base-x.js

// base-x encoding
// Forked from https://github.com/cryptocoinjs/bs58
// Originally written by Mike Hearn for BitcoinJ
// Copyright (c) 2011 Google Inc
// Ported to JavaScript by Stefan Thomas
// Merged Buffer refactorings from base58-native by Stephen Pair
// Copyright (c) 2013 BitPay Inc

var Buffer = require('safe-buffer').Buffer

module.exports = function base (ALPHABET) {
  var ALPHABET_MAP = {}
  var BASE = ALPHABET.length
  var LEADER = ALPHABET.charAt(0)

  // pre-compute lookup table
  for (var z = 0; z < ALPHABET.length; z++) {
    var x = ALPHABET.charAt(z)

    if (ALPHABET_MAP[x] !== undefined) throw new TypeError(x + ' is ambiguous')
    ALPHABET_MAP[x] = z
  }

  function encode (source) {
    if (source.length === 0) return ''

    var digits = [0]
    for (var i = 0; i < source.length; ++i) {
      for (var j = 0, carry = source[i]; j < digits.length; ++j) {
        carry += digits[j] << 8
        digits[j] = carry % BASE
        carry = (carry / BASE) | 0
      }

      while (carry > 0) {
        digits.push(carry % BASE)
        carry = (carry / BASE) | 0
      }
    }

    var string = ''

    // deal with leading zeros
    for (var k = 0; source[k] === 0 && k < source.length - 1; ++k) string += LEADER
    // convert digits to a string
    for (var q = digits.length - 1; q >= 0; --q) string += ALPHABET[digits[q]]

    return string
  }

  function decodeUnsafe (string) {
    if (typeof string !== 'string') throw new TypeError('Expected String')
    if (string.length === 0) return Buffer.allocUnsafe(0)

    var bytes = [0]
    for (var i = 0; i < string.length; i++) {
      var value = ALPHABET_MAP[string[i]]
      if (value === undefined) return

      for (var j = 0, carry = value; j < bytes.length; ++j) {
        carry += bytes[j] * BASE
        bytes[j] = carry & 0xff
        carry >>= 8
      }

      while (carry > 0) {
        bytes.push(carry & 0xff)
        carry >>= 8
      }
    }

    // deal with leading zeros
    for (var k = 0; string[k] === LEADER && k < string.length - 1; ++k) {
      bytes.push(0)
    }

    return Buffer.from(bytes.reverse())
  }

  function decode (string) {
    var buffer = decodeUnsafe(string)
    if (buffer) return buffer

    throw new Error('Non-base' + BASE + ' character')
  }

  return {
    encode: encode,
    decodeUnsafe: decodeUnsafe,
    decode: decode
  }
}