From: Yee Cheng Chin <ychin.git@gmail.com>
After a diff algorithm has been run, the compaction phase (xdl_change_compact()) shifts and merges change groups to produce a cleaner output. However, this shifting could create a new matched group where both sides now have matching lines. This results in a wrong-looking diff output which contains redundant lines that are the same on both files.
Fix this by detecting this situation, and re-diff the texts on each side to find similar lines, using the fall-back Myer's diff. Only do this for histogram diff as it's the only algorithm where this is relevant. Below contains an example, and more details.
For an example, consider two files below:
file1:
A A
A
A A
A
A file2:
A A
x
A A
A
AWhen using Myer's diff, the algorithm finds that only the "x" has been changed, and produces a final diff result (these are line diffs, but using word-diff syntax for ease of presentation):
A A[-A-]{+x+}A AAAWhen using histogram diff, the algorithm first discovers the LCS "A AAA", which it uses as anchor, then produces an intermediate diff:
{+A Ax+}A AAA[- AAA-].This is a longer diff than Myer's, but it's still self-consistent. However, the compaction phase attempts to shift the first file's diff group upwards (note that this shift crosses the anchor that histogram had used), leading to the final results for histogram diff:
[-A AA-]{+A Ax+}A AAAThis is a technically correct patch but looks clearly redundant to a human as the first 3 lines should not be in the diff.
The fix would detect that a shift has caused matching to a new group, and re-diff the "A AA" and "A Ax" parts, which results in "A A" correctly re-marked as unchanged. This creates the now correct histogram diff:
A A[-A-]{+x+}A AAAThis issue is not applicable to Myer's diff algorithm as it already generates a minimal diff, which means a shift cannot result in a smaller diff output (the default Myer's diff in xdiff is not guaranteed to be minimal for performance reasons, but it typically does a good enough job).
It's also not applicable to patience diff, because it uses only unique lines as anchor for its splits, and falls back to Myer's diff within each split. Shifting requires both ends having the same lines, and therefore cannot cross the unique line boundaries established by the patience algorithm. In contrast histogram diff uses non-unique lines as anchors, and therefore shifting can cross over them.
This issue is rare in a normal repository. Below is a table of repositories (`git log --no-merges -p --histogram -1000`), showing how many times a re-diff was done and how many times it resulted in finding matching lines (therefore addressing this issue) with the fix. In general it is fewer than 1% of diff's that exhibit this offending behavior:
| Repo (1k commits) | Re-diff | Found matching lines | |--------------------|---------|----------------------| | llvm-project | 45 | 11 | | vim | 110 | 9 | | git | 18 | 2 | | WebKit | 168 | 1 | | ripgrep | 22 | 1 | | cpython | 32 | 0 | | vscode | 13 | 0 |
Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
---
xdiff: re-diff shifted change groups when using histogram algorithm
This is a somewhat rare issue when using histogram to diff files, as the
algorithm will generate a diff output that looks redundant and wrong to
a human. I provided a synthetic example in the commit message, but for
one from the real world, do the following command in the Git repo:
git show -U0 --diff-algorithm=histogram 2c8999027c -- po/ga.po
Scroll to the line "@@ -7239,3 +5831,5 @@", and we can see the following
diff hunk:
-#: builtin/diff.c
-msgid "Not a git repository"
-msgstr "Ní stór git"
+msgid "cannot come back to cwd"
+msgstr "ní féidir teacht ar ais chuig cwd"
+
+msgid "Not a git repository"
+msgstr "Ní stór git é"
We can see that the "Not a git repository" line is identical on both
sides, which means it should not have been in the diff results to begin
with. Under other diff algorithms (or histogram diff with this fix),
said line is not considered to be part of the diff.
Also, when I was implementing this, an alternative I was considering was
to add a bespoke linear-time algorithm to remove matching lines on both
sides. Just calling the fall-back diff seems the easiest and cleanest
and so I went with that.Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2120%2Fychin%2Fxdiff-fix-compact-remove-redundant-lines-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2120/ychin/xdiff-fix-compact-remove-redundant-lines-v1 Pull-Request: https://github.com/git/git/pull/2120
t/meson.build | 1 + t/t4073-diff-shifted-matched-group.sh | 137 ++++++++++++++++++++++++++ xdiff/xdiffi.c | 43 ++++++++ 3 files changed, 181 insertions(+) create mode 100755 t/t4073-diff-shifted-matched-group.sh
Show changes to 3 files +181 −0
t/meson.build, t/t4073-diff-shifted-matched-group.sh, xdiff/xdiffi.c
