Improve x-subrip detection performance (#524)

* srt: improve performance by using bytes.Cut instead of bufio.Scanner

it helps with mem allocs
Before:
BenchmarkSrt-8   	  946042	      1089 ns/op	    4240 B/op	       5 allocs/op
After:
BenchmarkSrt-8   	 3235448	       368.8 ns/op	      64 B/op	       2 allocs/op

internal/magic/text.go

@@ -302,19 +302,21 @@ func Svg(raw []byte, limit uint32) bool {
 
 // Srt matches a SubRip file.
 func Srt(in []byte, _ uint32) bool {
-	s := bufio.NewScanner(bytes.NewReader(in))
-	if !s.Scan() {
+	line, in, found := scanLine(in)
+	if !found {
 		return false
 	}
+
 	// First line must be 1.
-	if s.Text() != "1" {
+	if string(line) != "1" {
 		return false
 	}
-
-	if !s.Scan() {
+	line, in, found = scanLine(in)
+	if !found {
 		return false
 	}
-	secondLine := s.Text()
+
+	secondLine := string(line)
 	// Timestamp format (e.g: 00:02:16,612 --> 00:02:19,376) limits secondLine
 	// length to exactly 29 characters.
 	if len(secondLine) != 29 {
@@ -325,14 +327,12 @@ func Srt(in []byte, _ uint32) bool {
 	if strings.Contains(secondLine, ".") {
 		return false
 	}
-	// For Go <1.17, comma is not recognised as a decimal separator by `time.Parse`.
-	secondLine = strings.ReplaceAll(secondLine, ",", ".")
 	// Second line must be a time range.
 	ts := strings.Split(secondLine, " --> ")
 	if len(ts) != 2 {
 		return false
 	}
-	const layout = "15:04:05.000"
+	const layout = "15:04:05,000"
 	t0, err := time.Parse(layout, ts[0])
 	if err != nil {
 		return false
@@ -345,8 +345,9 @@ func Srt(in []byte, _ uint32) bool {
 		return false
 	}
 
+	line, _, found = scanLine(in)
 	// A third line must exist and not be empty. This is the actual subtitle text.
-	return s.Scan() && len(s.Bytes()) != 0
+	return found && len(line) != 0
 }
 
 // Vtt matches a Web Video Text Tracks (WebVTT) file. See
@@ -373,3 +374,16 @@ func Vtt(raw []byte, limit uint32) bool {
 	return bytes.Equal(raw, []byte{0xEF, 0xBB, 0xBF, 0x57, 0x45, 0x42, 0x56, 0x54, 0x54}) || // UTF-8 BOM and "WEBVTT"
 		bytes.Equal(raw, []byte{0x57, 0x45, 0x42, 0x56, 0x54, 0x54}) // "WEBVTT"
 }
+
+func scanLine(in []byte) (line, remainder []byte, found bool) {
+	line, remainder, found = bytes.Cut(in, []byte("\n"))
+	if !found {
+		return
+	}
+
+	// Drop off any \r before \n.
+	if lenLine := len(line); lenLine > 0 && line[lenLine-1] == '\r' {
+		line = line[:lenLine-1]
+	}
+	return
+}

mimetype_test.go

@@ -176,18 +176,13 @@ var files = map[string]string{
 	"so.so":              "application/x-sharedlib",
 	"sqlite.sqlite":      "application/vnd.sqlite3",
 	"srt.srt":            "application/x-subrip",
-	// not.srt.txt uses periods instead of commas for the decimal separators of
-	// the timestamps.
-	"not.srt.txt": "text/plain; charset=utf-8",
-	// not.srt.2.txt does not specify milliseconds.
-	"not.srt.2.txt":  "text/plain; charset=utf-8",
-	"svg.1.svg":      "image/svg+xml",
-	"svg.svg":        "image/svg+xml",
-	"swf.swf":        "application/x-shockwave-flash",
-	"tar.tar":        "application/x-tar",
-	"tar.gnu.tar":    "application/x-tar",
-	"tar.oldgnu.tar": "application/x-tar",
-	"tar.posix.tar":  "application/x-tar",
+	"svg.1.svg":          "image/svg+xml",
+	"svg.svg":            "image/svg+xml",
+	"swf.swf":            "application/x-shockwave-flash",
+	"tar.tar":            "application/x-tar",
+	"tar.gnu.tar":        "application/x-tar",
+	"tar.oldgnu.tar":     "application/x-tar",
+	"tar.posix.tar":      "application/x-tar",
 	// tar.star.tar was generated with star 1.6.
 	"tar.star.tar":     "application/x-tar",
 	"tar.ustar.tar":    "application/x-tar",
@@ -491,6 +486,7 @@ func BenchmarkSliceRand(b *testing.B) {
 	}
 
 	b.ResetTimer()
+	b.ReportAllocs()
 
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
@@ -499,6 +495,24 @@ func BenchmarkSliceRand(b *testing.B) {
 	})
 }
 
+func BenchmarkAll(b *testing.B) {
+	r := rand.New(rand.NewSource(0))
+	data := make([]byte, 3072)
+	if _, err := io.ReadFull(r, data); err != io.ErrUnexpectedEOF && err != nil {
+		b.Fatal(err)
+	}
+	for _, m := range root.flatten() {
+		b.Run(m.String(), func(b *testing.B) {
+			b.ReportAllocs()
+			b.ResetTimer()
+			for n := 0; n < b.N; n++ {
+				m.detector(data, uint32(len(data)))
+			}
+		})
+	}
+
+}
+
 func BenchmarkCommon(b *testing.B) {
 	commonFiles := []string{
 		"xlsx.xlsx",
@@ -512,13 +526,15 @@ func BenchmarkCommon(b *testing.B) {
 		"gif.gif",
 		"xls.xls",
 		"webm.webm",
+		"csv.csv",
 	}
 	for _, file := range commonFiles {
+		f, err := os.ReadFile(filepath.Join(testDataDir, file))
+		if err != nil {
+			b.Fatal(err)
+		}
 		b.Run(filepath.Ext(file), func(b *testing.B) {
-			f, err := os.ReadFile(testDataDir + file)
-			if err != nil {
-				b.Fatal(err)
-			}
+			b.ReportAllocs()
 			b.ResetTimer()
 			for n := 0; n < b.N; n++ {
 				Detect(f)