Speed up NextLabel and PrevLabel (#1039)

* change NextLabel and PrevLabel to be faster

This reduces readability, but they are in the hot path of coredns.

* @redyeti pointed out, that my implementation disregarded triple backslashes

* add synthetic benchmark-tests for PrevLabel and NextLabel

* rename ii -> j

* invert compare

* PrevLabel: add empty string check + test case

* NextLabel: fix and add testcase for NextLabel("", offset>0)

labels.go

@@ -126,20 +126,23 @@ func Split(s string) []int {
 // The bool end is true when the end of the string has been reached.
 // Also see PrevLabel.
 func NextLabel(s string, offset int) (i int, end bool) {
-	quote := false
+	if s == "" {
+		return 0, true
+	}
 	for i = offset; i < len(s)-1; i++ {
-		switch s[i] {
-		case '\\':
-			quote = !quote
-		default:
-			quote = false
-		case '.':
-			if quote {
-				quote = !quote
-				continue
-			}
-			return i + 1, false
+		if s[i] != '.' {
+			continue
+		}
+		j := i - 1
+		for j >= 0 && s[j] == '\\' {
+			j--
+		}
+
+		if (j-i)%2 == 0 {
+			continue
 		}
+
+		return i + 1, false
 	}
 	return i + 1, true
 }
@@ -149,17 +152,38 @@ func NextLabel(s string, offset int) (i int, end bool) {
 // The bool start is true when the start of the string has been overshot.
 // Also see NextLabel.
 func PrevLabel(s string, n int) (i int, start bool) {
+	if s == "" {
+		return 0, true
+	}
 	if n == 0 {
 		return len(s), false
 	}
-	lab := Split(s)
-	if lab == nil {
-		return 0, true
+
+	l := len(s) - 1
+	if s[l] == '.' {
+		l--
 	}
-	if n > len(lab) {
-		return 0, true
+
+	for ; l >= 0 && n > 0; l-- {
+		if s[l] != '.' {
+			continue
+		}
+		j := l - 1
+		for j >= 0 && s[j] == '\\' {
+			j--
+		}
+
+		if (j-l)%2 == 0 {
+			continue
+		}
+
+		n--
+		if n == 0 {
+			return l + 1, false
+		}
 	}
-	return lab[len(lab)-n], false
+
+	return 0, n > 1
 }
 
 // equal compares a and b while ignoring case. It returns true when equal otherwise false.

labels_test.go

@@ -40,16 +40,17 @@ func TestCompareDomainName(t *testing.T) {
 
 func TestSplit(t *testing.T) {
 	splitter := map[string]int{
-		"www.miek.nl.":   3,
-		"www.miek.nl":    3,
-		"www..miek.nl":   4,
-		`www\.miek.nl.`:  2,
-		`www\\.miek.nl.`: 3,
-		".":              0,
-		"nl.":            1,
-		"nl":             1,
-		"com.":           1,
-		".com.":          2,
+		"www.miek.nl.":    3,
+		"www.miek.nl":     3,
+		"www..miek.nl":    4,
+		`www\.miek.nl.`:   2,
+		`www\\.miek.nl.`:  3,
+		`www\\\.miek.nl.`: 2,
+		".":               0,
+		"nl.":             1,
+		"nl":              1,
+		"com.":            1,
+		".com.":           2,
 	}
 	for s, i := range splitter {
 		if x := len(Split(s)); x != i {
@@ -79,12 +80,32 @@ func TestSplit2(t *testing.T) {
 	}
 }
 
+func TestNextLabel(t *testing.T) {
+	type next struct {
+		string
+		int
+	}
+	nexts := map[next]int{
+		{"", 1}:             0,
+		{"www.miek.nl.", 0}: 4,
+		{"www.miek.nl.", 4}: 9,
+		{"www.miek.nl.", 9}: 12,
+	}
+	for s, i := range nexts {
+		x, ok := NextLabel(s.string, s.int)
+		if i != x {
+			t.Errorf("label should be %d, got %d, %t: nexting %d, %s", i, x, ok, s.int, s.string)
+		}
+	}
+}
+
 func TestPrevLabel(t *testing.T) {
 	type prev struct {
 		string
 		int
 	}
 	prever := map[prev]int{
+		{"", 1}:             0,
 		{"www.miek.nl.", 0}: 12,
 		{"www.miek.nl.", 1}: 9,
 		{"www.miek.nl.", 2}: 4,
@@ -237,3 +258,63 @@ func BenchmarkIsSubDomain(b *testing.B) {
 		IsSubDomain("miek.nl.", "aa.example.com.")
 	}
 }
+
+func BenchmarkNextLabelSimple(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		NextLabel("www.example.com", 0)
+		NextLabel("www.example.com", 5)
+		NextLabel("www.example.com", 12)
+	}
+}
+
+func BenchmarkPrevLabelSimple(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		PrevLabel("www.example.com", 0)
+		PrevLabel("www.example.com", 5)
+		PrevLabel("www.example.com", 12)
+	}
+}
+
+func BenchmarkNextLabelComplex(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		NextLabel(`www\.example.com`, 0)
+		NextLabel(`www\\.example.com`, 0)
+		NextLabel(`www\\\.example.com`, 0)
+	}
+}
+
+func BenchmarkPrevLabelComplex(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		PrevLabel(`www\.example.com`, 10)
+		PrevLabel(`www\\.example.com`, 10)
+		PrevLabel(`www\\\.example.com`, 10)
+	}
+}
+
+func BenchmarkNextLabelMixed(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		NextLabel("www.example.com", 0)
+		NextLabel(`www\.example.com`, 0)
+		NextLabel("www.example.com", 5)
+		NextLabel(`www\\.example.com`, 0)
+		NextLabel("www.example.com", 12)
+		NextLabel(`www\\\.example.com`, 0)
+	}
+}
+
+func BenchmarkPrevLabelMixed(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		PrevLabel("www.example.com", 0)
+		PrevLabel(`www\.example.com`, 10)
+		PrevLabel("www.example.com", 5)
+		PrevLabel(`www\\.example.com`, 10)
+		PrevLabel("www.example.com", 12)
+		PrevLabel(`www\\\.example.com`, 10)
+	}
+}