import { Observable as ObservableT, Subscription } from './index'
import Observable from './observable'

/**
 * Creates an observable that merges the emits of many sources
 *
 * {@link merge} takes an arbitrary amount of source `Observable<T>s` and emits any value from any of those sources in
 * the returned Observable<*>. The output Observable<*> will only complete() when all sources complete(). Calling unsubscribe() on the output Observable<*> will call unsubscribe() on all source Observable<*>s.
 *
 * @example <caption>Marble diagram merging two sources</caption>
 *
 * // source a:    |--1-----2-----3-----4-----|
 * // source b:    |--9-----8--------7-----6--|
 * // merge(a, b): |--1-9---2-8---3--7--4--6--|
 *
 * @param {Array<ObservableT<T>>} sources the source observables
 * @returns {ObservableT<T>} the observable that merges all the emits of all the sources
 */
export default function merge<A, B>(
  a: ObservableT<A>,
  b: ObservableT<B>
): ObservableT<A | B>
export default function merge<A, B, C>(
  a: ObservableT<A>,
  b: ObservableT<B>,
  c: ObservableT<C>
): ObservableT<A | B | C>
export default function merge<A, B, C, D>(
  a: ObservableT<A>,
  b: ObservableT<B>,
  c: ObservableT<C>,
  d: ObservableT<D>
): ObservableT<A | B | C | D>
export default function merge<T>(
  ...sources: Array<ObservableT<T>>
): ObservableT<T> {
  return new Observable(
    ({ error, next, complete }): (() => void) => {
      let remaining = sources.length
      const subscriptions = sources.map(
        (source): Subscription =>
          source.subscribe({
            error,
            next,
            complete: (): false | void => (remaining -= 1) === 0 && complete()
          })
      )
      return (): void => {
        for (const { unsubscribe } of subscriptions) {
          unsubscribe()
        }
      }
    }
  )
}