bufio: add UnreadRune.

R=rsc
CC=golang-dev
https://golang.org/cl/2103046

src/pkg/bufio/bufio.go

@@ -27,6 +27,7 @@ type Error struct {
 
 var (
 	ErrInvalidUnreadByte os.Error = &Error{"bufio: invalid use of UnreadByte"}
+	ErrInvalidUnreadRune os.Error = &Error{"bufio: invalid use of UnreadRune"}
 	ErrBufferFull        os.Error = &Error{"bufio: buffer full"}
 	ErrNegativeCount     os.Error = &Error{"bufio: negative count"}
 	errInternal          os.Error = &Error{"bufio: internal error"}
@@ -48,7 +49,8 @@ type Reader struct {
 	rd           io.Reader
 	r, w         int
 	err          os.Error
-	lastbyte int
+	lastByte     int
+	lastRuneSize int
 }
 
 // NewReaderSize creates a new Reader whose buffer has the specified size,
@@ -67,7 +69,8 @@ func NewReaderSize(rd io.Reader, size int) (*Reader, os.Error) {
 	b = new(Reader)
 	b.buf = make([]byte, size)
 	b.rd = rd
-	b.lastbyte = -1
+	b.lastByte = -1
+	b.lastRuneSize = -1
 	return b, nil
 }
 
@@ -141,7 +144,8 @@ func (b *Reader) Read(p []byte) (nn int, err os.Error) {
 				// Read directly into p to avoid copy.
 				n, b.err = b.rd.Read(p)
 				if n > 0 {
-					b.lastbyte = int(p[n-1])
+					b.lastByte = int(p[n-1])
+					b.lastRuneSize = -1
 				}
 				p = p[n:]
 				nn += n
@@ -156,7 +160,8 @@ func (b *Reader) Read(p []byte) (nn int, err os.Error) {
 		copy(p[0:n], b.buf[b.r:])
 		p = p[n:]
 		b.r += n
-		b.lastbyte = int(b.buf[b.r-1])
+		b.lastByte = int(b.buf[b.r-1])
+		b.lastRuneSize = -1
 		nn += n
 	}
 	return nn, nil
@@ -165,6 +170,7 @@ func (b *Reader) Read(p []byte) (nn int, err os.Error) {
 // ReadByte reads and returns a single byte.
 // If no byte is available, returns an error.
 func (b *Reader) ReadByte() (c byte, err os.Error) {
+	b.lastRuneSize = -1
 	for b.w == b.r {
 		if b.err != nil {
 			return 0, b.err
@@ -173,24 +179,25 @@ func (b *Reader) ReadByte() (c byte, err os.Error) {
 	}
 	c = b.buf[b.r]
 	b.r++
-	b.lastbyte = int(c)
+	b.lastByte = int(c)
 	return c, nil
 }
 
 // UnreadByte unreads the last byte.  Only the most recently read byte can be unread.
 func (b *Reader) UnreadByte() os.Error {
-	if b.r == b.w && b.lastbyte >= 0 {
+	b.lastRuneSize = -1
+	if b.r == b.w && b.lastByte >= 0 {
 		b.w = 1
 		b.r = 0
-		b.buf[0] = byte(b.lastbyte)
-		b.lastbyte = -1
+		b.buf[0] = byte(b.lastByte)
+		b.lastByte = -1
 		return nil
 	}
 	if b.r <= 0 {
 		return ErrInvalidUnreadByte
 	}
 	b.r--
-	b.lastbyte = -1
+	b.lastByte = -1
 	return nil
 }
 
@@ -208,10 +215,25 @@ func (b *Reader) ReadRune() (rune int, size int, err os.Error) {
 		rune, size = utf8.DecodeRune(b.buf[b.r:b.w])
 	}
 	b.r += size
-	b.lastbyte = int(b.buf[b.r-1])
+	b.lastByte = int(b.buf[b.r-1])
+	b.lastRuneSize = size
 	return rune, size, nil
 }
 
+// UnreadRune unreads the last rune.  If the most recent read operation on
+// the buffer was not a ReadRune, UnreadRune returns an error.  (In this
+// regard it is stricter than UnreadByte, which will unread the last byte
+// from any read operation.)
+func (b *Reader) UnreadRune() os.Error {
+	if b.lastRuneSize < 0 {
+		return ErrInvalidUnreadRune
+	}
+	b.r -= b.lastRuneSize
+	b.lastByte = -1
+	b.lastRuneSize = -1
+	return nil
+}
+
 // Buffered returns the number of bytes that can be read from the current buffer.
 func (b *Reader) Buffered() int { return b.w - b.r }
 

src/pkg/bufio/bufio_test.go

@@ -226,6 +226,99 @@ func TestReadRune(t *testing.T) {
 	}
 }
 
+func TestUnreadRune(t *testing.T) {
+	got := ""
+	segments := []string{"Hello, world:", "日本語"}
+	data := strings.Join(segments, "")
+	r := NewReader(&StringReader{data: segments})
+	// Normal execution.
+	for {
+		rune, _, err := r.ReadRune()
+		if err != nil {
+			if err != os.EOF {
+				t.Error("unexpected EOF")
+			}
+			break
+		}
+		got += string(rune)
+		// Put it back and read it again
+		if err = r.UnreadRune(); err != nil {
+			t.Error("unexpected error on UnreadRune:", err)
+		}
+		rune1, _, err := r.ReadRune()
+		if err != nil {
+			t.Error("unexpected error reading after unreading:", err)
+		}
+		if rune != rune1 {
+			t.Error("incorrect rune after unread: got %c wanted %c", rune1, rune)
+		}
+	}
+	if got != data {
+		t.Errorf("want=%q got=%q", data, got)
+	}
+}
+
+// Test that UnreadRune fails if the preceding operation was not a ReadRune.
+func TestUnreadRuneError(t *testing.T) {
+	buf := make([]byte, 3) // All runes in this test are 3 bytes long
+	r := NewReader(&StringReader{data: []string{"日本語日本語日本語"}})
+	if r.UnreadRune() == nil {
+		t.Error("expected error on UnreadRune from fresh buffer")
+	}
+	_, _, err := r.ReadRune()
+	if err != nil {
+		t.Error("unexpected error on ReadRune (1):", err)
+	}
+	if err = r.UnreadRune(); err != nil {
+		t.Error("unexpected error on UnreadRune (1):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after UnreadRune (1)")
+	}
+	// Test error after Read.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (2):", err)
+	}
+	_, err = r.Read(buf)
+	if err != nil {
+		t.Error("unexpected error on Read (2):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after Read (2)")
+	}
+	// Test error after ReadByte.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (2):", err)
+	}
+	for _ = range buf {
+		_, err = r.ReadByte()
+		if err != nil {
+			t.Error("unexpected error on ReadByte (2):", err)
+		}
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after ReadByte")
+	}
+	// Test error after UnreadByte.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (3):", err)
+	}
+	_, err = r.ReadByte()
+	if err != nil {
+		t.Error("unexpected error on ReadByte (3):", err)
+	}
+	err = r.UnreadByte()
+	if err != nil {
+		t.Error("unexpected error on UnreadByte (3):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after UnreadByte (3)")
+	}
+}
+
 func TestReadWriteRune(t *testing.T) {
 	const NRune = 1000
 	byteBuf := new(bytes.Buffer)