diff --git a/t/meson.build b/t/meson.build index 7c994d4643..ee233e80da 100644 --- a/t/meson.build +++ b/t/meson.build @@ -497,6 +497,7 @@ integration_tests = [ 't4070-diff-pairs.sh', 't4071-diff-minimal.sh', 't4072-diff-max-depth.sh', + 't4073-diff-shifted-matched-group.sh', 't4100-apply-stat.sh', 't4101-apply-nonl.sh', 't4102-apply-rename.sh', diff --git a/t/t4073-diff-shifted-matched-group.sh b/t/t4073-diff-shifted-matched-group.sh new file mode 100755 index 0000000000..0e915b78a6 --- /dev/null +++ b/t/t4073-diff-shifted-matched-group.sh @@ -0,0 +1,137 @@ +#!/bin/sh + +test_description='shifted diff groups re-diffing during histogram diff' + +. ./test-lib.sh + +test_expect_success 'shifted diff group should re-diff to minimize patch' ' + test_write_lines A x A A A x A A A >file1 && + test_write_lines A x A Z A x A A A >file2 && + + file1_h=$(git rev-parse --short $(git hash-object file1)) && + file2_h=$(git rev-parse --short $(git hash-object file2)) && + + cat >expect <<-EOF && + diff --git a/file1 b/file2 + index $file1_h..$file2_h 100644 + --- a/file1 + +++ b/file2 + @@ -1,7 +1,7 @@ + A + x + A + -A + +Z + A + x + A + EOF + + test_expect_code 1 git diff --no-index --histogram file1 file2 >output && + test_cmp expect output +' + +test_expect_success 're-diff should preserve diff flags' ' + test_write_lines a b c a b c >file1 && + test_write_lines x " b" z a b c >file2 && + + file1_h=$(git rev-parse --short $(git hash-object file1)) && + file2_h=$(git rev-parse --short $(git hash-object file2)) && + + cat >expect <<-EOF && + diff --git a/file1 b/file2 + index $file1_h..$file2_h 100644 + --- a/file1 + +++ b/file2 + @@ -1,6 +1,6 @@ + -a + -b + -c + +x + + b + +z + a + b + c + EOF + + test_expect_code 1 git diff --no-index --histogram file1 file2 >output && + test_cmp expect output && + + cat >expect_iwhite <<-EOF && + diff --git a/file1 b/file2 + index $file1_h..$file2_h 100644 + --- a/file1 + +++ b/file2 + @@ -1,6 +1,6 @@ + -a + +x + b + -c + +z + a + b + c + EOF + + test_expect_code 1 git diff --no-index --histogram --ignore-all-space file1 file2 >output_iwhite && + test_cmp expect_iwhite output_iwhite +' + +test_expect_success 'shifting on either side should trigger re-diff properly' ' + test_write_lines a b c a b c a b c >file1 && + test_write_lines a b c a1 a2 a3 b c1 a b c >file2 && + + file1_h=$(git rev-parse --short $(git hash-object file1)) && + file2_h=$(git rev-parse --short $(git hash-object file2)) && + + cat >expect1 <<-EOF && + diff --git a/file1 b/file2 + index $file1_h..$file2_h 100644 + --- a/file1 + +++ b/file2 + @@ -1,9 +1,11 @@ + a + b + c + -a + +a1 + +a2 + +a3 + b + -c + +c1 + a + b + c + EOF + + test_expect_code 1 git diff --no-index --histogram file1 file2 >output1 && + test_cmp expect1 output1 && + + cat >expect2 <<-EOF && + diff --git a/file2 b/file1 + index $file2_h..$file1_h 100644 + --- a/file2 + +++ b/file1 + @@ -1,11 +1,9 @@ + a + b + c + -a1 + -a2 + -a3 + +a + b + -c1 + +c + a + b + c + EOF + + test_expect_code 1 git diff --no-index --histogram file2 file1 >output2 && + test_cmp expect2 output2 +' + +test_done diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c index 6f3998ee54..5d9c7b5434 100644 --- a/xdiff/xdiffi.c +++ b/xdiff/xdiffi.c @@ -793,6 +793,7 @@ static int group_slide_up(xdfile_t *xdf, struct xdlgroup *g) */ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) { struct xdlgroup g, go; + struct xdlgroup g_orig, go_orig; long earliest_end, end_matching_other; long groupsize; @@ -806,6 +807,9 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) { if (g.end == g.start) goto next; + g_orig = g; + go_orig = go; + /* * Now shift the change up and then down as far as possible in * each direction. If it bumps into any other changes, merge @@ -915,6 +919,45 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) { } } + /* + * If this has a matching group from the other file, it could + * either be the original match from the diff algorithm, or + * arrived at by shifting and joining groups. When it's the + * latter, it's possible for the two newly joined sides to have + * matching lines. Re-diff the group to mark these matching + * lines as unchanged and remove from the diff output. + * + * Only do this for histogram diff as its LCS algorithm makes + * this scenario possible. In contrast, patience diff finds LCS + * of unique lines that groups cannot be shifted across. + * Myer's diff (standalone or used as fall-back in patience + * diff) already finds minimal edits so it is not possible for + * shifted groups to result in a smaller diff. (Without + * XDF_NEED_MINIMAL, Myer's isn't technically guaranteed to be + * minimal, but it should be so most of the time) + */ + if (end_matching_other != -1 && + XDF_DIFF_ALG(flags) == XDF_HISTOGRAM_DIFF && + (g.start != g_orig.start || + g.end != g_orig.end || + go.start != go_orig.start || + go.end != go_orig.end)) { + xpparam_t xpp; + xdfenv_t xe; + + memset(&xpp, 0, sizeof(xpp)); + xpp.flags = flags & ~XDF_DIFF_ALGORITHM_MASK; + + memcpy(&xe.xdf1, xdf, sizeof(xdfile_t)); + memcpy(&xe.xdf2, xdfo, sizeof(xdfile_t)); + + if (xdl_fall_back_diff(&xe, &xpp, + g.start + 1, g.end - g.start, + go.start + 1, go.end - go.start)) { + return -1; + } + } + next: /* Move past the just-processed group: */ if (group_next(xdf, &g)) base-commit: f0ef5b6d9bcc258e4cbef93839d1b7465d5212b9
-- gitgitgadget
