6 ======= X C Y ">>>>>>> right^0" E 7 8 9 >expect &&
                 test_cmp expect basic &&