tee.ts

// Tees an iterable into two iterables while keeping memory costs low.
// Functional style.

export function tee<T>(iterable: Iterable<T>): [Generator<T>, Generator<T>] {
  const iterator = iterable[Symbol.iterator]()
  let isDone = false

  const aQueue: T[] = []
  const bQueue: T[] = []

  function* teePart(myQueue: T[], otherQueue: T[]) {
    while (true) {
      if (myQueue.length != 0) {
        yield myQueue.shift()!
        continue
      }

      if (isDone) {
        return
      }

      const result = iterator.next()

      if (result.done) {
        isDone = true
        return
      }

      otherQueue.push(result.value)
      yield result.value
    }
  }

  return [teePart(aQueue, bQueue), teePart(bQueue, aQueue)]
}