class Observable {

  constructor(subscriber) {
    this.subscriber = subscriber
  }

  subscribe(observer) {
    if (typeof observer == 'function') observer = {next: observer}
    if (!observer.next)     observer.next     = () => {}
    if (!observer.complete) observer.complete = () => {}
    if (!observer.error)    observer.error    = () => {}
    return {unsubscribe: this.subscriber(observer) || (() => {})}
  }

  static fromEvent(object, event) {
    return new Observable(observer => {
      const handler = event => observer.next(event)
      object.addEventListener(event, handler)
      return () => object.removeEventListener(event, handler)
    })
  }

  do(cb) {
    return new Observable(observer => {
      const subscription = this.subscribe({
        next: value => {
          cb(value)
          observer.next(value)
        },
        complete: () => observer.complete(),
        error: err => observer.error(err),
      })
      return () => subscription.unsubscribe()
    })
  }

  map(cb) {
    return new Observable(observer => {
      const subscription = this.subscribe({
        next: value => observer.next(cb(value)),
        complete: () => observer.complete(),
        error: err => observer.error(err),
      })
      return () => subscription.unsubscribe()
    })
  }

  filter(cb) {
    return new Observable(observer => {
      const subscription = this.subscribe({
        next: value => { if (cb(value)) observer.next(value) },
        complete: () => observer.complete(),
        error: err => observer.error(err),
      })
      return () => subscription.unsubscribe()
    })
  }

  filterMap(cb) {
    return new Observable(observer => {
      const subscription = this.subscribe({
        next: value => {
          value = cb(value)
          if (value) observer.next(value)
        },
        complete: () => observer.complete(),
        error: err => observer.error(err),
      })
      return () => subscription.unsubscribe()
    })
  }

  debounceTime(time) {
    return new Observable(observer => {
      let i
      const subscription = this.subscribe({
        next: value => {
          if (i) clearTimeout(i)
          i = setTimeout(() => observer.next(value), time)
        },
        complete: () => setTimeout(() => observer.complete(), time),
        error: err => setTimeout(() => observer.error(err), time),
      })
      return () => subscription.unsubscribe()
    })
  }

  distinctUntilChanged() {
    return new Observable(observer => {
      let last = null
      const subscription = this.subscribe({
        next: value => {
          if (value == last) return
          last = value
          observer.next(value)
        },
        complete: () => observer.complete(),
        error: err => observer.error(err),
      })
      return () => subscription.unsubscribe()
    })
  }

}