김동현님 질문 코드

solution.js

function* filter(f, iter) {
  for (const a of iter) if (f(a)) yield a;
}

function* take(limit, iter) {
  for (const a of iter) if (limit--) yield a;
}

const head = iter => take(1, iter).next().value;

const find = (f, iter) => head(filter(f, iter));

function* map(f, iter) {
  for (const a of iter) yield f(a);
}

const zipWithCount = (iter, cnt = 1) => map(a => [a, cnt++], iter);

function* infinity(f, acc) {
  while (true) yield (acc = f(acc));
}

const isOdd = a => a % 2;

const calc = a => isOdd(a) ? a * 3 + 1 : a / 2;

const iter = zipWithCount(infinity(calc, 1));
console.log(iter.next().value); // [4, 1]
console.log(iter.next().value); // [2, 2]
console.log(iter.next().value); // [1, 3]
console.log(iter.next().value); // [4, 4]
console.log(iter.next().value); // [2, 5]

const [num, count] =
  find(([a]) => a == 1,
    zipWithCount(
      infinity(calc, 1)));

console.log(num, count); // 1, 3