add bytes.IndexByte; common case we can make fast later.

also pick off the special case in strings.Index.   don't want strings.IndexByte
because the call site will very rarely need to allocate and we can handle the
test in the code itself.   bytes.IndexByte can avoid a common allocation.

R=rsc
CC=golang-dev
https://golang.org/cl/156091

src/pkg/bytes/bytes.go

@@ -98,6 +98,16 @@ func Index(s, sep []byte) int {
 	return -1;
 }
 
+// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
+func IndexByte(s []byte, c byte) int {
+	for i, b := range s {
+		if b == c {
+			return i
+		}
+	}
+	return -1;
+}
+
 // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
 func LastIndex(s, sep []byte) int {
 	n := len(sep);

src/pkg/bytes/bytes_test.go

@@ -39,41 +39,83 @@ var faces = "☺☻☹"
 var commas = "1,2,3,4"
 var dots = "1....2....3....4"
 
-type CompareTest struct {
+type BinOpTest struct {
 	a	string;
 	b	string;
-	cmp	int;
+	i	int;
 }
 
-var comparetests = []CompareTest{
-	CompareTest{"", "", 0},
-	CompareTest{"a", "", 1},
-	CompareTest{"", "a", -1},
-	CompareTest{"abc", "abc", 0},
-	CompareTest{"ab", "abc", -1},
-	CompareTest{"abc", "ab", 1},
-	CompareTest{"x", "ab", 1},
-	CompareTest{"ab", "x", -1},
-	CompareTest{"x", "a", 1},
-	CompareTest{"b", "x", -1},
+var comparetests = []BinOpTest{
+	BinOpTest{"", "", 0},
+	BinOpTest{"a", "", 1},
+	BinOpTest{"", "a", -1},
+	BinOpTest{"abc", "abc", 0},
+	BinOpTest{"ab", "abc", -1},
+	BinOpTest{"abc", "ab", 1},
+	BinOpTest{"x", "ab", 1},
+	BinOpTest{"ab", "x", -1},
+	BinOpTest{"x", "a", 1},
+	BinOpTest{"b", "x", -1},
 }
 
 func TestCompare(t *testing.T) {
-	for i := 0; i < len(comparetests); i++ {
-		tt := comparetests[i];
+	for _, tt := range comparetests {
 		a := strings.Bytes(tt.a);
 		b := strings.Bytes(tt.b);
 		cmp := Compare(a, b);
 		eql := Equal(a, b);
-		if cmp != tt.cmp {
+		if cmp != tt.i {
 			t.Errorf(`Compare(%q, %q) = %v`, tt.a, tt.b, cmp)
 		}
-		if eql != (tt.cmp == 0) {
+		if eql != (tt.i == 0) {
 			t.Errorf(`Equal(%q, %q) = %v`, tt.a, tt.b, eql)
 		}
 	}
 }
 
+var indextests = []BinOpTest{
+	BinOpTest{"", "", 0},
+	BinOpTest{"a", "", 0},
+	BinOpTest{"", "a", -1},
+	BinOpTest{"abc", "abc", 0},
+	BinOpTest{"ab", "abc", -1},
+	BinOpTest{"abc", "bc", 1},
+	BinOpTest{"x", "ab", -1},
+	// one-byte tests for IndexByte
+	BinOpTest{"ab", "x", -1},
+	BinOpTest{"", "a", -1},
+	BinOpTest{"x", "a", -1},
+	BinOpTest{"x", "x", 0},
+	BinOpTest{"abc", "a", 0},
+	BinOpTest{"abc", "b", 1},
+	BinOpTest{"abc", "c", 2},
+	BinOpTest{"abc", "x", -1},
+}
+
+func TestIndex(t *testing.T) {
+	for _, tt := range indextests {
+		a := strings.Bytes(tt.a);
+		b := strings.Bytes(tt.b);
+		pos := Index(a, b);
+		if pos != tt.i {
+			t.Errorf(`Index(%q, %q) = %v`, tt.a, tt.b, pos)
+		}
+	}
+}
+
+func TestIndexByte(t *testing.T) {
+	for _, tt := range indextests {
+		if len(tt.b) != 1 {
+			continue
+		}
+		a := strings.Bytes(tt.a);
+		b := tt.b[0];
+		pos := IndexByte(a, b);
+		if pos != tt.i {
+			t.Errorf(`IndexByte(%q, '%c') = %v`, tt.a, b, pos)
+		}
+	}
+}
 
 type ExplodeTest struct {
 	s	string;

src/pkg/strings/strings.go

@@ -56,6 +56,15 @@ func Index(s, sep string) int {
 		return 0
 	}
 	c := sep[0];
+	if n == 1 {
+		// special case worth making fast
+		for i := 0; i < len(s); i++ {
+			if s[i] == c {
+				return i
+			}
+		}
+		return -1;
+	}
 	for i := 0; i+n <= len(s); i++ {
 		if s[i] == c && (n == 1 || s[i:i+n] == sep) {
 			return i
@@ -71,6 +80,15 @@ func LastIndex(s, sep string) int {
 		return len(s)
 	}
 	c := sep[0];
+	if n == 1 {
+		// special case worth making fast
+		for i := len(s) - 1; i >= 0; i-- {
+			if s[i] == c {
+				return i
+			}
+		}
+		return -1;
+	}
 	for i := len(s) - n; i >= 0; i-- {
 		if s[i] == c && (n == 1 || s[i:i+n] == sep) {
 			return i

src/pkg/strings/strings_test.go

@@ -46,6 +46,14 @@ var indexTests = []IndexTest{
 	IndexTest{"foo", "", 0},
 	IndexTest{"foo", "o", 1},
 	IndexTest{"abcABCabc", "A", 3},
+	// cases with one byte strings - test special case in Index()
+	IndexTest{"", "a", -1},
+	IndexTest{"x", "a", -1},
+	IndexTest{"x", "x", 0},
+	IndexTest{"abc", "a", 0},
+	IndexTest{"abc", "b", 1},
+	IndexTest{"abc", "c", 2},
+	IndexTest{"abc", "x", -1},
 }
 
 var lastIndexTests = []IndexTest{
@@ -54,6 +62,7 @@ var lastIndexTests = []IndexTest{
 	IndexTest{"", "foo", -1},
 	IndexTest{"fo", "foo", -1},
 	IndexTest{"foo", "foo", 0},
+	IndexTest{"foo", "f", 0},
 	IndexTest{"oofofoofooo", "f", 7},
 	IndexTest{"oofofoofooo", "foo", 7},
 	IndexTest{"barfoobarfoo", "foo", 9},