bytes, strings: optimize Split*

The relevant benchmark results on linux/amd64:

bytes:

SplitSingleByteSeparator-4   25.7ms ± 5%   9.1ms ± 4%  -64.40%  (p=0.000 n=10+10)
SplitMultiByteSeparator-4    13.8ms ±20%   4.3ms ± 8%  -69.23%  (p=0.000 n=10+10)
SplitNSingleByteSeparator-4  1.88µs ± 9%  0.88µs ± 4%  -53.25%  (p=0.000 n=10+10)
SplitNMultiByteSeparator-4   4.83µs ±10%  1.32µs ± 9%  -72.65%  (p=0.000 n=10+10)

strings:

name                         old time/op  new time/op  delta
SplitSingleByteSeparator-4   21.4ms ± 8%   8.5ms ± 5%  -60.19%  (p=0.000 n=10+10)
SplitMultiByteSeparator-4    13.2ms ± 9%   3.9ms ± 4%  -70.29%  (p=0.000 n=10+10)
SplitNSingleByteSeparator-4  1.54µs ± 5%  0.75µs ± 7%  -51.21%  (p=0.000 n=10+10)
SplitNMultiByteSeparator-4   3.57µs ± 8%  1.01µs ±11%  -71.76%  (p=0.000 n=10+10)

Fixes #18973

Change-Id: Ie4bc010c6cc389001e72eab530497c81e5b26f34
Reviewed-on: https://go-review.googlesource.com/36510Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/bytes.go

@@ -215,20 +215,21 @@ func genSplit(s, sep []byte, sepSave, n int) [][]byte {
 	if n < 0 {
 		n = Count(s, sep) + 1
 	}
-	c := sep[0]
-	start := 0
+
 	a := make([][]byte, n)
-	na := 0
-	for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
-		if s[i] == c && (len(sep) == 1 || Equal(s[i:i+len(sep)], sep)) {
-			a[na] = s[start : i+sepSave]
-			na++
-			start = i + len(sep)
-			i += len(sep) - 1
+	n--
+	i := 0
+	for i < n {
+		m := Index(s, sep)
+		if m < 0 {
+			break
 		}
+		a[i] = s[:m+sepSave]
+		s = s[m+len(sep):]
+		i++
 	}
-	a[na] = s[start:]
-	return a[0 : na+1]
+	a[i] = s
+	return a[:i+1]
 }
 
 // SplitN slices s into subslices separated by sep and returns a slice of

src/bytes/bytes_test.go

@@ -1432,6 +1432,59 @@ func BenchmarkTrimSpace(b *testing.B) {
 	}
 }
 
+func makeBenchInputHard() []byte {
+	tokens := [...]string{
+		"<a>", "<p>", "<b>", "<strong>",
+		"</a>", "</p>", "</b>", "</strong>",
+		"hello", "world",
+	}
+	x := make([]byte, 0, 1<<20)
+	for {
+		i := rand.Intn(len(tokens))
+		if len(x)+len(tokens[i]) >= 1<<20 {
+			break
+		}
+		x = append(x, tokens[i]...)
+	}
+	return x
+}
+
+var benchInputHard = makeBenchInputHard()
+
+func BenchmarkSplitEmptySeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, nil)
+	}
+}
+
+func BenchmarkSplitSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, sep)
+	}
+}
+
+func BenchmarkSplitMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, sep)
+	}
+}
+
+func BenchmarkSplitNSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, sep, 10)
+	}
+}
+
+func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, sep, 10)
+	}
+}
+
 func BenchmarkRepeat(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Repeat([]byte("-"), 80)

src/strings/strings.go

@@ -239,20 +239,21 @@ func genSplit(s, sep string, sepSave, n int) []string {
 	if n < 0 {
 		n = Count(s, sep) + 1
 	}
-	c := sep[0]
-	start := 0
+
 	a := make([]string, n)
-	na := 0
-	for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
-		if s[i] == c && (len(sep) == 1 || s[i:i+len(sep)] == sep) {
-			a[na] = s[start : i+sepSave]
-			na++
-			start = i + len(sep)
-			i += len(sep) - 1
-		}
+	n--
+	i := 0
+	for i < n {
+		m := Index(s, sep)
+		if m < 0 {
+			break
+		}
+		a[i] = s[:m+sepSave]
+		s = s[m+len(sep):]
+		i++
 	}
-	a[na] = s[start:]
-	return a[0 : na+1]
+	a[i] = s
+	return a[:i+1]
 }
 
 // SplitN slices s into substrings separated by sep and returns a slice of

src/strings/strings_test.go

@@ -1476,24 +1476,36 @@ func BenchmarkFieldsFunc(b *testing.B) {
 	}
 }
 
-func BenchmarkSplit1(b *testing.B) {
+func BenchmarkSplitEmptySeparator(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Split(benchInputHard, "")
 	}
 }
 
-func BenchmarkSplit2(b *testing.B) {
+func BenchmarkSplitSingleByteSeparator(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Split(benchInputHard, "/")
 	}
 }
 
-func BenchmarkSplit3(b *testing.B) {
+func BenchmarkSplitMultiByteSeparator(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Split(benchInputHard, "hello")
 	}
 }
 
+func BenchmarkSplitNSingleByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, "/", 10)
+	}
+}
+
+func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, "hello", 10)
+	}
+}
+
 func BenchmarkRepeat(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Repeat("-", 80)