module.exports = function base64_encode(stringToEncode) {
  //  discuss at: https://locutus.io/php/base64_encode/
  // original by: Tyler Akins (https://rumkin.com)
  // improved by: Bayron Guevara
  // improved by: Thunder.m
  // improved by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Rafał Kukawski (https://blog.kukawski.pl)
  // bugfixed by: Pellentesque Malesuada
  // improved by: Indigo744
  //   example 1: base64_encode('Kevin van Zonneveld')
  //   returns 1: 'S2V2aW4gdmFuIFpvbm5ldmVsZA=='
  //   example 2: base64_encode('a')
  //   returns 2: 'YQ=='
  //   example 3: base64_encode('✓ à la mode')
  //   returns 3: '4pyTIMOgIGxhIG1vZGU='

  // encodeUTF8string()
  // Internal function to encode properly UTF8 string
  // Adapted from Solution #1 at https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding
  const encodeUTF8string = function (str) {
    // first we use encodeURIComponent to get percent-encoded UTF-8,
    // then we convert the percent encodings into raw bytes which
    // can be fed into the base64 encoding algorithm.
    return encodeURIComponent(str).replace(/%([0-9A-F]{2})/g, function toSolidBytes(match, p1) {
      return String.fromCharCode('0x' + p1)
    })
  }

  if (typeof window !== 'undefined') {
    if (typeof window.btoa !== 'undefined') {
      return window.btoa(encodeUTF8string(stringToEncode))
    }
  } else {
    return new Buffer(stringToEncode).toString('base64')
  }

  const b64 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
  let o1
  let o2
  let o3
  let h1
  let h2
  let h3
  let h4
  let bits
  let i = 0
  let ac = 0
  let enc = ''
  const tmpArr = []

  if (!stringToEncode) {
    return stringToEncode
  }

  stringToEncode = encodeUTF8string(stringToEncode)

  do {
    // pack three octets into four hexets
    o1 = stringToEncode.charCodeAt(i++)
    o2 = stringToEncode.charCodeAt(i++)
    o3 = stringToEncode.charCodeAt(i++)

    bits = (o1 << 16) | (o2 << 8) | o3

    h1 = (bits >> 18) & 0x3f
    h2 = (bits >> 12) & 0x3f
    h3 = (bits >> 6) & 0x3f
    h4 = bits & 0x3f

    // use hexets to index into b64, and append result to encoded string
    tmpArr[ac++] = b64.charAt(h1) + b64.charAt(h2) + b64.charAt(h3) + b64.charAt(h4)
  } while (i < stringToEncode.length)

  enc = tmpArr.join('')

  const r = stringToEncode.length % 3

  return (r ? enc.slice(0, r - 3) : enc) + '==='.slice(r || 3)
}