strings: add Replacer, NewReplacer

This is just a new API to do many replacements at once.

While the point of this API is to be faster than doing replacements one
at a time, the implementation in this CL has the optimizations removed
and may actually be slower.

Future CLs will bring back & add optimizations.

R=r, rsc, rogpeppe
CC=golang-dev
https://golang.org/cl/5081042

src/pkg/strings/Makefile

@@ -7,6 +7,7 @@ include ../../Make.inc
 TARG=strings
 GOFILES=\
 	reader.go\
+	replace.go\
 	strings.go\
 
 include ../../Make.pkg

src/pkg/strings/replace.go

@@ -0,0 +1,84 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"io"
+	"os"
+)
+
+// Can't import ioutil for ioutil.Discard, due to ioutil/tempfile.go -> strconv -> strings
+var discard io.Writer = devNull(0)
+
+type devNull int
+
+func (devNull) Write(p []byte) (int, os.Error) {
+	return len(p), nil
+}
+
+type pair struct{ old, new string }
+
+// A Replacer replaces a list of strings with replacements.
+type Replacer struct {
+	p []pair
+}
+
+// NewReplacer returns a new Replacer from a list of old, new string pairs.
+// Replacements are performed in order, without overlapping matches.
+func NewReplacer(oldnew ...string) *Replacer {
+	if len(oldnew)%2 == 1 {
+		panic("strings.NewReplacer: odd argument count")
+	}
+	r := new(Replacer)
+	for len(oldnew) >= 2 {
+		r.p = append(r.p, pair{oldnew[0], oldnew[1]})
+		oldnew = oldnew[2:]
+	}
+	return r
+}
+
+type appendSliceWriter struct {
+	b []byte
+}
+
+func (w *appendSliceWriter) Write(p []byte) (int, os.Error) {
+	w.b = append(w.b, p...)
+	return len(p), nil
+}
+
+// Replace returns a copy of s with all replacements performed.
+func (r *Replacer) Replace(s string) string {
+	// TODO(bradfitz): optimized version
+	n, _ := r.WriteString(discard, s)
+	w := appendSliceWriter{make([]byte, 0, n)}
+	r.WriteString(&w, s)
+	return string(w.b)
+}
+
+// WriteString writes s to w with all replacements performed.
+func (r *Replacer) WriteString(w io.Writer, s string) (n int, err os.Error) {
+Input:
+	// TODO(bradfitz): optimized version
+	for i := 0; i < len(s); {
+		for _, p := range r.p {
+			if HasPrefix(s[i:], p.old) {
+				wn, err := w.Write([]byte(p.new))
+				n += wn
+				if err != nil {
+					return n, err
+				}
+				i += len(p.old)
+				continue Input
+			}
+		}
+		wn, err := w.Write([]byte{s[i]})
+		n += wn
+		if err != nil {
+			return n, err
+		}
+		i++
+	}
+	return n, nil
+}

src/pkg/strings/replace_test.go

@@ -0,0 +1,75 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	. "strings"
+	"testing"
+)
+
+type ReplacerTest struct {
+	m   *Replacer
+	in  string
+	out string
+}
+
+var htmlEscaper = NewReplacer("&", "&amp;", "<", "&lt;", ">", "&gt;", "\"", "&quot;")
+
+// The http package's old HTML escaping function.
+func oldhtmlEscape(s string) string {
+	s = Replace(s, "&", "&amp;", -1)
+	s = Replace(s, "<", "&lt;", -1)
+	s = Replace(s, ">", "&gt;", -1)
+	s = Replace(s, "\"", "&quot;", -1)
+	s = Replace(s, "'", "&apos;", -1)
+	return s
+}
+
+var replacer = NewReplacer("aaa", "3[aaa]", "aa", "2[aa]", "a", "1[a]", "i", "i",
+	"longerst", "most long", "longer", "medium", "long", "short",
+	"X", "Y", "Y", "Z")
+
+var ReplacerTests = []ReplacerTest{
+	{htmlEscaper, "No changes", "No changes"},
+	{htmlEscaper, "I <3 escaping & stuff", "I &lt;3 escaping &amp; stuff"},
+	{htmlEscaper, "&&&", "&amp;&amp;&amp;"},
+	{replacer, "fooaaabar", "foo3[aaa]b1[a]r"},
+	{replacer, "long, longerst, longer", "short, most long, medium"},
+	{replacer, "XiX", "YiY"},
+}
+
+func TestReplacer(t *testing.T) {
+	for i, tt := range ReplacerTests {
+		if s := tt.m.Replace(tt.in); s != tt.out {
+			t.Errorf("%d. Replace(%q) = %q, want %q", i, tt.in, s, tt.out)
+		}
+	}
+}
+
+var slowReplacer = NewReplacer("&&", "&amp;", "<<", "&lt;", ">>", "&gt;", "\"\"", "&quot;", "''", "&apos;")
+
+func BenchmarkReplacerSingleByte(b *testing.B) {
+	str := "I <3 benchmarking html & other stuff too >:D"
+	n := 0
+	for i := 0; i < b.N; i++ {
+		n += len(htmlEscaper.Replace(str))
+	}
+}
+
+func BenchmarkReplaceMap(b *testing.B) {
+	str := "I <<3 benchmarking html && other stuff too >>:D"
+	n := 0
+	for i := 0; i < b.N; i++ {
+		n += len(slowReplacer.Replace(str))
+	}
+}
+
+func BenchmarkOldHTTPHTMLReplace(b *testing.B) {
+	str := "I <3 benchmarking html & other stuff too >:D"
+	n := 0
+	for i := 0; i < b.N; i++ {
+		n += len(oldhtmlEscape(str))
+	}
+}