Provably Fair System

import { createHash } from 'crypto';

const TICKETS_AMOUNT = 5000;
const BLOCK_HASHES = [
  '000000000000000000abcd1234ef567890abcdef1234567890abcdef1234567890',
  '000000000000000000bcde2345ef678901abcdef1234567890abcdef1234567890',
  '000000000000000000cdef3456ef789012abcdef1234567890abcdef1234567890'
];

/**
 * Combine block hashes using a clear separator to avoid any ambiguity.
 *
 * @param {string[]} hashes - Array of block hashes.
 * @returns {string} Combined hash string.
 */
function combineBlockHashes(hashes) {
  return hashes.join(':');
}

/**
 * Generate a SHA-256 hash from the combined block hashes and return it as a BigInt.
 *
 * @returns {BigInt} The BigInt representation of the SHA-256 hash.
 */
function getFinalHashBigInt() {
  const combinedHash = combineBlockHashes(BLOCK_HASHES);
  const hashBuffer = createHash('sha256').update(combinedHash).digest();
  return BigInt('0x' + hashBuffer.toString('hex'));
}

/**
 * Generate a random ticket number between 1 and TICKETS_AMOUNT (inclusive)
 * using rejection sampling to eliminate any modulo bias.
 *
 * @returns {number} The winning ticket number.
 */
function getRandomTicketNumber() {
  const maxValue = BigInt(2) ** BigInt(256); // Maximum possible value for a 256-bit hash.
  let candidate = getFinalHashBigInt();

  // Define the limit to eliminate modulo bias:
  // Only values strictly below 'limit' are accepted.
  const limit = maxValue - (maxValue % BigInt(TICKETS_AMOUNT));

  // Rejection sampling: rehash if candidate is in the biased zone.
  while (candidate >= limit) {
    // Rehash candidate to generate a new candidate.
    const candidateBuffer = Buffer.from(candidate.toString(16).padStart(64, '0'), 'hex');
    candidate = BigInt('0x' + createHash('sha256').update(candidateBuffer).digest('hex'));
  }

  // Map candidate uniformly to [1, TICKETS_AMOUNT]
  return Number(candidate % BigInt(TICKETS_AMOUNT)) + 1;
}

const winningTicketNumber = getRandomTicketNumber();
console.log(`Winning Ticket Number: ${winningTicketNumber}`);