src/tee.js

import count    from './count';
import filter   from './filter';
import map      from './map';
import range    from './range';
import repeat   from './repeat';
import sequence from './sequence';

/**
 * イテレーターを複数に分けます。
 *
 * @param {number} n - イテレーターの個数
 * @param {*[] | iterator | null} coll - 集合
 *
 * @return {iterator} イテレーターの集合
 */
export default function tee(n, coll) {
  const iter = sequence(coll);

  const queue = [];
  const cupPositions = Array.from(repeat(n, 0));

  return map(function*(i) {
    for (;;) {
      const maxPosition = Math.max(...cupPositions);
      const minPosition = Math.min(...cupPositions);

      if (cupPositions[i] === maxPosition) {
        queue.push(iter.next());
      }

      const next = queue[cupPositions[i] - minPosition];
      if (next.done) {
        break;
      }

      if (cupPositions[i] === minPosition && count(filter(position => position === minPosition, cupPositions)) == 1) {
        queue.shift();
      }

      cupPositions[i]++;

      yield next.value;
    }
  }, range(n));
}