export async function concat<T, R>(arr: T[], iterator: ((item: T) => Promise<R[]>)): Promise<R[]> {
  let awaits: Promise<R[]>[] = []
  for (let item of arr) {
    awaits.push(iterator(item))
  }

  let results: R[] = []

  for (let item of awaits) {
    results = results.concat(await item)
  }

  return results
}

export async function concatSeries<T, R>(arr: T[], iterator: ((item: T) => Promise<R[]>)): Promise<R[]> {
  let results: R[] = []

  for (let item of arr) {
    results = results.concat(await iterator(item))
  }

  return results
}

export async function concatLimit<T, R>(limit: number, arr: T[], iterator: ((item: T) => Promise<R[]>)): Promise<R[]> {
  if (limit < 1) {
    throw new Error('limit must be greater then 0.')
  }

  let awaits: Promise<R[]>[] = []
  let results: R[] = []

  for (let i = 0; i < arr.length; i++) {
    let item = arr[i]
    awaits.push(iterator(item))

    if (i % limit === 0) {
      for (let item of awaits) {
        results = results.concat(await item)
      }

      awaits = []
    }
  }

  return results
}