// 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
}