Promise.filter = function (iterable, filterer, options = {}) {
  let concurrency = options.concurrency || Infinity

  let index = 0
  const results = []
  const predicates = []
  const pending = []
  const iterator = iterable[Symbol.iterator]()

  while (concurrency-- > 0) {
    const pending = wrappedFilterer()
    if (pending) pending.push(promise)
    else break
  }

  return Promise.all(pending).then(() => results.filter((v, i) => predicates[i]))

  function wrappedFilterer () {
    const next = iterator.next()
    if (next.done) return null
    const i = index++
    results.push(next.value)
    const predicate = filterer(next.value, i)
    return Promise.resolve(predicate).then(resolved => {
      predicates[i] = resolved
      return wrappedFilterer()
    })
  }
}