// https://en.bitcoin.it/wiki/Base58Check_encoding

var BigInteger = require('./jsbn/jsbn')
var Crypto = require('crypto-js')
var convert = require('./convert')
var SHA256 = Crypto.SHA256

var alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
var base = BigInteger.valueOf(58)

var positions = {}
for (var i=0; i<alphabet.length; ++i) {
  positions[alphabet[i]] = i
}

// Convert a byte array to a base58-encoded string.
// Written by Mike Hearn for BitcoinJ.
// Copyright (c) 2011 Google Inc.
// Ported to JavaScript by Stefan Thomas.
function encode(input) {
  var bi = BigInteger.fromByteArrayUnsigned(input)
  var chars = []

  while (bi.compareTo(base) >= 0) {
    var mod = bi.mod(base)
    chars.push(alphabet[mod.intValue()])
    bi = bi.subtract(mod).divide(base)
  }

  chars.push(alphabet[bi.intValue()])

  // Convert leading zeros too.
  for (var i=0; i<input.length; i++) {
    if (input[i] == 0x00) {
      chars.push(alphabet[0])
    } else break
  }

  return chars.reverse().join('')
}

// decode a base58 string into a byte array
// input should be a base58 encoded string
// @return Array
function decode(input) {
  var base = BigInteger.valueOf(58)

  var length = input.length
  var num = BigInteger.valueOf(0)
  var leading_zero = 0
  var seen_other = false
  for (var i=0; i<length; ++i) {
    var chr = input[i]
    var p = positions[chr]

    // if we encounter an invalid character, decoding fails
    if (p === undefined) {
      throw new Error('invalid base58 string: ' + input)
    }

    num = num.multiply(base).add(BigInteger.valueOf(p))

    if (chr == '1' && !seen_other) {
      ++leading_zero
    } else {
      seen_other = true
    }
  }

  var bytes = num.toByteArrayUnsigned()

  // remove leading zeros
  while (leading_zero-- > 0) {
    bytes.unshift(0)
  }

  return bytes
}

function checkEncode(input, vbyte) {
  vbyte = vbyte || 0

  var front = [vbyte].concat(input)
  return encode(front.concat(getChecksum(front)))
}

function checkDecode(input) {
  var bytes = decode(input),
  front = bytes.slice(0, bytes.length-4),
  back = bytes.slice(bytes.length-4)

  var checksum = getChecksum(front)

  if ("" + checksum != "" + back) {
    throw new Error("Checksum failed")
  }

  var o = front.slice(1)
  o.version = front[0]
  return o
}

function getChecksum(bytes) {
  var wordArray = convert.bytesToWordArray(bytes)
  return convert.hexToBytes(SHA256(SHA256(wordArray)).toString()).slice(0, 4)
}

module.exports = {
  encode: encode,
  decode: decode,
  checkEncode: checkEncode,
  checkDecode: checkDecode,
  getChecksum: getChecksum
}