# Re: \[PATCH v2 1/2\] xdiff: implement a zealous diff3, or "zdiff3"

- From: Phillip Wood \<hidden\>
- Date: 2021-09-15 11:22:05 
- List: git
- Page: https://ratatoskr.run/git/2021/09/7902548
- Thread: https://ratatoskr.run/git/2021/06/7891996/t.md

```text
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 &&
 
```