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, 46 insertions, 4 deletions

diff --git a/src/pkg/strings/example_test.go b/src/pkg/strings/example_test.go
index 733caf5f2d..36e0a42fb0 100644
--- a/src/pkg/strings/example_test.go
+++ b/src/pkg/strings/example_test.go
@@ -179,3 +179,19 @@ func ExampleToLower() {
 	fmt.Println(strings.ToLower("Gopher"))
 	// Output: gopher
 }
+
+func ExampleTrimSuffix() {
+	var s = "Hello, goodbye, etc!"
+	s = strings.TrimSuffix(s, "goodbye, etc!")
+	s = strings.TrimSuffix(s, "planet")
+	fmt.Print(s, "world!")
+	// Output: Hello, world!
+}
+
+func ExampleTrimPrefix() {
+	var s = "Goodbye,, world!"
+	s = strings.TrimPrefix(s, "Goodbye,")
+	s = strings.TrimPrefix(s, "Howdy,")
+	fmt.Print("Hello" + s)
+	// Output: Hello, world!
+}
diff --git a/src/pkg/strings/strings.go b/src/pkg/strings/strings.go
index b411ba5d8b..d4b3f03473 100644
--- a/src/pkg/strings/strings.go
+++ b/src/pkg/strings/strings.go
@@ -558,6 +558,24 @@ func TrimSpace(s string) string {
 	return TrimFunc(s, unicode.IsSpace)
 }
 
+// TrimPrefix returns s without the provided leading prefix string.
+// If s doesn't start with prefix, s is returned unchanged.
+func TrimPrefix(s, prefix string) string {
+	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 string) string {
+	if HasSuffix(s, suffix) {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}
+
 // Replace returns a copy of the string s with the first n
 // non-overlapping instances of old replaced by new.
 // If n < 0, there is no limit on the number of replacements.
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index 7be41a8dca..e222af14a7 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -496,8 +496,8 @@ func TestSpecialCase(t *testing.T) {
 func TestTrimSpace(t *testing.T) { runStringTests(t, TrimSpace, "TrimSpace", trimSpaceTests) }
 
 var trimTests = []struct {
-	f               string
-	in, cutset, out string
+	f            string
+	in, arg, out string
 }{
 	{"Trim", "abba", "a", "bb"},
 	{"Trim", "abba", "ab", ""},
@@ -520,6 +520,10 @@ var trimTests = []struct {
 	{"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) {
@@ -533,12 +537,16 @@ func TestTrim(t *testing.T) {
 			f = TrimLeft
 		case "TrimRight":
 			f = TrimRight
+		case "TrimPrefix":
+			f = TrimPrefix
+		case "TrimSuffix":
+			f = TrimSuffix
 		default:
 			t.Errorf("Undefined trim function %s", name)
 		}
-		actual := f(tc.in, tc.cutset)
+		actual := f(tc.in, 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)
 		}
 	}
 }