bytes, string: add Reset method to Reader

Currently, there is no easy allocation-free way to turn a
[]byte or string into an io.Reader. Thus, we add a Reset method
to bytes.Reader and strings.Reader to allow the reuse of these
Readers with another []byte or string.

This is consistent with the fact that many standard library io.Readers
already support a Reset method of some type:
	bufio.Reader
	flate.Reader
	gzip.Reader
	zlib.Reader
	debug/dwarf.LineReader
	bytes.Buffer
	crypto/rc4.Cipher

Fixes #15033

Change-Id: I456fd1af77af6ef0b4ac6228b058ac1458ff3d19
Reviewed-on: https://go-review.googlesource.com/21386
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/reader.go

@@ -146,5 +146,8 @@ func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+// Reset resets the Reader to be reading from b.
+func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
+
 // NewReader returns a new Reader reading from b.
 func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }

src/bytes/reader_test.go

@@ -256,3 +256,23 @@ func TestReaderLenSize(t *testing.T) {
 		t.Errorf("Size = %d; want 3", r.Size())
 	}
 }
+
+func TestReaderReset(t *testing.T) {
+	r := NewReader([]byte("世界"))
+	if _, _, err := r.ReadRune(); err != nil {
+		t.Errorf("ReadRune: unexpected error: %v", err)
+	}
+
+	const want = "abcdef"
+	r.Reset([]byte(want))
+	if err := r.UnreadRune(); err == nil {
+		t.Errorf("UnreadRune: expected error, got nil")
+	}
+	buf, err := ioutil.ReadAll(r)
+	if err != nil {
+		t.Errorf("ReadAll: unexpected error: %v", err)
+	}
+	if got := string(buf); got != want {
+		t.Errorf("ReadAll: got %q, want %q", got, want)
+	}
+}

src/strings/reader.go

@@ -145,6 +145,9 @@ func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+// Reset resets the Reader to be reading from s.
+func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }
+
 // 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, 0, -1} }

src/strings/reader_test.go

@@ -170,3 +170,23 @@ func TestReaderLenSize(t *testing.T) {
 		t.Errorf("Size = %d; want 3", r.Size())
 	}
 }
+
+func TestReaderReset(t *testing.T) {
+	r := strings.NewReader("世界")
+	if _, _, err := r.ReadRune(); err != nil {
+		t.Errorf("ReadRune: unexpected error: %v", err)
+	}
+
+	const want = "abcdef"
+	r.Reset(want)
+	if err := r.UnreadRune(); err == nil {
+		t.Errorf("UnreadRune: expected error, got nil")
+	}
+	buf, err := ioutil.ReadAll(r)
+	if err != nil {
+		t.Errorf("ReadAll: unexpected error: %v", err)
+	}
+	if got := string(buf); got != want {
+		t.Errorf("ReadAll: got %q, want %q", got, want)
+	}
+}