let crypto = require('crypto')

let { urlAlphabet } = require('./url-alphabet/index.cjs')

// We reuse buffers with the same size to avoid memory fragmentations
// for better performance.
let buffers = {}
let random = bytes => {
  let buffer = buffers[bytes]
  if (!buffer) {
    // `Buffer.allocUnsafe()` is faster because it doesn’t flush the memory.
    // Memory flushing is unnecessary since the buffer allocation itself resets
    // the memory with the new bytes.
    buffer = Buffer.allocUnsafe(bytes)
    if (bytes <= 255) buffers[bytes] = buffer
  }
  return crypto.randomFillSync(buffer)
}

let customRandom = (alphabet, size, getRandom) => {
  // First, a bitmask is necessary to generate the ID. The bitmask makes bytes
  // values closer to the alphabet size. The bitmask calculates the closest
  // `2^31 - 1` number, which exceeds the alphabet size.
  // For example, the bitmask for the alphabet size 30 is 31 (00011111).
  let mask = (2 << (31 - Math.clz32((alphabet.length - 1) | 1))) - 1
  // Though, the bitmask solution is not perfect since the bytes exceeding
  // the alphabet size are refused. Therefore, to reliably generate the ID,
  // the random bytes redundancy has to be satisfied.

  // Note: every hardware random generator call is performance expensive,
  // because the system call for entropy collection takes a lot of time.
  // So, to avoid additional system calls, extra bytes are requested in advance.

  // Next, a step determines how many random bytes to generate.
  // The number of random bytes gets decided upon the ID size, mask,
  // alphabet size, and magic number 1.6 (using 1.6 peaks at performance
  // according to benchmarks).
  let step = Math.ceil((1.6 * mask * size) / alphabet.length)

  return () => {
    let id = ''
    while (true) {
      let bytes = getRandom(step)
      // A compact alternative for `for (var i = 0; i < step; i++)`.
      let i = step
      while (i--) {
        // Adding `|| ''` refuses a random byte that exceeds the alphabet size.
        id += alphabet[bytes[i] & mask] || ''
        // `id.length + 1 === size` is a more compact option.
        if (id.length === +size) return id
      }
    }
  }
}

let customAlphabet = (alphabet, size) => customRandom(alphabet, size, random)

let nanoid = (size = 21) => {
  let bytes = random(size)
  let id = ''
  // A compact alternative for `for (var i = 0; i < step; i++)`.
  while (size--) {
    // It is incorrect to use bytes exceeding the alphabet size.
    // The following mask reduces the random byte in the 0-255 value
    // range to the 0-63 value range. Therefore, adding hacks, such
    // as empty string fallback or magic numbers, is unneccessary because
    // the bitmask trims bytes down to the alphabet size.
    id += urlAlphabet[bytes[size] & 63]
  }
  return id
}

module.exports = { nanoid, customAlphabet, customRandom, urlAlphabet, random }