strings: add IndexRune tests, ASCII fast path

$ gotest -test.v -test.run=IndexRune -test.bench=.*
=== RUN  strings_test.TestIndexRune
--- PASS: strings_test.TestIndexRune (0.0 seconds)
PASS
strings_test.BenchmarkIndexRune	20000000   105 ns/op
strings_test.BenchmarkIndexByte	50000000    48 ns/op

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/4267050

src/pkg/strings/strings.go

@@ -119,9 +119,19 @@ func LastIndex(s, sep string) int {
 // IndexRune returns the index of the first instance of the Unicode code point
 // rune, or -1 if rune is not present in s.
 func IndexRune(s string, rune int) int {
-	for i, c := range s {
+	switch {
-		if c == rune {
+	case rune < 0x80:
-			return i
+		b := byte(rune)
+		for i := 0; i < len(s); i++ {
+			if s[i] == b {
+				return i
+			}
+		}
+	default:
+		for i, c := range s {
+			if c == rune {
+				return i
+			}
 		}
 	}
 	return -1

src/pkg/strings/strings_test.go

@@ -6,6 +6,7 @@ package strings_test
 
 import (
 	"os"
+	"strconv"
 	. "strings"
 	"testing"
 	"unicode"
@@ -116,6 +117,45 @@ func TestLastIndex(t *testing.T)    { runIndexTests(t, LastIndex, "LastIndex", l
 func TestIndexAny(t *testing.T)     { runIndexTests(t, IndexAny, "IndexAny", indexAnyTests) }
 func TestLastIndexAny(t *testing.T) { runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests) }
 
+type IndexRuneTest struct {
+	s    string
+	rune int
+	out  int
+}
+
+var indexRuneTests = []IndexRuneTest{
+	{"a A x", 'A', 2},
+	{"some_text=some_value", '=', 9},
+	{"☺a", 'a', 3},
+	{"a☻☺b", '☺', 4},
+}
+
+func TestIndexRune(t *testing.T) {
+	for _, test := range indexRuneTests {
+		if actual := IndexRune(test.s, test.rune); actual != test.out {
+			t.Errorf("IndexRune(%q,%d)= %v; want %v", test.s, test.rune, actual, test.out)
+		}
+	}
+}
+
+func BenchmarkIndexRune(b *testing.B) {
+	if got := IndexRune("some_text=some☺value", '☺'); got != 14 {
+		panic("wrong index: got=" + strconv.Itoa(got))
+	}
+	for i := 0; i < b.N; i++ {
+		IndexRune("some_text=some☺value", '☺')
+	}
+}
+
+func BenchmarkIndexByte(b *testing.B) {
+	if got := IndexRune("some_text=some☺value", 'v'); got != 17 {
+		panic("wrong index: got=" + strconv.Itoa(got))
+	}
+	for i := 0; i < b.N; i++ {
+		IndexRune("some_text=some☺value", 'v')
+	}
+}
+
 type ExplodeTest struct {
 	s string
 	n int