import * as crypto from 'crypto';

function v4_uuid() {
  let rand_bytes = crypto.randomBytes(16);
  let rand_bytes_int = Uint8Array.from(rand_bytes);

  // replace 7th byte with 4 (0100)
  // & 0b00001111 replaces the first 4 bits with 0s
  // | 0b01000000 replaces the first 4 bits with 0100
  rand_bytes_int[6] = (rand_bytes_int[6] & 0b00001111) | 0b01000000;

  // set 2 significant bits of 9th byte to 2 (10)
  // & 0b00111111 replaces the first 2 bits with 0s
  // | 0b10000000 replaces the first 2 bits with 10
  rand_bytes_int[8] = (rand_bytes_int[8] & 0b00111111) | 0b10000000;
  
  let output = '';
  const hyphen_points = [2, 4, 6, 8];
  for (let i = 0; i < rand_bytes_int.length; i++) {
    output += rand_bytes_int[i].toString(16).padStart(2, '0');
    if (hyphen_points.includes(i)) {
      output += '-';
    }
  }

  return output;
}