Reduce diff allocations and fix line counting

diff/diff.go

@@ -2,7 +2,6 @@
 package diff
 
 import (
-	"bufio"
 	"bytes"
 	"fmt"
 	"io"
@@ -107,9 +106,9 @@ func Compare(oldReader, newReader archives.Reader) (*CompareResult, error) {
 			result.FilesAdded++
 		case TypeModified:
 			result.FilesChanged++
-			result.TotalAdded += fileDiff.LinesAdded
-			result.TotalDeleted += fileDiff.LinesDeleted
 		}
+		result.TotalAdded += fileDiff.LinesAdded
+		result.TotalDeleted += fileDiff.LinesDeleted
 
 		result.Files = append(result.Files, fileDiff)
 	}
@@ -125,7 +124,15 @@ func compareFile(path string, oldInfo, newInfo archives.FileInfo, oldReader, new
 
 	switch {
 	case inOld && !inNew:
-		return FileDiff{Path: path, Type: TypeDeleted}, true
+		fd := FileDiff{Path: path, Type: TypeDeleted}
+		if content, err := readFileContent(oldReader, path); err == nil {
+			if !isDiffableText(content) {
+				fd.IsBinary = true
+			} else {
+				fd.LinesDeleted = countLines(content)
+			}
+		}
+		return fd, true
 
 	case !inOld && inNew:
 		fd := FileDiff{Path: path, Type: TypeAdded}
@@ -248,25 +255,25 @@ func generateSimpleDiff(path string, oldContent, newContent []byte) (string, int
 
 	// Context before
 	for i := hunkOldStart; i < oldStart && i < len(oldLines); i++ {
-		hunk.WriteString(" " + oldLines[i] + "\n")
+		writeDiffLine(&hunk, ' ', oldLines[i])
 	}
 
 	// Deleted lines
 	for i := oldStart; i < oldStart+oldCount && i < len(oldLines); i++ {
-		hunk.WriteString("-" + oldLines[i] + "\n")
+		writeDiffLine(&hunk, '-', oldLines[i])
 		linesDeleted++
 	}
 
 	// Added lines
 	for i := newStart; i < newStart+newCount && i < len(newLines); i++ {
-		hunk.WriteString("+" + newLines[i] + "\n")
+		writeDiffLine(&hunk, '+', newLines[i])
 		linesAdded++
 	}
 
 	// Context after
 	afterStart := oldStart + oldCount
 	for i := 0; i < contextAfter && afterStart+i < len(oldLines); i++ {
-		hunk.WriteString(" " + oldLines[afterStart+i] + "\n")
+		writeDiffLine(&hunk, ' ', oldLines[afterStart+i])
 	}
 
 	// Calculate hunk size
@@ -291,24 +298,36 @@ func generateAddedDiff(path string, content []byte) string {
 	fmt.Fprintf(&buf, "@@ -0,0 +1,%d @@\n", len(lines))
 
 	for _, line := range lines {
-		buf.WriteString("+" + string(line) + "\n")
+		buf.WriteByte('+')
+		buf.Write(line)
+		buf.WriteByte('\n')
 	}
 
 	return buf.String()
 }
 
+func writeDiffLine(b *strings.Builder, prefix byte, line string) {
+	b.WriteByte(prefix)
+	b.WriteString(line)
+	b.WriteByte('\n')
+}
+
 var diffPathReplacer = strings.NewReplacer("\n", "", "\r", "")
 
 func sanitizeDiffPath(path string) string {
 	return diffPathReplacer.Replace(path)
 }
 
-// countLines counts the number of lines in content.
+// countLines counts the number of lines in content. bufio.Scanner is not
+// used here because it silently stops on any line longer than 64 KiB
+// (minified JS, source maps) and returns a short count.
 func countLines(content []byte) int {
-	scanner := bufio.NewScanner(bytes.NewReader(content))
-	count := 0
-	for scanner.Scan() {
-		count++
+	if len(content) == 0 {
+		return 0
+	}
+	n := bytes.Count(content, []byte{'\n'})
+	if content[len(content)-1] != '\n' {
+		n++
 	}
-	return count
+	return n
 }

diff/diff_test.go

@@ -86,11 +86,28 @@ func TestCompare(t *testing.T) {
 	// Check deleted file
 	if f, ok := fileMap["deleted.txt"]; !ok || f.Type != TypeDeleted {
 		t.Error("deleted.txt should be marked as deleted")
+	} else if f.LinesDeleted != 1 {
+		t.Errorf("deleted.txt LinesDeleted = %d, want 1", f.LinesDeleted)
 	}
 
 	// Check added file
 	if f, ok := fileMap["added.txt"]; !ok || f.Type != TypeAdded {
 		t.Error("added.txt should be marked as added")
+	} else if f.LinesAdded != 1 {
+		t.Errorf("added.txt LinesAdded = %d, want 1", f.LinesAdded)
+	}
+
+	// Totals include added and deleted files as well as modified hunks.
+	var wantAdded, wantDeleted int
+	for _, f := range result.Files {
+		wantAdded += f.LinesAdded
+		wantDeleted += f.LinesDeleted
+	}
+	if result.TotalAdded != wantAdded {
+		t.Errorf("TotalAdded = %d, want sum of per-file %d", result.TotalAdded, wantAdded)
+	}
+	if result.TotalDeleted != wantDeleted {
+		t.Errorf("TotalDeleted = %d, want sum of per-file %d", result.TotalDeleted, wantDeleted)
 	}
 
 	// Check modified files
@@ -195,6 +212,8 @@ func TestCountLines(t *testing.T) {
 		{"one line", []byte("hello"), 1},
 		{"three lines", []byte("line1\nline2\nline3"), 3},
 		{"trailing newline", []byte("line1\nline2\n"), 2},
+		{"only newline", []byte("\n"), 1},
+		{"long line", append(bytes.Repeat([]byte{'x'}, 100_000), '\n', 'y'), 2},
 	}
 
 	for _, tt := range tests {
@@ -207,6 +226,26 @@ func TestCountLines(t *testing.T) {
 	}
 }
 
+func BenchmarkGenerateAddedDiff(b *testing.B) {
+	content := bytes.Repeat([]byte("the quick brown fox jumps over the lazy dog\n"), 10_000)
+	b.SetBytes(int64(len(content)))
+	b.ReportAllocs()
+	for b.Loop() {
+		_ = generateAddedDiff("bench.txt", content)
+	}
+}
+
+func BenchmarkGenerateSimpleDiff(b *testing.B) {
+	line := []byte("the quick brown fox jumps over the lazy dog\n")
+	oldContent := bytes.Repeat(line, 5000)
+	newContent := append(bytes.Repeat(line, 2500), bytes.Repeat([]byte("THE QUICK BROWN FOX\n"), 2500)...)
+	b.SetBytes(int64(len(oldContent) + len(newContent)))
+	b.ReportAllocs()
+	for b.Loop() {
+		_, _, _ = generateSimpleDiff("bench.txt", oldContent, newContent)
+	}
+}
+
 func TestCompareIdentical(t *testing.T) {
 	files := map[string]string{
 		"README.md": "# Test\n",