diff: support anchoring line(s)

Teach diff a new algorithm, one that attempts to prevent user-specified
lines from appearing as a deletion or addition in the end result. The
end user can use this by specifying "--anchored=<text>" one or more
times when using Git commands like "diff" and "show".

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 94 insertions, 0 deletions

diff --git a/t/t4065-diff-anchored.sh b/t/t4065-diff-anchored.sh
new file mode 100755
index 0000000000..b3f510f040
--- /dev/null
+++ b/t/t4065-diff-anchored.sh
@@ -0,0 +1,94 @@
+#!/bin/sh
+
+test_description='anchored diff algorithm'
+
+. ./test-lib.sh
+
+test_expect_success '--anchored' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	# normally, c is moved to produce the smallest diff
+	test_expect_code 1 git diff --no-index pre post >diff &&
+	grep "^+c" diff &&
+
+	# with anchor, a is moved
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+a" diff
+'
+
+test_expect_success '--anchored multiple' '
+	printf "a\nb\nc\nd\ne\nf\n" >pre &&
+	printf "c\na\nb\nf\nd\ne\n" >post &&
+
+	# with 1 anchor, c is not moved, but f is moved
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+a" diff && # a is moved instead of c
+	grep "^+f" diff &&
+
+	# with 2 anchors, c and f are not moved
+	test_expect_code 1 git diff --no-index --anchored=c --anchored=f pre post >diff &&
+	grep "^+a" diff &&
+	grep "^+d" diff # d is moved instead of f
+'
+
+test_expect_success '--anchored with nonexistent line has no effect' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=x pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success '--anchored with non-unique line has no effect' '
+	printf "a\nb\nc\nd\ne\nc\n" >pre &&
+	printf "c\na\nb\nc\nd\ne\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success 'diff still produced with impossible multiple --anchored' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=a --anchored=c pre post >diff &&
+	mv post expected_post &&
+
+	# Ensure that the diff is correct by applying it and then
+	# comparing the result with the original
+	git apply diff &&
+	diff expected_post post
+'
+
+test_expect_success 'later algorithm arguments override earlier ones' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --patience --anchored=c pre post >diff &&
+	grep "^+a" diff &&
+
+	test_expect_code 1 git diff --no-index --anchored=c --patience pre post >diff &&
+	grep "^+c" diff &&
+
+	test_expect_code 1 git diff --no-index --histogram --anchored=c pre post >diff &&
+	grep "^+a" diff &&
+
+	test_expect_code 1 git diff --no-index --anchored=c --histogram pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success '--anchored works with other commands like "git show"' '
+	printf "a\nb\nc\n" >file &&
+	git add file &&
+	git commit -m foo &&
+	printf "c\na\nb\n" >file &&
+	git add file &&
+	git commit -m foo &&
+
+	# with anchor, a is moved
+	git show --patience --anchored=c >diff &&
+	grep "^+a" diff
+'
+
+test_done