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

src/cmd/api/goapi.go

@@ -778,8 +778,7 @@ func (w *Walker) walkConst(vs *ast.ValueSpec) {
 				}
 			}
 		}
-		if strings.HasPrefix(litType, constDepPrefix) {
-			dep := litType[len(constDepPrefix):]
+		if dep := strings.TrimPrefix(litType, constDepPrefix); dep != litType {
 			w.constDep[ident.Name] = dep
 			continue
 		}

src/cmd/cgo/gcc.go

@@ -1542,8 +1542,8 @@ func godefsFields(fld []*ast.Field) {
 	npad := 0
 	for _, f := range fld {
 		for _, n := range f.Names {
-			if strings.HasPrefix(n.Name, prefix) && n.Name != prefix {
-				n.Name = n.Name[len(prefix):]
+			if n.Name != prefix {
+				n.Name = strings.TrimPrefix(n.Name, prefix)
 			}
 			if n.Name == "_" {
 				// Use exported name instead.

src/cmd/cgo/godefs.go

@@ -180,7 +180,7 @@ func (p *Package) cdefs(f *File, srcfile string) string {
 			for _, line := range lines {
 				line = strings.TrimSpace(line)
 				if strings.HasPrefix(line, "type ") && strings.HasSuffix(line, " struct {") {
-					s := line[len("type ") : len(line)-len(" struct {")]
+					s := strings.TrimSuffix(strings.TrimPrefix(line, "type "), " struct {")
 					printf("typedef struct %s %s;\n", s, s)
 				}
 			}

src/cmd/fix/typecheck.go

@@ -395,9 +395,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 			// Field or method.
 			name := n.Sel.Name
 			if t := typeof[n.X]; t != "" {
-				if strings.HasPrefix(t, "*") {
-					t = t[1:] // implicit *
-				}
+				t = strings.TrimPrefix(t, "*") // implicit *
 				if typ := cfg.Type[t]; typ != nil {
 					if t := typ.dot(cfg, name); t != "" {
 						typeof[n] = t

src/cmd/go/testflag.go

@@ -195,9 +195,7 @@ func testFlag(args []string, i int) (f *testFlagSpec, value string, extra bool)
 	}
 	name := arg[1:]
 	// If there's already "test.", drop it for now.
-	if strings.HasPrefix(name, "test.") {
-		name = name[5:]
-	}
+	name = strings.TrimPrefix(name, "test.")
 	equals := strings.Index(name, "=")
 	if equals >= 0 {
 		value = name[equals+1:]

src/cmd/godoc/dirtrees.go

@@ -229,9 +229,7 @@ func (dir *Directory) lookupLocal(name string) *Directory {
 }
 
 func splitPath(p string) []string {
-	if strings.HasPrefix(p, "/") {
-		p = p[1:]
-	}
+	p = strings.TrimPrefix(p, "/")
 	if p == "" {
 		return nil
 	}
@@ -310,14 +308,9 @@ func (root *Directory) listing(skipRoot bool) *DirList {
 		// the path is relative to root.Path - remove the root.Path
 		// prefix (the prefix should always be present but avoid
 		// crashes and check)
-		path := d.Path
-		if strings.HasPrefix(d.Path, root.Path) {
-			path = d.Path[len(root.Path):]
-		}
+		path := strings.TrimPrefix(d.Path, root.Path)
 		// remove leading separator if any - path must be relative
-		if len(path) > 0 && path[0] == '/' {
-			path = path[1:]
-		}
+		path = strings.TrimPrefix(path, "/")
 		p.Path = path
 		p.Name = d.Name
 		p.HasPkg = d.HasPkg

src/cmd/godoc/filesystem.go

@@ -459,9 +459,7 @@ func (ns nameSpace) ReadDir(path string) ([]os.FileInfo, error) {
 		if hasPathPrefix(old, path) && old != path {
 			// Find next element after path in old.
 			elem := old[len(path):]
-			if strings.HasPrefix(elem, "/") {
-				elem = elem[1:]
-			}
+			elem = strings.TrimPrefix(elem, "/")
 			if i := strings.Index(elem, "/"); i >= 0 {
 				elem = elem[:i]
 			}

