Subject: [PATCH] diffcore-delta: avoid ignoring final 'line' of file

From: "Elijah Newren via GitGitGadget" <[email-masked]>
Date: Thu, 11 Jan 2024 20:47:54 +0000

From: Elijah Newren <[email-masked]>

hash_chars() would hash lines to integers, and store them in a spanhash,
but cut lines at 64 characters.  Thus, whenever it reached 64 characters
or a newline, it would create a new spanhash.  The problem is, the final
part of the file might not end 64 characters after the previous 'line'
and might not end with a newline.  This could, for example, cause an
85-byte file with 12 lines and only the first character in the file
differing to appear merely 23% similar rather than the expected 97%.
Ensure the last line is included, and add a testcase that would have
caught this problem.

Signed-off-by: Elijah Newren <[email-masked]>
---
    diffcore-delta: avoid ignoring final 'line' of file
    
    Found while experimenting with converting portions of diffcore-delta to
    Rust.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1637%2Fnewren%2Ffix-diffcore-final-line-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1637/newren/fix-diffcore-final-line-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1637

 diffcore-delta.c       |  4 ++++
 t/t4001-diff-rename.sh | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/diffcore-delta.c b/diffcore-delta.c
index c30b56e983b..7136c3dd203 100644
--- a/diffcore-delta.c
+++ b/diffcore-delta.c
@@ -159,6 +159,10 @@ static struct spanhash_top *hash_chars(struct repository *r,
 		n = 0;
 		accum1 = accum2 = 0;
 	}
+	if (n > 0) {
+		hashval = (accum1 + accum2 * 0x61) % HASHBASE;
+		hash = add_spanhash(hash, hashval, n);
+	}
 	QSORT(hash->data, (size_t)1ul << hash->alloc_log2, spanhash_cmp);
 	return hash;
 }
diff --git a/t/t4001-diff-rename.sh b/t/t4001-diff-rename.sh
index 85be1367de6..29299acbce7 100755
--- a/t/t4001-diff-rename.sh
+++ b/t/t4001-diff-rename.sh
@@ -286,4 +286,23 @@ test_expect_success 'basename similarity vs best similarity' '
 	test_cmp expected actual
 '
 
+test_expect_success 'last line matters too' '
+	test_write_lines a 0 1 2 3 4 5 6 7 8 9 >nonewline &&
+	printf "git ignores final up to 63 characters if not newline terminated" >>nonewline &&
+	git add nonewline &&
+	git commit -m "original version of file with no final newline" &&
+
+	# Change ONLY the first character of the whole file
+	test_write_lines b 0 1 2 3 4 5 6 7 8 9 >nonewline &&
+	printf "git ignores final up to 63 characters if not newline terminated" >>nonewline &&
+	git add nonewline &&
+	git mv nonewline still-no-newline &&
+	git commit -a -m "rename nonewline -> still-no-newline" &&
+	git diff-tree -r -M01 --name-status HEAD^ HEAD >actual &&
+	cat >expected <<-\EOF &&
+	R097	nonewline	still-no-newline
+	EOF
+	test_cmp expected actual
+'
+
 test_done

base-commit: 055bb6e9969085777b7fab83e3fee0017654f134
-- 
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