scan.ts

import {
  Event,
  EventStream,
  EventStreamSeed,
  isValue,
  Observer,
  Property,
  PropertySeed,
  valueEvent,
  Scope,
} from "./abstractions"
import { applyScopeMaybe } from "./applyscope"
import { PropertySeedImpl } from "./property"

export function scan<A, B>(
  initial: B,
  fn: (state: B, next: A) => B,
  scope: Scope
): (stream: EventStream<A> | EventStreamSeed<A>) => Property<B>
export function scan<A, B>(
  initial: B,
  fn: (state: B, next: A) => B
): (stream: EventStream<A> | EventStreamSeed<A>) => PropertySeed<B>

export function scan<A, B>(
  initial: B,
  fn: (state: B, next: A) => B,
  scope?: Scope
): any {
  return (seed: EventStream<A> | EventStreamSeed<A>) => {
    const source = seed.consume()
    let current = initial
    return applyScopeMaybe(
      new PropertySeedImpl(
        [source, "scan", [fn]],
        () => initial,
        (onValue: Observer<B>, onEnd?: Observer<void>) => {
          const unsub = source.subscribe((event) => {
            current = fn(current, event)
            onValue(current)
          }, onEnd)
          return unsub
        }
      ),
      scope
    )
  }
}