src/cmd/godoc/godoc.go

@@ -419,9 +419,7 @@ func pkgLinkFunc(path string) string {
 	relpath := path[1:]
 	// because of the irregular mapping under goroot
 	// we need to correct certain relative paths
-	if strings.HasPrefix(relpath, "src/pkg/") {
-		relpath = relpath[len("src/pkg/"):]
-	}
+	relpath = strings.TrimPrefix(relpath, "src/pkg/")
 	return pkgHandler.pattern[1:] + relpath // remove trailing '/' for relative URL
 }
 

src/cmd/godoc/main.go

@@ -347,7 +347,7 @@ func main() {
 		fs.Bind(target, OS(path), "/", bindReplace)
 		abspath = target
 	} else if strings.HasPrefix(path, cmdPrefix) {
-		path = path[len(cmdPrefix):]
+		path = strings.TrimPrefix(path, cmdPrefix)
 		forceCmd = true
 	} else if bp, _ := build.Import(path, "", build.FindOnly); bp.Dir != "" && bp.ImportPath != "" {
 		fs.Bind(target, OS(bp.Dir), "/", bindReplace)

src/cmd/vet/method.go

@@ -90,9 +90,7 @@ func (f *File) checkCanonicalMethod(id *ast.Ident, t *ast.FuncType) {
 			fmt.Fprintf(&f.b, "<%s>", err)
 		}
 		actual := f.b.String()
-		if strings.HasPrefix(actual, "func(") {
-			actual = actual[4:]
-		}
+		actual = strings.TrimPrefix(actual, "func(")
 		actual = id.Name + actual
 
 		f.Warnf(id.Pos(), "method %s should have signature %s", actual, expectFmt)

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.

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)
 		}
 	}
 }

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!
+}

src/pkg/exp/html/parse_test.go

@@ -42,10 +42,7 @@ func readParseTest(r *bufio.Reader) (text, want, context string, err error) {
 		}
 		b = append(b, line...)
 	}
-	text = string(b)
-	if strings.HasSuffix(text, "\n") {
-		text = text[:len(text)-1]
-	}
+	text = strings.TrimSuffix(string(b), "\n")
 	b = b[:0]
 
 	// Skip the error list.

src/pkg/go/printer/printer.go

@@ -551,9 +551,7 @@ func stripCommonPrefix(lines []string) {
 			}
 			// Shorten the computed common prefix by the length of
 			// suffix, if it is found as suffix of the prefix.
-			if strings.HasSuffix(prefix, string(suffix)) {
-				prefix = prefix[0 : len(prefix)-len(suffix)]
-			}
+			prefix = strings.TrimSuffix(prefix, string(suffix))
 		}
 	}
 

src/pkg/go/types/gcimporter.go

@@ -44,10 +44,7 @@ func FindPkg(path, srcDir string) (filename, id string) {
 		if bp.PkgObj == "" {
 			return
 		}
-		noext = bp.PkgObj
-		if strings.HasSuffix(noext, ".a") {
-			noext = noext[:len(noext)-len(".a")]
-		}
+		noext = strings.TrimSuffix(bp.PkgObj, ".a")
 
 	case build.IsLocalImport(path):
 		// "./x" -> "/this/directory/x.ext", "/this/directory/x"

src/pkg/net/http/pprof/pprof.go

@@ -172,7 +172,7 @@ func (name handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // listing the available profiles.
 func Index(w http.ResponseWriter, r *http.Request) {
 	if strings.HasPrefix(r.URL.Path, "/debug/pprof/") {
-		name := r.URL.Path[len("/debug/pprof/"):]
+		name := strings.TrimPrefix(r.URL.Path, "/debug/pprof/")
 		if name != "" {
 			handler(name).ServeHTTP(w, r)
 			return

src/pkg/net/http/response.go

@@ -198,9 +198,7 @@ func (r *Response) Write(w io.Writer) error {
 	}
 	protoMajor, protoMinor := strconv.Itoa(r.ProtoMajor), strconv.Itoa(r.ProtoMinor)
 	statusCode := strconv.Itoa(r.StatusCode) + " "
-	if strings.HasPrefix(text, statusCode) {
-		text = text[len(statusCode):]
-	}
+	text = strings.TrimPrefix(text, statusCode)
 	io.WriteString(w, "HTTP/"+protoMajor+"."+protoMinor+" "+statusCode+text+"\r\n")
 
 	// Process Body,ContentLength,Close,Trailer

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!
+}

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.

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)
 		}
 	}
 }