strings: implement UnreadByte, UnreadRune

Added corresponding tests.

R=rsc
CC=golang-dev
https://golang.org/cl/4560045

src/pkg/strings/reader.go

@@ -9,50 +9,78 @@ import (
 	"utf8"
 )
 
-// A Reader satisfies calls to Read, ReadByte, and ReadRune by
-// reading from a string.
-type Reader string
+// A Reader implements the io.Reader, io.ByteScanner, and
+// io.RuneScanner interfaces by reading from a string.
+type Reader struct {
+	s        string
+	i        int // current reading index
+	prevRune int // index of previous rune; or < 0
+}
 
 func (r *Reader) Read(b []byte) (n int, err os.Error) {
-	s := *r
-	if len(s) == 0 {
+	if r.i >= len(r.s) {
 		return 0, os.EOF
 	}
-	n = copy(b, s)
-	*r = s[n:]
+	n = copy(b, r.s[r.i:])
+	r.i += n
+	r.prevRune = -1
 	return
 }
 
 func (r *Reader) ReadByte() (b byte, err os.Error) {
-	s := *r
-	if len(s) == 0 {
+	if r.i >= len(r.s) {
 		return 0, os.EOF
 	}
-	b = s[0]
-	*r = s[1:]
+	b = r.s[r.i]
+	r.i++
+	r.prevRune = -1
 	return
 }
 
+
+// UnreadByte moves the reading position back by one byte.
+// It is an error to call UnreadByte if nothing has been
+// read yet.
+func (r *Reader) UnreadByte() os.Error {
+	if r.i <= 0 {
+		return os.ErrorString("strings.Reader: at beginning of string")
+	}
+	r.i--
+	r.prevRune = -1
+	return nil
+}
+
 // ReadRune reads and returns the next UTF-8-encoded
 // Unicode code point from the buffer.
 // If no bytes are available, the error returned is os.EOF.
 // If the bytes are an erroneous UTF-8 encoding, it
 // consumes one byte and returns U+FFFD, 1.
 func (r *Reader) ReadRune() (rune int, size int, err os.Error) {
-	s := *r
-	if len(s) == 0 {
+	if r.i >= len(r.s) {
 		return 0, 0, os.EOF
 	}
-	c := s[0]
-	if c < utf8.RuneSelf {
-		*r = s[1:]
+	r.prevRune = r.i
+	if c := r.s[r.i]; c < utf8.RuneSelf {
+		r.i++
 		return int(c), 1, nil
 	}
-	rune, size = utf8.DecodeRuneInString(string(s))
-	*r = s[size:]
+	rune, size = utf8.DecodeRuneInString(r.s[r.i:])
+	r.i += size
 	return
 }
 
+// UnreadRune causes the next call to ReadRune to return the same rune
+// as the previous call to ReadRune.
+// The last method called on r must have been ReadRune.
+func (r *Reader) UnreadRune() os.Error {
+	if r.prevRune < 0 {
+		return os.ErrorString("strings.Reader: previous operation was not ReadRune")
+	}
+	r.i = r.prevRune
+	r.prevRune = -1
+	return nil
+}
+
 // NewReader returns a new Reader reading from s.
 // It is similar to bytes.NewBufferString but more efficient and read-only.
-func NewReader(s string) *Reader { return (*Reader)(&s) }
+func NewReader(s string) *Reader { return &Reader{s, 0, -1} }

src/pkg/strings/strings_test.go

@@ -5,6 +5,7 @@
 package strings_test
 
 import (
+	"bytes"
 	"os"
 	"reflect"
 	"strconv"
@@ -751,13 +752,56 @@ func TestRunes(t *testing.T) {
 	}
 }
 
+func TestReadByte(t *testing.T) {
+	testStrings := []string{"", abcd, faces, commas}
+	for _, s := range testStrings {
+		reader := NewReader(s)
+		if e := reader.UnreadByte(); e == nil {
+			t.Errorf("Unreading %q at beginning: expected error", s)
+		}
+		var res bytes.Buffer
+		for {
+			b, e := reader.ReadByte()
+			if e == os.EOF {
+				break
+			}
+			if e != nil {
+				t.Errorf("Reading %q: %s", s, e)
+				break
+			}
+			res.WriteByte(b)
+			// unread and read again
+			e = reader.UnreadByte()
+			if e != nil {
+				t.Errorf("Unreading %q: %s", s, e)
+				break
+			}
+			b1, e := reader.ReadByte()
+			if e != nil {
+				t.Errorf("Reading %q after unreading: %s", s, e)
+				break
+			}
+			if b1 != b {
+				t.Errorf("Reading %q after unreading: want byte %q, got %q", s, b, b1)
+				break
+			}
+		}
+		if res.String() != s {
+			t.Errorf("Reader(%q).ReadByte() produced %q", s, res.String())
+		}
+	}
+}
+
 func TestReadRune(t *testing.T) {
 	testStrings := []string{"", abcd, faces, commas}
 	for _, s := range testStrings {
 		reader := NewReader(s)
+		if e := reader.UnreadRune(); e == nil {
+			t.Errorf("Unreading %q at beginning: expected error", s)
+		}
 		res := ""
 		for {
-			r, _, e := reader.ReadRune()
+			r, z, e := reader.ReadRune()
 			if e == os.EOF {
 				break
 			}
@@ -766,6 +810,25 @@ func TestReadRune(t *testing.T) {
 				break
 			}
 			res += string(r)
+			// unread and read again
+			e = reader.UnreadRune()
+			if e != nil {
+				t.Errorf("Unreading %q: %s", s, e)
+				break
+			}
+			r1, z1, e := reader.ReadRune()
+			if e != nil {
+				t.Errorf("Reading %q after unreading: %s", s, e)
+				break
+			}
+			if r1 != r {
+				t.Errorf("Reading %q after unreading: want rune %q, got %q", s, r, r1)
+				break
+			}
+			if z1 != z {
+				t.Errorf("Reading %q after unreading: want size %d, got %d", s, z, z1)
+				break
+			}
 		}
 		if res != s {
 			t.Errorf("Reader(%q).ReadRune() produced %q", s, res)