// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package hex implements hexadecimal encoding and decoding.
package hex

import (
	"bytes"
	"io"
	"os"
	"strconv"
)

const hextable = "0123456789abcdef"

// EncodedLen returns the length of an encoding of n source bytes.
func EncodedLen(n int) int { return n * 2 }

// Encode encodes src into EncodedLen(len(src))
// bytes of dst.  As a convenience, it returns the number
// of bytes written to dst, but this value is always EncodedLen(len(src)).
// Encode implements hexadecimal encoding.
func Encode(dst, src []byte) int {
	for i, v := range src {
		dst[i*2] = hextable[v>>4]
		dst[i*2+1] = hextable[v&0x0f]
	}

	return len(src) * 2
}

// OddLengthInputError results from decoding an odd length slice.
type OddLengthInputError struct{}

func (OddLengthInputError) String() string { return "odd length hex string" }

// InvalidHexCharError results from finding an invalid character in a hex string.
type InvalidHexCharError byte

func (e InvalidHexCharError) String() string {
	return "invalid hex char: " + strconv.Itoa(int(e))
}

func DecodedLen(x int) int { return x / 2 }

// Decode decodes src into DecodedLen(len(src)) bytes, returning the actual
// number of bytes written to dst.
//
// If Decode encounters invalid input, it returns an OddLengthInputError or an
// InvalidHexCharError.
func Decode(dst, src []byte) (int, os.Error) {
	if len(src)%2 == 1 {
		return 0, OddLengthInputError{}
	}

	for i := 0; i < len(src)/2; i++ {
		a, ok := fromHexChar(src[i*2])
		if !ok {
			return 0, InvalidHexCharError(src[i*2])
		}
		b, ok := fromHexChar(src[i*2+1])
		if !ok {
			return 0, InvalidHexCharError(src[i*2+1])
		}
		dst[i] = (a << 4) | b
	}

	return len(src) / 2, nil
}

// fromHexChar converts a hex character into its value and a success flag.
func fromHexChar(c byte) (byte, bool) {
	switch {
	case '0' <= c && c <= '9':
		return c - '0', true
	case 'a' <= c && c <= 'f':
		return c - 'a' + 10, true
	case 'A' <= c && c <= 'F':
		return c - 'A' + 10, true
	}

	return 0, false
}

// EncodeToString returns the hexadecimal encoding of src.
func EncodeToString(src []byte) string {
	dst := make([]byte, EncodedLen(len(src)))
	Encode(dst, src)
	return string(dst)
}

// DecodeString returns the bytes represented by the hexadecimal string s.
func DecodeString(s string) ([]byte, os.Error) {
	src := []byte(s)
	dst := make([]byte, DecodedLen(len(src)))
	_, err := Decode(dst, src)
	if err != nil {
		return nil, err
	}
	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 > 126 {
		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
}