function fib (n, cache) {
  cache = cache || {};

  if(n >= -1 && n <= 1) return BigInt(n);
  let a, b;

  if(n > 0) {
    a = cache[n-1] ? cache[n-1] : cache[n-1] = fib(n-1, cache);
    b = cache[n-2] ? cache[n-2] : cache[n-2] = fib(n-2, cache);

  } else {
    a = cache[n+1] ? cache[n+1] : cache[n+1] = fib(n+1, cache);
    b = cache[n+2] ? cache[n+2] : cache[n+2] = fib(n+2, cache);
  }

  return BigInt(a + b);
}