encoding/hex: add hex dumping.

I found this useful, esp with an io.MultiWriter. But I fear that
it may be bloat in such a low-level package so please feel free to
decline if you feel likewise.

R=rsc, ality
CC=golang-dev
https://golang.org/cl/4530088

src/pkg/encoding/hex/hex.go

@@ -6,6 +6,8 @@
 package hex
 
 import (
+	"bytes"
+	"io"
 	"os"
 	"strconv"
 )
@@ -99,3 +101,117 @@ func DecodeString(s string) ([]byte, os.Error) {
 	}
 	return dst, nil
 }
+
+// Dump returns a string that contains a hex dump of the given data. The format
+// of the hex dump matches the output of `hexdump -C` on the command line.
+func Dump(data []byte) string {
+	buf := bytes.NewBuffer(nil)
+	dumper := Dumper(buf)
+	dumper.Write(data)
+	dumper.Close()
+	return string(buf.Bytes())
+}
+
+// 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.
+func Dumper(w io.Writer) io.WriteCloser {
+	return &dumper{w: w}
+}
+
+type dumper struct {
+	w          io.Writer
+	rightChars [18]byte
+	buf        [14]byte
+	used       int  // number of bytes in the current line
+	n          uint // number of bytes, total
+}
+
+func toChar(b byte) byte {
+	if b < 32 || b > 127 {
+		return '.'
+	}
+	return b
+}
+
+func (h *dumper) Write(data []byte) (n int, err os.Error) {
+	// Output lines look like:
+	// 00000010  2e 2f 30 31 32 33 34 35  36 37 38 39 3a 3b 3c 3d  |./0123456789:;<=|
+	// ^ offset                          ^ extra space              ^ ASCII of line.
+	for i := range data {
+		if h.used == 0 {
+			// At the beginning of a line we print the current
+			// offset in hex.
+			h.buf[0] = byte(h.n >> 24)
+			h.buf[1] = byte(h.n >> 16)
+			h.buf[2] = byte(h.n >> 8)
+			h.buf[3] = byte(h.n)
+			Encode(h.buf[4:], h.buf[:4])
+			h.buf[12] = ' '
+			h.buf[13] = ' '
+			_, err = h.w.Write(h.buf[4:])
+		}
+		Encode(h.buf[:], data[i:i+1])
+		h.buf[2] = ' '
+		l := 3
+		if h.used == 7 {
+			// There's an additional space after the 8th byte.
+			h.buf[3] = ' '
+			l = 4
+		} else if h.used == 15 {
+			// At the end of the line there's an extra space and
+			// the bar for the right column.
+			h.buf[3] = ' '
+			h.buf[4] = '|'
+			l = 5
+		}
+		_, err = h.w.Write(h.buf[:l])
+		if err != nil {
+			return
+		}
+		n++
+		h.rightChars[h.used] = toChar(data[i])
+		h.used++
+		h.n++
+		if h.used == 16 {
+			h.rightChars[16] = '|'
+			h.rightChars[17] = '\n'
+			_, err = h.w.Write(h.rightChars[:])
+			if err != nil {
+				return
+			}
+			h.used = 0
+		}
+	}
+	return
+}
+
+func (h *dumper) Close() (err os.Error) {
+	// See the comments in Write() for the details of this format.
+	if h.used == 0 {
+		return
+	}
+	h.buf[0] = ' '
+	h.buf[1] = ' '
+	h.buf[2] = ' '
+	h.buf[3] = ' '
+	h.buf[4] = '|'
+	nBytes := h.used
+	for h.used < 16 {
+		l := 3
+		if h.used == 7 {
+			l = 4
+		} else if h.used == 15 {
+			l = 5
+		}
+		_, err = h.w.Write(h.buf[:l])
+		if err != nil {
+			return
+		}
+		h.used++
+	}
+	h.rightChars[nBytes] = '|'
+	h.rightChars[nBytes+1] = '\n'
+	_, err = h.w.Write(h.rightChars[:nBytes+2])
+	return
+}

src/pkg/encoding/hex/hex_test.go

@@ -147,3 +147,46 @@ func TestDecodeString(t *testing.T) {
 		}
 	}
 }
+
+func TestDumper(t *testing.T) {
+	var in [40]byte
+	for i := range in {
+		in[i] = byte(i + 30)
+	}
+
+	for stride := 1; stride < len(in); stride++ {
+		out := bytes.NewBuffer(nil)
+		dumper := Dumper(out)
+		done := 0
+		for done < len(in) {
+			todo := done + stride
+			if todo > len(in) {
+				todo = len(in)
+			}
+			dumper.Write(in[done:todo])
+			done = todo
+		}
+
+		dumper.Close()
+		if !bytes.Equal(out.Bytes(), expectedHexDump) {
+			t.Errorf("stride: %d failed. got:\n%s\nwant:\n%s", stride, out.Bytes(), expectedHexDump)
+		}
+	}
+}
+
+func TestDump(t *testing.T) {
+	var in [40]byte
+	for i := range in {
+		in[i] = byte(i + 30)
+	}
+
+	out := []byte(Dump(in[:]))
+	if !bytes.Equal(out, expectedHexDump) {
+		t.Errorf("got:\n%s\nwant:\n%s", out, expectedHexDump)
+	}
+}
+
+var expectedHexDump = []byte(`00000000  1e 1f 20 21 22 23 24 25  26 27 28 29 2a 2b 2c 2d  |.. !"#$%&'()*+,-|
+00000010  2e 2f 30 31 32 33 34 35  36 37 38 39 3a 3b 3c 3d  |./0123456789:;<=|
+00000020  3e 3f 40 41 42 43 44 45                           |>?@ABCDE|
+`)