/**
 * A polled function used by [fromPoll](../globals.html#frompoll)
 */
import {
  EventLike,
  EventStream,
  EventStreamSeed,
  isEnd,
  isValue,
  Scope,
  toEvents,
  valueEvent,
} from "./abstractions"
import { fromSubscribe } from "./fromsubscribe"
import GlobalScheduler from "./scheduler"
import { nop, rename } from "./util"

export type PollFunction<V> = () => EventLike<V>

/**
 Polls given function with given interval.
 When there are subscribers to the stream. Polling ends permanently when
 `f` returns an End event.

 * @param delay poll interval in milliseconds
 * @param poll function to be polled. Can return either V, Event<V> or an array of Event<V>. In the latter cases the End event can be used to stop polling.
 * @typeparam V Type of stream elements
 */
export function fromPoll<V>(
  delay: number,
  poll: PollFunction<V>,
  scope: Scope
): EventStream<V>
export function fromPoll<V>(
  delay: number,
  poll: PollFunction<V>
): EventStreamSeed<V>

export function fromPoll<V>(
  delay: number,
  poll: PollFunction<V>,
  scope?: Scope
): any {
  return rename(
    `fromPoll(${delay},fn)`,
    fromSubscribe((onValue, onEnd = nop) => {
      const interval = GlobalScheduler.scheduler.setInterval(() => {
        const events = toEvents(poll())
        for (const event of events) {
          if (isValue(event)) {
            onValue(event.value)
          } else {
            GlobalScheduler.scheduler.clearInterval(interval)
            onEnd()
          }
        }
      }, delay)
      return () => GlobalScheduler.scheduler.clearInterval(interval)
    }, scope as any)
  )
}