compress/lzw: fix the stupidity of allocating and zeroing a new buffer

on each loop iteration, yielding a 20x performance improvement.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/4240044

src/pkg/compress/lzw/reader.go

@@ -99,6 +99,9 @@ func decode(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os.
 		// The c == hi case is a special case.
 		suffix [1 << maxWidth]uint8
 		prefix [1 << maxWidth]uint16
+		// buf is a scratch buffer for reconstituting the bytes that a code expands to.
+		// Code suffixes are written right-to-left from the end of the buffer.
+		buf [1 << maxWidth]byte
 	)
 
 	// Loop over the code stream, converting codes into decompressed bytes.
@@ -131,9 +134,6 @@ func decode(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os.
 		case code == eof:
 			return w.Flush()
 		case code <= hi:
-			// buf is a scratch buffer for reconstituting the bytes that a code expands to.
-			// Code suffixes are written right-to-left from the end of the buffer.
-			var buf [1 << maxWidth]byte
 			c, i := code, len(buf)-1
 			if code == hi {
 				// code == hi is a special case which expands to the last expansion

src/pkg/compress/lzw/reader_test.go

@@ -7,6 +7,7 @@ package lzw
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
 	"os"
 	"strconv"
 	"strings"
@@ -109,3 +110,23 @@ func TestReader(t *testing.T) {
 		}
 	}
 }
+
+type devNull struct{}
+
+func (devNull) Write(p []byte) (int, os.Error) {
+	return len(p), nil
+}
+
+func BenchmarkDecoder(b *testing.B) {
+	b.StopTimer()
+	buf0, _ := ioutil.ReadFile("../testdata/e.txt")
+	compressed := bytes.NewBuffer(nil)
+	w := NewWriter(compressed, LSB, 8)
+	io.Copy(w, bytes.NewBuffer(buf0))
+	w.Close()
+	buf1 := compressed.Bytes()
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		io.Copy(devNull{}, NewReader(bytes.NewBuffer(buf1), LSB, 8))
+	}
+}

src/pkg/compress/lzw/writer_test.go

@@ -98,3 +98,14 @@ func TestWriter(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkEncoder(b *testing.B) {
+	b.StopTimer()
+	buf, _ := ioutil.ReadFile("../testdata/e.txt")
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		w := NewWriter(devNull{}, LSB, 8)
+		w.Write(buf)
+		w.Close()
+	}
+}