iterator.js

/**
 * Utility module to create and manipulate Iterators.
 *
 * @module iterator
 */

/**
 * @template T,R
 * @param {Iterator<T>} iterator
 * @param {function(T):R} f
 * @return {IterableIterator<R>}
 */
export const mapIterator = (iterator, f) => ({
  [Symbol.iterator] () {
    return this
  },
  // @ts-ignore
  next () {
    const r = iterator.next()
    return { value: r.done ? undefined : f(r.value), done: r.done }
  }
})

/**
 * @template T
 * @param {function():IteratorResult<T>} next
 * @return {IterableIterator<T>}
 */
export const createIterator = next => ({
  /**
   * @return {IterableIterator<T>}
   */
  [Symbol.iterator] () {
    return this
  },
  // @ts-ignore
  next
})

/**
 * @template T
 * @param {Iterator<T>} iterator
 * @param {function(T):boolean} filter
 */
export const iteratorFilter = (iterator, filter) => createIterator(() => {
  let res
  do {
    res = iterator.next()
  } while (!res.done && !filter(res.value))
  return res
})

/**
 * @template T,M
 * @param {Iterator<T>} iterator
 * @param {function(T):M} fmap
 */
export const iteratorMap = (iterator, fmap) => createIterator(() => {
  const { done, value } = iterator.next()
  return { done, value: done ? undefined : fmap(value) }
})