export const intoObservable = <T>(value: T | Promise<T> | Observable<T>): Observable<T> => {
  if (isObservable(value)) {
    return Observable.from(value)
  }
  else if (isPromise(value)) {
    return new Observable<T>(observer => {
      value.then(
        value => {
          if (observer.closed) return
          observer.next(value)
          observer.complete()
        },
        (error: Error) => {
          if (observer.closed) return
          observer.error(error)
          observer.complete()
        }
      )
      return () => {}
    })
  }
  else {
    return new Observable<T>(observer => {
      observer.next(value)
      observer.complete()
      return () => {}
    })
  }
}

const isObservable = <T>(value: any): value is Observable<T> =>
     value != null
  && typeof value === 'object'
  && value[Symbol['observable']]

const isPromise = <T>(value: any): value is Promise<T> =>
     value != null
  && typeof value === 'object'
  && typeof value.then === 'function'

export const debounce = <T>(observable: Observable<T>, timeout: number = 0): Observable<T> =>
  new Observable<T>(observer => {
    let timeoutID: any
    let lastValue: T

    const subscription = observable.subscribe(
      nextValue => {
        lastValue = nextValue
        clearTimeout(timeoutID)
        timeoutID = setTimeout(() => observer.next(lastValue), timeout)
      },
      error => observer.error(error),
      () => {
        clearTimeout(timeoutID)
        observer.next(lastValue)
        observer.complete()
      }
    )
    return () => {
      clearTimeout(timeoutID)
      subscription.unsubscribe()
    }
  })