import { concreteChunkId } from "@tsplus/stdlib/collections/Chunk"

/**
 * Statefully and effectfully maps over the elements of this chunk to produce
 * new elements.
 *
 * @tsplus static effect/core/io/Effect.Ops mapAccum
 */
export function mapAccum<A, B, R, E, S>(
  self: Collection<A>,
  s: S,
  f: (s: S, a: A) => Effect<R, E, readonly [S, B]>
): Effect<R, E, readonly [S, Chunk<B>]> {
  return Effect.suspendSucceed(() => {
    const chunk = Chunk.from(self)
    const iterator = concreteChunkId(chunk)._arrayLikeIterator()
    let dest: Effect<R, E, S> = Effect.succeed(s)
    let builder = Chunk.empty<B>()
    let next
    while ((next = iterator.next()) && !next.done) {
      const array = next.value
      const length = array.length
      let i = 0
      while (i < length) {
        const a = array[i]!
        dest = dest.flatMap((state) =>
          f(state, a).map(([s, b]) => {
            builder = builder.append(b)
            return s
          })
        )
        i++
      }
    }
    return dest.map((s) => [s, builder] as const)
  })
}