import {merge, Observable, of as observableOf, Subject} from 'rxjs'
import {filter, map, mergeMap} from 'rxjs/operators'

import {orientationChange$} from './orientationChange'
import {resize$} from './resize'
import {scroll$} from './scroll'

const ROOT_MARGIN_PX = 150

/*
  Adapted from the polyfill at https://github.com/WICG/IntersectionObserver
*/
function isIntersectionObserverSupported() {
  if (
    typeof window !== 'undefined' &&
    'IntersectionObserver' in window &&
    'IntersectionObserverEntry' in window &&
    'intersectionRatio' in IntersectionObserverEntry.prototype
  ) {
    // Minimal polyfill for Edge 15's lack of `isIntersecting`
    // See: https://github.com/WICG/IntersectionObserver/issues/211
    if (!('isIntersecting' in IntersectionObserverEntry.prototype)) {
      Object.defineProperty(IntersectionObserverEntry.prototype, 'isIntersecting', {
        get() {
          return this.intersectionRatio > 0
        },
      })
    }
    return true
  }
  return false
}

type IntersectionEvent = {isIntersecting: boolean}

export const intersectionObservableFor = isIntersectionObserverSupported()
  ? createIntersectionObserverBased()
  : createLegacyBased()

type IntersectionObservableFor = (element: Element) => Observable<IntersectionEvent>

function createIntersectionObserverBased(): IntersectionObservableFor {
  const intersectionObserverEntriesSubject = new Subject<IntersectionObserverEntry>()

  const intersectionObserver = new IntersectionObserver(
    (entries) => {
      entries.forEach((entry) => {
        intersectionObserverEntriesSubject.next(entry)
      })
    },
    {
      threshold: 0,
      rootMargin: `${ROOT_MARGIN_PX}px`,
    },
  )

  // eslint-disable-next-line @typescript-eslint/no-shadow
  return function intersectionObservableFor(element) {
    return new Observable((observer) => {
      intersectionObserver.observe(element)
      observer.next()
      return () => intersectionObserver.unobserve(element)
    }).pipe(
      mergeMap(() => intersectionObserverEntriesSubject.asObservable()),
      filter((entry: IntersectionObserverEntry) => entry.target === element),
      map((ev) => ({
        isIntersecting: ev.isIntersecting,
      })),
    )
  }
}

// This can be removed when intersection observer are supported by the browsers we support
function createLegacyBased() {
  function getViewport() {
    return {
      left: 0,
      right: window.innerWidth,
      top: 0,
      bottom: window.innerHeight,
    }
  }

  function intersects(
    rect: DOMRect,
    viewport: {left: number; right: number; top: number; bottom: number},
    margin: number,
  ) {
    return (
      rect.left <= viewport.right + margin &&
      rect.right >= viewport.left - margin &&
      rect.top <= viewport.bottom + margin &&
      rect.bottom >= viewport.top - margin
    )
  }

  function inViewport(element: HTMLElement) {
    return () => intersects(element.getBoundingClientRect(), getViewport(), ROOT_MARGIN_PX)
  }

  // eslint-disable-next-line @typescript-eslint/no-shadow
  return function intersectionObservableFor(element: HTMLElement): Observable<IntersectionEvent> {
    const isElementInViewport = inViewport(element)
    return merge(observableOf(isElementInViewport()), resize$, scroll$, orientationChange$).pipe(
      // @todo: consider "faking" more of the IntersectionObserverEntry api if possible
      map(isElementInViewport),
      map((isIntersecting) => ({isIntersecting})),
    )
  }
}