// This file replaces `async/index.js` in bundlers like webpack or Rollup,
// according to `browser` config in `package.json`.

var crypto = self.crypto || self.msCrypto

// This alphabet uses a-z A-Z 0-9 _- symbols.
// Symbols are generated for smaller size.
// -_zyxwvutsrqponmlkjihgfedcba9876543210ZYXWVUTSRQPONMLKJIHGFEDCBA
var url = '-_'
// Loop from 36 to 0 (from z to a and 9 to 0 in Base36).
var i = 36
while (i--) {
  // 36 is radix. Number.prototype.toString(36) returns number
  // in Base36 representation. Base36 is like hex, but it uses 0–9 and a-z.
  url += i.toString(36)
}
// Loop from 36 to 10 (from Z to A in Base36).
i = 36
while (i-- - 10) {
  url += i.toString(36).toUpperCase()
}

module.exports = function (size) {
  var id = ''
  var bytes = crypto.getRandomValues(new Uint8Array(size || 21))
  i = size || 21

  // Compact alternative for `for (var i = 0; i < size; i++)`
  while (i--) {
    // We can’t use bytes bigger than the alphabet. 63 is 00111111 bitmask.
    // This mask reduces random byte 0-255 to 0-63 values.
    // There is no need in `|| ''` and `* 1.6` hacks in here,
    // because bitmask trim bytes exact to alphabet size.
    id += url[bytes[i] & 63]
  }
  return Promise.resolve(id)
}