bytes, strings: add TrimPrefix and TrimSuffix

Everybody either gets confused and thinks this is
TrimLeft/TrimRight or does this by hand which gets
repetitive looking.

R=rsc, kevlar
CC=golang-dev
https://golang.org/cl/7239044

3 files changed, 53 insertions, 4 deletions

diff --git a/src/pkg/bytes/bytes.go b/src/pkg/bytes/bytes.go
index e3ee5b1d88..31cf89ea87 100644
--- a/src/pkg/bytes/bytes.go
+++ b/src/pkg/bytes/bytes.go
@@ -515,6 +515,24 @@ func TrimFunc(s []byte, f func(r rune) bool) []byte {
 	return TrimRightFunc(TrimLeftFunc(s, f), f)
 }
 
+// TrimPrefix returns s without the provided leading prefix string.
+// If s doesn't start with prefix, s is returned unchanged.
+func TrimPrefix(s, prefix []byte) []byte {
+	if HasPrefix(s, prefix) {
+		return s[len(prefix):]
+	}
+	return s
+}
+
+// TrimSuffix returns s without the provided trailing suffix string.
+// If s doesn't end with suffix, s is returned unchanged.
+func TrimSuffix(s, suffix []byte) []byte {
+	if HasSuffix(s, suffix) {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}
+
 // IndexFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
 // It returns the byte index in s of the first Unicode
 // code point satisfying f(c), or -1 if none do.
diff --git a/src/pkg/bytes/bytes_test.go b/src/pkg/bytes/bytes_test.go
index 05956d460a..1d073b143b 100644
--- a/src/pkg/bytes/bytes_test.go
+++ b/src/pkg/bytes/bytes_test.go
@@ -794,8 +794,8 @@ func TestRunes(t *testing.T) {
 }
 
 type TrimTest struct {
-	f               string
-	in, cutset, out string
+	f            string
+	in, arg, out string
 }
 
 var trimTests = []TrimTest{
@@ -820,12 +820,17 @@ var trimTests = []TrimTest{
 	{"TrimRight", "", "123", ""},
 	{"TrimRight", "", "", ""},
 	{"TrimRight", "☺\xc0", "☺", "☺\xc0"},
+	{"TrimPrefix", "aabb", "a", "abb"},
+	{"TrimPrefix", "aabb", "b", "aabb"},
+	{"TrimSuffix", "aabb", "a", "aabb"},
+	{"TrimSuffix", "aabb", "b", "aab"},
 }
 
 func TestTrim(t *testing.T) {
 	for _, tc := range trimTests {
 		name := tc.f
 		var f func([]byte, string) []byte
+		var fb func([]byte, []byte) []byte
 		switch name {
 		case "Trim":
 			f = Trim
@@ -833,12 +838,21 @@ func TestTrim(t *testing.T) {
 			f = TrimLeft
 		case "TrimRight":
 			f = TrimRight
+		case "TrimPrefix":
+			fb = TrimPrefix
+		case "TrimSuffix":
+			fb = TrimSuffix
 		default:
 			t.Errorf("Undefined trim function %s", name)
 		}
-		actual := string(f([]byte(tc.in), tc.cutset))
+		var actual string
+		if f != nil {
+			actual = string(f([]byte(tc.in), tc.arg))
+		} else {
+			actual = string(fb([]byte(tc.in), []byte(tc.arg)))
+		}
 		if actual != tc.out {
-			t.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.cutset, actual, tc.out)
+			t.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.arg, actual, tc.out)
 		}
 	}
 }
diff --git a/src/pkg/bytes/example_test.go b/src/pkg/bytes/example_test.go
index dc66b6a40f..ad2dbc69b7 100644
--- a/src/pkg/bytes/example_test.go
+++ b/src/pkg/bytes/example_test.go
@@ -66,3 +66,20 @@ func ExampleCompare_search() {
 		// Found it!
 	}
 }
+
+func ExampleTrimSuffix() {
+	var b = []byte("Hello, goodbye, etc!")
+	b = bytes.TrimSuffix(b, []byte("goodbye, etc!"))
+	b = bytes.TrimSuffix(b, []byte("gopher"))
+	b = append(b, bytes.TrimSuffix([]byte("world!"), []byte("x!"))...)
+	os.Stdout.Write(b)
+	// Output: Hello, world!
+}
+
+func ExampleTrimPrefix() {
+	var b = []byte("Goodbye,, world!")
+	b = bytes.TrimPrefix(b, []byte("Goodbye,"))
+	b = bytes.TrimPrefix(b, []byte("See ya,"))
+	fmt.Printf("Hello%s", b)
+	// Output: Hello, world!
+}