package modhex

import (
	"errors"
	"fmt"
)

const hexChars = "0123456789abcdef"
const modhexChars = "cbdefghijklnrtuv"

var modhex2HexMap map[byte]byte
var hex2ModhexMap map[byte]byte

func init() {
	hex2ModhexMap = make(map[byte]byte)
	for i, h := range []byte(hexChars) {
		hex2ModhexMap[h] = modhexChars[i]
	}

	modhex2HexMap = make(map[byte]byte)
	for i, m := range []byte(modhexChars) {
		modhex2HexMap[m] = hexChars[i]
	}
}

// EncodeHex encodes a hex string into a modhex string
func EncodeHex(hex string) (string, error) {
	size := len([]byte(hex))

	if size%2 == 1 {
		return "", errors.New("size of input hex input not even")
	}

	var modhex = make([]byte, size)

	for i, h := range []byte(hex) {
		if m, ok := hex2ModhexMap[h]; ok {
			modhex[i] = m
		} else {
			return "", fmt.Errorf("input not hex encoded; position %d", i)
		}
	}

	return string(modhex), nil
}

// DecodeHex decodes a modhex string into a hex string
func DecodeHex(modhex string) (string, error) {
	size := len([]byte(modhex))

	if size%2 == 1 {
		return "", errors.New("size of modhex input not even")
	}

	var hex = make([]byte, size)

	for i, m := range []byte(modhex) {
		if h, ok := modhex2HexMap[m]; ok {
			hex[i] = h
		} else {
			return "", fmt.Errorf("input not modhex encoded; position %d", i)
		}
	}

	return string(hex), nil
}