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perf(gitdiff): optimize inline diff highlighting using cache (#38706)

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This pull request optimizes code diff highlighting in the PR Files
Changed view, which is a major CPU bottleneck and hot path in
production.

Previously, Gitea executed the expensive `DiffMatchPatch` algorithm
twice for every matching deleted/added line pair (once for the deleted
line, and once for the added line). We now run the diff algorithm once
and cache the resulting `DiffInline` on the `DiffLine` struct itself,
allowing the second line to render via a 0-cost cache lookup.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
This commit is contained in:
Shudhanshu Singh
2026-07-30 22:13:36 +05:30
committed by GitHub
parent 11d0ed699b
commit d87d26d735
11 changed files with 186 additions and 157 deletions
Download Patch File Download Diff File Expand all files Collapse all files

39
services/gitdiff/highlightdiff.go
View File

@@ -150,7 +150,7 @@ func (hcd *highlightCodeDiff) diffEqualPartIsSpaceOnly(s string) bool {
return true
}
func (hcd *highlightCodeDiff) diffLineWithHighlight(lineType DiffLineType, codeA, codeB template.HTML) template.HTML {
func (hcd *highlightCodeDiff) diffLineWithHighlight(codeA, codeB template.HTML) (del, add template.HTML) {
hcd.collectUsedRunes(codeA)
hcd.collectUsedRunes(codeB)
@@ -161,8 +161,6 @@ func (hcd *highlightCodeDiff) diffLineWithHighlight(lineType DiffLineType, codeA
diffs := dmp.DiffMain(convertedCodeA, convertedCodeB, true)
diffs = dmp.DiffCleanupSemantic(diffs)
buf := bytes.NewBuffer(nil)
if hcd.diffCodeClose == 0 {
// tests can pre-set the placeholders
hcd.diffCodeAddedOpen = hcd.registerTokenAsPlaceholder(`<span class="added-code">`)
@@ -183,32 +181,37 @@ func (hcd *highlightCodeDiff) diffLineWithHighlight(lineType DiffLineType, codeA
// only add "added"/"removed" tags when needed:
// * non-space contents appear in the DiffEqual parts (not a full-line add/del)
// * placeholder map still works (not exhausted, can get the closing tag placeholder)
bufDel := bytes.NewBuffer(nil)
bufAdd := bytes.NewBuffer(nil)
addDiffTags := !equalPartSpaceOnly && hcd.diffCodeClose != 0
if addDiffTags {
for _, diff := range diffs {
switch {
case diff.Type == diffmatchpatch.DiffEqual:
buf.WriteString(diff.Text)
case diff.Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd:
buf.WriteRune(hcd.diffCodeAddedOpen)
buf.WriteString(diff.Text)
buf.WriteRune(hcd.diffCodeClose)
case diff.Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel:
buf.WriteRune(hcd.diffCodeRemovedOpen)
buf.WriteString(diff.Text)
buf.WriteRune(hcd.diffCodeClose)
switch diff.Type {
case diffmatchpatch.DiffEqual:
bufDel.WriteString(diff.Text)
bufAdd.WriteString(diff.Text)
case diffmatchpatch.DiffInsert:
bufAdd.WriteRune(hcd.diffCodeAddedOpen)
bufAdd.WriteString(diff.Text)
bufAdd.WriteRune(hcd.diffCodeClose)
case diffmatchpatch.DiffDelete:
bufDel.WriteRune(hcd.diffCodeRemovedOpen)
bufDel.WriteString(diff.Text)
bufDel.WriteRune(hcd.diffCodeClose)
}
}
} else {
// the caller will still add added/removed backgrounds for the whole line
for _, diff := range diffs {
take := diff.Type == diffmatchpatch.DiffEqual || (diff.Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd) || (diff.Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel)
if take {
buf.WriteString(diff.Text)
if diff.Type == diffmatchpatch.DiffEqual || diff.Type == diffmatchpatch.DiffDelete {
bufDel.WriteString(diff.Text)
}
if diff.Type == diffmatchpatch.DiffEqual || diff.Type == diffmatchpatch.DiffInsert {
bufAdd.WriteString(diff.Text)
}
}
}
return hcd.recoverOneDiff(buf.String())
return hcd.recoverOneDiff(util.UnsafeBytesToString(bufDel.Bytes())), hcd.recoverOneDiff(util.UnsafeBytesToString(bufAdd.Bytes()))
}
func (hcd *highlightCodeDiff) registerTokenAsPlaceholder(token string) rune {