// Package modhex implements modhex encoding and decoding.
package modhex

import (
	"errors"
	"fmt"
)

const modhexTable = "cbdefghijklnrtuv"

var modhexMap map[byte]byte

func init() {
	modhexMap = make(map[byte]byte)
	for i, v := range []byte(modhexTable) {
		modhexMap[v] = byte(i)
	}
}

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

// Encode encodes src into EncodedLen(len(src)) bytes of dst.
// It returns the number of bytes written to dst, but this is
// always EncodedLen(len(src)).
// Encode implements modhex encoding.
func Encode(dst, src []byte) int {
	i := 0
	for _, v := range src {
		dst[i] = modhexTable[v>>4]
		dst[i+1] = modhexTable[v&0x0f]
		i += 2
	}
	return len(src) * 2
}

// EncodeToString returns the modhex encoding of src.
func EncodeToString(src []byte) string {
	// make a new slice to hold the encoded bytes
	dst := make([]byte, EncodedLen(len(src)))

	// encode it!
	Encode(dst, src)

	// return the encoded bytes as a string
	return string(dst)
}

// DecodedLen returns the length of a decoding of n source bytes.
// Specifically, it returns n / 2.
func DecodedLen(n int) int {
	return n / 2
}

// Decode decodes src into DecodedLen(len(src)) bytes, returning
// the actual number of bytes written to dst.
//
// Decode expects that src contains only modhex characters and that
// src has an even length.
// If the input is malformed, Decode returns the number
// of bytes decoded before the error.
func Decode(dst, src []byte) (int, error) {
	i, j := 0, 1
	for ; j < len(src); j += 2 {
		a, ok := modhexMap[src[j-1]]
		if !ok {
			return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j-1]))
		}

		b, ok := modhexMap[src[j]]
		if !ok {
			return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j]))
		}

		dst[i] = (a << 4) | b
		i++
	}

	if len(src)%2 == 1 {
		if _, ok := modhexMap[src[j-1]]; !ok {
			return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j-1]))
		}

		return i, errors.New("modhex: odd length modhex string")
	}

	return i, nil
}

// DecodeString returns the bytes represesented by the modhex string s.
//
// Decode expects that s contains only modhex characters and that s
// has an even length.
// If the input is malformed, Decode returns the number
// of bytes decoded before the error.
func DecodeString(s string) ([]byte, error) {
	// convert the input string to an array of bytes
	src := []byte(s)

	// make a new slice to hold the decoded bytes
	dst := make([]byte, DecodedLen(len(src)))

	// decode it!
	n, err := Decode(dst, src)

	// return the decoded bytes along with any errors that may have occured
	return dst[:n], err
}