diff --color-moved colors the two sides of the diff separately. It
walks through the diff and tries to find matches on the other side of
the diff for the current line. When it finds one or more matches it
starts a "potential moved block" (pmb) and marks the current line as
moved. Then as it walks through the diff it only looks for matches for
the current line in the lines following those in the pmb. When none of
the lines in the pmb match it checks how long the match is and if it
is too short it unmarks the lines as matched and goes back to finding
all the lines that match the current line. As the process of finding
matching lines restarts from the end of the block that was too short
it is possible to miss the start of a matching block on on side but
not the other. In the test added here "-two" would not be colored as
moved but "+two" would be.

Fix this by rewinding the current line when we reach the end of a
block that is too short. This is quadratic in the length of the
discarded block. While the discarded blocks are quite short on a large
diff this still has a significant impact on the performance of
--color-moved-ws=allow-indentation-change. The following commits
optimize the performance of the --color-moved machinery which
mitigates the performance impact of this commit. After the
optimization this commit has a negligible impact on performance.

Test                                                                 HEAD^               HEAD
------------------------------------------------------------------------------------------------------------------
4002.1: diff --no-color-moved --no-color-moved-ws large change        0.38 (0.33+0.05)    0.39 (0.34+0.04)  +2.6%
4002.2: diff --color-moved --no-color-moved-ws large change           0.80 (0.76+0.03)    0.86 (0.82+0.04)  +7.5%
4002.3: diff --color-moved-ws=allow-indentation-change large change  14.22(14.17+0.04)   19.01(18.93+0.05) +33.7%
4002.4: log --no-color-moved --no-color-moved-ws                      1.16 (1.06+0.09)    1.16 (1.07+0.07)  +0.0%
4002.5: log --color-moved --no-color-moved-ws                         1.31 (1.22+0.09)    1.32 (1.22+0.09)  +0.8%
4002.6: log --color-moved-ws=allow-indentation-change                 1.71 (1.61+0.09)    1.72 (1.63+0.08)  +0.6%

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 diff.c                     | 28 ++++++++++++++++++-----
 t/t4015-diff-whitespace.sh | 46 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/diff.c b/diff.c
index 09af94e018c..1e1b5127d15 100644
--- a/diff.c
+++ b/diff.c
@@ -1205,7 +1205,15 @@ static void mark_color_as_moved(struct diff_options *o,
 		if (!match) {
 			int i;
 
-			adjust_last_block(o, n, block_length);
+			if (!adjust_last_block(o, n, block_length) &&
+			    block_length > 1) {
+				/*
+				 * Rewind in case there is another match
+				 * starting at the second line of the block
+				 */
+				match = NULL;
+				n -= block_length;
+			}
 			for(i = 0; i < pmb_nr; i++)
 				moved_block_clear(&pmb[i]);
 			pmb_nr = 0;
@@ -1230,10 +1238,20 @@ static void mark_color_as_moved(struct diff_options *o,
 		pmb_nr = shrink_potential_moved_blocks(pmb, pmb_nr);
 
 		if (pmb_nr == 0) {
-			fill_potential_moved_blocks(
-				o, hm, match, l, &pmb, &pmb_alloc, &pmb_nr);
-			if (adjust_last_block(o, n, block_length) &&
-			    pmb_nr && last_symbol != l->s)
+			int contiguous = adjust_last_block(o, n, block_length);
+
+			if (!contiguous && block_length > 1)
+				/*
+				 * Rewind in case there is another match
+				 * starting at the second line of the block
+				 */
+				n -= block_length;
+			else
+				fill_potential_moved_blocks(o, hm, match, l,
+							    &pmb, &pmb_alloc,
+							    &pmb_nr);
+
+			if (contiguous && pmb_nr && last_symbol != l->s)
 				flipped_block = (flipped_block + 1) % 2;
 			else
 				flipped_block = 0;
diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
index 2c13b62d3c6..308dc136596 100755
--- a/t/t4015-diff-whitespace.sh
+++ b/t/t4015-diff-whitespace.sh
@@ -1833,6 +1833,52 @@ test_expect_success '--color-moved treats adjacent blocks as separate for MIN_AL
 	test_cmp expected actual
 '
 
+test_expect_success '--color-moved rewinds for MIN_ALNUM_COUNT' '
+	git reset --hard &&
+	test_write_lines >file \
+		A B C one two three four five six seven D E F G H I J &&
+	git add file &&
+	test_write_lines >file \
+		one two A B C D E F G H I J two three four five six seven &&
+	git diff --color-moved=zebra -- file &&
+
+	git diff --color-moved=zebra --color -- file >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
+	cat >expected <<-\EOF &&
+	<BOLD>diff --git a/file b/file<RESET>
+	<BOLD>--- a/file<RESET>
+	<BOLD>+++ b/file<RESET>
+	<CYAN>@@ -1,13 +1,8 @@<RESET>
+	<GREEN>+<RESET><GREEN>one<RESET>
+	<GREEN>+<RESET><GREEN>two<RESET>
+	 A<RESET>
+	 B<RESET>
+	 C<RESET>
+	<RED>-one<RESET>
+	<BOLD;MAGENTA>-two<RESET>
+	<BOLD;MAGENTA>-three<RESET>
+	<BOLD;MAGENTA>-four<RESET>
+	<BOLD;MAGENTA>-five<RESET>
+	<BOLD;MAGENTA>-six<RESET>
+	<BOLD;MAGENTA>-seven<RESET>
+	 D<RESET>
+	 E<RESET>
+	 F<RESET>
+	<CYAN>@@ -15,3 +10,9 @@<RESET> <RESET>G<RESET>
+	 H<RESET>
+	 I<RESET>
+	 J<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>two<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>three<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>four<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>five<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>six<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>seven<RESET>
+	EOF
+
+	test_cmp expected actual
+'
+
 test_expect_success 'move detection with submodules' '
 	test_create_repo bananas &&
 	echo ripe >bananas/recipe &&
