interface ReduceResult<T> {
  done: true;
  value: T;
}

export function reduceLargeArray<T, R>(
  arr: T[],
  processEachFn: (acc: R, x: T) => ReduceResult<R> | R,
  initialValue: R,
  onCompleteFn: (r: R) => void
) {
  let frameTime = 1000 / 60; // 60 FPS
  let accumulator = initialValue;

  function isDone<T>(value: unknown): value is ReduceResult<T> {
    return value && value['done'];
  }

  function nextBatch(start: number) {
    let tstart = Date.now();

    for (let i = start; i < arr.length; i++) {
      let tdelta = Date.now() - tstart;

      if (tdelta >= frameTime) {
        requestAnimationFrame(() => nextBatch(i));
        break;
      }

      let resultOrDone = processEachFn(accumulator, arr[i]);

      if (isDone(resultOrDone)) {
        onCompleteFn(resultOrDone.value);
        break;
      } else if (i === arr.length - 1) {
        onCompleteFn(resultOrDone);
        break;
      }

      accumulator = resultOrDone;
    }
  }

  if (arr.length === 0) {
    onCompleteFn(accumulator);
  } else {
    nextBatch(0);
  }
}