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

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>

modules/charset/escape_status.go

@@ -10,14 +10,8 @@ type EscapeStatus struct {
 	HasAmbiguous bool
 }
 
-// Or combines two EscapeStatus structs into one representing the conjunction of the two
-func (status *EscapeStatus) Or(other *EscapeStatus) *EscapeStatus {
-	st := status
-	if status == nil {
-		st = &EscapeStatus{}
-	}
+func (st *EscapeStatus) Combine(other *EscapeStatus) {
 	st.Escaped = st.Escaped || other.Escaped
 	st.HasAmbiguous = st.HasAmbiguous || other.HasAmbiguous
 	st.HasInvisible = st.HasInvisible || other.HasInvisible
-	return st
 }

modules/highlight/benchmark_test.go

@@ -0,0 +1,48 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package highlight
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/alecthomas/chroma/v2/lexers"
+)
+
+func BenchmarkDetectChromaLexerByFileName(b *testing.B) {
+	for b.Loop() {
+		// BenchmarkDetectChromaLexerByFileName-12    	18214717	        61.35 ns/op
+		DetectChromaLexerByFileName("a.sql", "")
+	}
+}
+
+func BenchmarkDetectChromaLexerWithAnalyze(b *testing.B) {
+	code := []byte(strings.Repeat("SELECT * FROM table;\n", 1000))
+	b.ResetTimer()
+	for b.Loop() {
+		// BenchmarkRenderCodeSlowGuess-12    	   87946	     13310 ns/op
+		detectChromaLexerWithAnalyze("a", "", code)
+	}
+}
+
+func BenchmarkChromaAnalyze(b *testing.B) {
+	code := strings.Repeat("SELECT * FROM table;\n", 1000)
+	b.ResetTimer()
+	for b.Loop() {
+		// comparing to detectChromaLexerWithAnalyze (go-enry), "chroma/lexers.Analyse" is very slow
+		// BenchmarkChromaAnalyze-12    	     519	   2247104 ns/op
+		lexers.Analyse(code)
+	}
+}
+
+func BenchmarkRenderCodeByLexer(b *testing.B) {
+	code := strings.Repeat("SELECT * FROM table;\n", 1000)
+	lexer := DetectChromaLexerByFileName("a.sql", "")
+	b.ResetTimer()
+	for b.Loop() {
+		// HINT: CODE-HIGHLIGHT-PERFORMANCE: Really slow ....... the regexp2 used by Chroma takes most of the time
+		// BenchmarkRenderCodeByLexer-12    	      22	  47159038 ns/op
+		RenderCodeByLexer(lexer, code)
+	}
+}

modules/highlight/lexerdetect_test.go

@@ -4,53 +4,11 @@
 package highlight
 
 import (
-	"strings"
 	"testing"
 
-	"github.com/alecthomas/chroma/v2/lexers"
 	"github.com/stretchr/testify/assert"
 )
 
-func BenchmarkDetectChromaLexerByFileName(b *testing.B) {
-	for b.Loop() {
-		// BenchmarkDetectChromaLexerByFileName-12    	18214717	        61.35 ns/op
-		DetectChromaLexerByFileName("a.sql", "")
-	}
-}
-
-func BenchmarkDetectChromaLexerWithAnalyze(b *testing.B) {
-	b.StopTimer()
-	code := []byte(strings.Repeat("SELECT * FROM table;\n", 1000))
-	b.StartTimer()
-	for b.Loop() {
-		// BenchmarkRenderCodeSlowGuess-12    	   87946	     13310 ns/op
-		detectChromaLexerWithAnalyze("a", "", code)
-	}
-}
-
-func BenchmarkChromaAnalyze(b *testing.B) {
-	b.StopTimer()
-	code := strings.Repeat("SELECT * FROM table;\n", 1000)
-	b.StartTimer()
-	for b.Loop() {
-		// comparing to detectChromaLexerWithAnalyze (go-enry), "chroma/lexers.Analyse" is very slow
-		// BenchmarkChromaAnalyze-12    	     519	   2247104 ns/op
-		lexers.Analyse(code)
-	}
-}
-
-func BenchmarkRenderCodeByLexer(b *testing.B) {
-	b.StopTimer()
-	code := strings.Repeat("SELECT * FROM table;\n", 1000)
-	lexer := DetectChromaLexerByFileName("a.sql", "")
-	b.StartTimer()
-	for b.Loop() {
-		// Really slow ....... the regexp2 used by Chroma takes most of the time
-		// BenchmarkRenderCodeByLexer-12    	      22	  47159038 ns/op
-		RenderCodeByLexer(lexer, code)
-	}
-}
-
 func TestDetectChromaLexer(t *testing.T) {
 	globalVars().highlightMapping[".my-html"] = "HTML"
 	t.Cleanup(func() { delete(globalVars().highlightMapping, ".my-html") })