export function extremaBy<TSource, TKey>(
  source: Iterable<TSource>,
  selector: (item: TSource) => TKey,
  comparer: (left: TKey, right: TKey) => number
): TSource[] {
  let result = [];
  const it = source[Symbol.iterator]();
  const { value, done } = it.next();
  if (done) {
    throw new Error('Sequence contains no elements');
  }

  let resKey = selector(value);
  result.push(value);

  let next: IteratorResult<TSource>;
  while (!(next = it.next()).done) {
    const current = next.value;
    const key = selector(current);
    const cmp = comparer(key, resKey);

    if (cmp === 0) {
      result.push(current);
    } else if (cmp > 0) {
      result = [current];
      resKey = key;
    }
  }

  return result;
}