encoding/hex: add NewEncoder, NewDecoder

NewEncoder returns an io.Writer that writes all incoming bytes as
hexadecimal characters to the underlying io.Writer. NewDecoder returns an
io.Reader that does the inverse.

Fixes #21590

Change-Id: Iebe0813faf365b42598f19a9aa41768f571dc0a8
Reviewed-on: https://go-review.googlesource.com/70210
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/hex/hex.go

@@ -58,11 +58,11 @@ func Decode(dst, src []byte) (int, error) {
 	for i := 0; i < len(src)/2; i++ {
 		a, ok := fromHexChar(src[i*2])
 		if !ok {
-			return 0, InvalidByteError(src[i*2])
+			return i, InvalidByteError(src[i*2])
 		}
 		b, ok := fromHexChar(src[i*2+1])
 		if !ok {
-			return 0, InvalidByteError(src[i*2+1])
+			return i, InvalidByteError(src[i*2+1])
 		}
 		dst[i] = (a << 4) | b
 	}
@@ -113,6 +113,77 @@ func Dump(data []byte) string {
 	return buf.String()
 }
 
+// bufferSize is the number of hexadecimal characters to buffer in encoder and decoder.
+const bufferSize = 1024
+
+type encoder struct {
+	w   io.Writer
+	err error
+	out [bufferSize]byte // output buffer
+}
+
+// NewEncoder returns an io.Writer that writes lowercase hexadecimal characters to w.
+func NewEncoder(w io.Writer) io.Writer {
+	return &encoder{w: w}
+}
+
+func (e *encoder) Write(p []byte) (n int, err error) {
+	for len(p) > 0 && e.err == nil {
+		chunkSize := bufferSize / 2
+		if len(p) < chunkSize {
+			chunkSize = len(p)
+		}
+
+		var written int
+		encoded := Encode(e.out[:], p[:chunkSize])
+		written, e.err = e.w.Write(e.out[:encoded])
+		n += written / 2
+		p = p[chunkSize:]
+	}
+	return n, e.err
+}
+
+type decoder struct {
+	r   io.Reader
+	err error
+	in  []byte           // input buffer (encoded form)
+	arr [bufferSize]byte // backing array for in
+}
+
+// NewDecoder returns an io.Reader that decodes hexadecimal characters from r.
+// NewDecoder expects that r contain only an even number of hexadecimal characters.
+func NewDecoder(r io.Reader) io.Reader {
+	return &decoder{r: r}
+}
+
+func (d *decoder) Read(p []byte) (n int, err error) {
+	// Fill internal buffer with sufficient bytes to decode
+	if len(d.in) < 2 && d.err == nil {
+		var numCopy, numRead int
+		numCopy = copy(d.arr[:], d.in) // Copies either 0 or 1 bytes
+		numRead, d.err = d.r.Read(d.arr[numCopy:])
+		d.in = d.arr[:numCopy+numRead]
+		if d.err == io.EOF && len(d.in)%2 != 0 {
+			d.err = io.ErrUnexpectedEOF
+		}
+	}
+
+	// Decode internal buffer into output buffer
+	if numAvail := len(d.in) / 2; len(p) > numAvail {
+		p = p[:numAvail]
+	}
+	numDec, err := Decode(p, d.in[:len(p)*2])
+	d.in = d.in[2*numDec:]
+	if err != nil {
+		d.in, d.err = nil, err // Decode error; discard input remainder
+	}
+
+	if len(d.in) < 2 {
+		return numDec, d.err // Only expose errors when buffer fully consumed
+	}
+	return numDec, nil
+}
+
 // Dumper returns a WriteCloser that writes a hex dump of all written data to
 // w. The format of the dump matches the output of `hexdump -C` on the command
 // line.

src/encoding/hex/hex_test.go

@@ -7,6 +7,9 @@ package hex
 import (
 	"bytes"
 	"fmt"
+	"io"
+	"io/ioutil"
+	"strings"
 	"testing"
 )
 
@@ -111,6 +114,63 @@ func TestInvalidStringErr(t *testing.T) {
 	}
 }
 
+func TestEncoderDecoder(t *testing.T) {
+	for _, multiplier := range []int{1, 128, 192} {
+		for _, test := range encDecTests {
+			input := bytes.Repeat(test.dec, multiplier)
+			output := strings.Repeat(test.enc, multiplier)
+
+			var buf bytes.Buffer
+			enc := NewEncoder(&buf)
+			r := struct{ io.Reader }{bytes.NewReader(input)} // io.Reader only; not io.WriterTo
+			if n, err := io.CopyBuffer(enc, r, make([]byte, 7)); n != int64(len(input)) || err != nil {
+				t.Errorf("encoder.Write(%q*%d) = (%d, %v), want (%d, nil)", test.dec, multiplier, n, err, len(input))
+				continue
+			}
+
+			if encDst := buf.String(); encDst != output {
+				t.Errorf("buf(%q*%d) = %v, want %v", test.dec, multiplier, encDst, output)
+				continue
+			}
+
+			dec := NewDecoder(&buf)
+			var decBuf bytes.Buffer
+			w := struct{ io.Writer }{&decBuf} // io.Writer only; not io.ReaderFrom
+			if _, err := io.CopyBuffer(w, dec, make([]byte, 7)); err != nil || decBuf.Len() != len(input) {
+				t.Errorf("decoder.Read(%q*%d) = (%d, %v), want (%d, nil)", test.enc, multiplier, decBuf.Len(), err, len(input))
+			}
+
+			if !bytes.Equal(decBuf.Bytes(), input) {
+				t.Errorf("decBuf(%q*%d) = %v, want %v", test.dec, multiplier, decBuf.Bytes(), input)
+				continue
+			}
+		}
+	}
+}
+
+func TestDecodeErr(t *testing.T) {
+	tests := []struct {
+		in      string
+		wantOut string
+		wantErr error
+	}{
+		{"", "", nil},
+		{"0", "", io.ErrUnexpectedEOF},
+		{"0g", "", InvalidByteError('g')},
+		{"00gg", "\x00", InvalidByteError('g')},
+		{"0\x01", "", InvalidByteError('\x01')},
+		{"ffeed", "\xff\xee", io.ErrUnexpectedEOF},
+	}
+
+	for _, tt := range tests {
+		dec := NewDecoder(strings.NewReader(tt.in))
+		got, err := ioutil.ReadAll(dec)
+		if string(got) != tt.wantOut || err != tt.wantErr {
+			t.Errorf("NewDecoder(%q) = (%q, %v), want (%q, %v)", tt.in, got, err, tt.wantOut, tt.wantErr)
+		}
+	}
+}
+
 func TestDumper(t *testing.T) {
 	var in [40]byte
 	for i := range in {