bytes: encode size of rune read by ReadRune into lastRead to speed up UnreadRune

In ReadRune store the size of the rune that was read into lastRead
to avoid the need to call DecodeRuneLast in UnreadRune.

fmt:
name        old time/op  new time/op  delta
ScanInts-4   481µs ± 4%   458µs ± 3%  -4.64%  (p=0.000 n=20+20)

Change-Id: I500848e663a975f426402a4b3d27a541e5cac06c
Reviewed-on: https://go-review.googlesource.com/28817Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/buffer.go

@@ -22,14 +22,18 @@ type Buffer struct {
 }
 
 // The readOp constants describe the last action performed on
-// the buffer, so that UnreadRune and UnreadByte can
-// check for invalid usage.
+// the buffer, so that UnreadRune and UnreadByte can check for
+// invalid usage. opReadRuneX constants are choosen such that
+// converted to int they correspond to the rune size that was read.
 type readOp int
 
 const (
-	opInvalid  readOp = iota // Non-read operation.
-	opReadRune               // Read rune.
-	opRead                   // Any other read operation.
+	opRead      readOp = -1 // Any other read operation.
+	opInvalid          = 0  // Non-read operation.
+	opReadRune1        = 1  // Read rune of size 1.
+	opReadRune2        = 2  // Read rune of size 2.
+	opReadRune3        = 3  // Read rune of size 3.
+	opReadRune4        = 4  // Read rune of size 4.
 )
 
 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
@@ -319,14 +323,15 @@ func (b *Buffer) ReadRune() (r rune, size int, err error) {
 		b.Truncate(0)
 		return 0, 0, io.EOF
 	}
-	b.lastRead = opReadRune
 	c := b.buf[b.off]
 	if c < utf8.RuneSelf {
 		b.off++
+		b.lastRead = opReadRune1
 		return rune(c), 1, nil
 	}
 	r, n := utf8.DecodeRune(b.buf[b.off:])
 	b.off += n
+	b.lastRead = readOp(n)
 	return r, n, nil
 }
 
@@ -336,14 +341,13 @@ func (b *Buffer) ReadRune() (r rune, size int, err error) {
 // it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Buffer) UnreadRune() error {
-	if b.lastRead != opReadRune {
+	if b.lastRead <= opInvalid {
 		return errors.New("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
 	}
-	b.lastRead = opInvalid
-	if b.off > 0 {
-		_, n := utf8.DecodeLastRune(b.buf[0:b.off])
-		b.off -= n
+	if b.off >= int(b.lastRead) {
+		b.off -= int(b.lastRead)
 	}
+	b.lastRead = opInvalid
 	return nil
 }
 
@@ -351,7 +355,7 @@ func (b *Buffer) UnreadRune() error {
 // read operation. If write has happened since the last read, UnreadByte
 // returns an error.
 func (b *Buffer) UnreadByte() error {
-	if b.lastRead != opReadRune && b.lastRead != opRead {
+	if b.lastRead == opInvalid {
 		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a read")
 	}
 	b.lastRead = opInvalid