import { SLIDES_SELECTOR } from '../utils/constants.js'
import { queryAll, createStyleSheet } from '../utils/util.js'

/**
 * The reader mode lets you read a reveal.js presentation
 * as a linear scrollable page.
 */
export default class Reader {

	constructor( Reveal ) {

		this.Reveal = Reveal;

		this.active = false;
		this.activatedCallbacks = [];

	}

	/**
	 * Activates the reader mode. This rearranges the presentation DOM
	 * by—among other things—wrapping each slide in a page element.
	 */
	async activate() {

		if( this.active ) return;

		this.active = true;

		this.slideHTMLBeforeActivation = this.Reveal.getSlidesElement().innerHTML;

		const viewportElement = this.Reveal.getViewportElement();
		const slides = queryAll( this.Reveal.getRevealElement(), SLIDES_SELECTOR );

		viewportElement.classList.add( 'loading-scroll-mode', 'reveal-reader' );
		viewportElement.addEventListener( 'scroll', this.onScroll.bind( this ) );

		let presentationBackground;
		if( viewportElement ) {
			const viewportStyles = window.getComputedStyle( viewportElement );
			if( viewportStyles && viewportStyles.background ) {
				presentationBackground = viewportStyles.background;
			}
		}

		const pageElements = [];
		const pageContainer = slides[0].parentNode;

		// Slide and slide background layout
		slides.forEach( function( slide ) {

			// Vertical stacks are not centred since their section
			// children will be
			if( slide.classList.contains( 'stack' ) === false ) {
				// Wrap the slide in a page element and hide its overflow
				// so that no page ever flows onto another
				const page = document.createElement( 'div' );
				page.className = 'reader-page';
				pageElements.push( page );

				// Copy the presentation-wide background to each individual
				// page when printing
				if( presentationBackground ) {
					page.style.background = presentationBackground;
				}

				const stickyContainer = document.createElement( 'div' );
				stickyContainer.className = 'reader-page-sticky';
				page.appendChild( stickyContainer );

				const contentContainer = document.createElement( 'div' );
				contentContainer.className = 'reader-page-content';
				stickyContainer.appendChild( contentContainer );

				contentContainer.appendChild( slide );

				slide.classList.remove( 'past', 'future' );

				if( slide.slideBackgroundElement ) {
					slide.slideBackgroundElement.remove( 'past', 'future' );
					contentContainer.insertBefore( slide.slideBackgroundElement, slide );
				}

			}

		}, this );

		// Remove leftover stacks
		queryAll( this.Reveal.getRevealElement(), '.stack' ).forEach( stack => stack.remove() );

		await new Promise( requestAnimationFrame );

		pageElements.forEach( page => pageContainer.appendChild( page ) );

		// Re-run JS-based content layout after the slide is added to page DOM
		this.Reveal.slideContent.layout( this.Reveal.getSlidesElement() );

		this.Reveal.layout();

		viewportElement.classList.remove( 'loading-scroll-mode' );

		this.activatedCallbacks.forEach( callback => callback() );
		this.activatedCallbacks = [];

	}

	/**
	 * Deactivates the reader mode and restores the standard slide-based
	 * presentation.
	 */
	deactivate() {

		if( !this.active ) return;

		this.active = false;

		this.Reveal.getViewportElement().classList.remove( 'reveal-reader' );
		this.Reveal.getSlidesElement().innerHTML = this.slideHTMLBeforeActivation;
		this.Reveal.sync();

		// TODO Navigate to the slide that is currently scrolled into view
		this.Reveal.slide( 0 );

	}

	toggle() {

		if( this.active === true ) {
			this.deactivate();
		}
		else {
			this.activate();
		}

	}

	/**
	 * Checks if the reader mode is currently active.
	 */
	isActive() {

		return this.active;

	}

	/**
	 * Updates our reader pages to match the latest configuration and
	 * presentation size.
	 */
	sync() {

		const config = this.Reveal.getConfig();

		const slideSize = this.Reveal.getComputedSlideSize( window.innerWidth, window.innerHeight );
		const scale = this.Reveal.getScale();
		const readerLayout = config.readerLayout;

		const viewportElement = this.Reveal.getViewportElement();
		const viewportHeight = viewportElement.offsetHeight;
		const compactHeight = slideSize.height * scale;
		const pageHeight = readerLayout === 'full' ? viewportHeight : compactHeight;

		// The height that needs to be scrolled between scroll triggers
		const scrollTriggerHeight = viewportHeight / 2;

		viewportElement.style.setProperty( '--page-height', pageHeight + 'px' );
		viewportElement.style.scrollSnapType = typeof config.readerScrollSnap === 'string' ?
												`y ${config.readerScrollSnap}` : '';

		const pageElements = Array.from( this.Reveal.getRevealElement().querySelectorAll( '.reader-page' ) );

		this.pages = pageElements.map( pageElement => {
			const page = {
				pageElement: pageElement,
				stickyElement: pageElement.querySelector( '.reader-page-sticky' ),
				slideElement: pageElement.querySelector( 'section' ),
				backgroundElement: pageElement.querySelector( '.slide-background' ),
				top: pageElement.offsetTop,
				scrollTriggers: []
			};

			page.slideElement.style.width = slideSize.width + 'px';
			page.slideElement.style.height = config.center === true ? '' : slideSize.height + 'px';

			// Each fragment 'group' is an array containing one or more
			// fragments. Multiple fragments that appear at the same time
			// are part of the same group.
			page.fragments = this.Reveal.fragments.sort( pageElement.querySelectorAll( '.fragment:not(.disabled)' ) );
			page.fragmentGroups = this.Reveal.fragments.sort( pageElement.querySelectorAll( '.fragment' ), true );

			// Create scroll triggers that show/hide fragments
			if( page.fragmentGroups.length ) {
				const segmentSize = 1 / ( page.fragmentGroups.length + 1 );
				page.scrollTriggers.push(
					// Trigger for the initial state with no fragments visible
					{ range: [ 0, segmentSize ], fragmentIndex: -1 },

					// Triggers for each fragment group
					...page.fragmentGroups.map( ( fragments, i ) => ({
						range: [ segmentSize * ( i + 1 ), segmentSize * ( i + 2 ) ],
						fragmentIndex: i
					}))
				);
			}

			// Add scroll padding based on how many scroll triggers we have
			page.scrollPadding = scrollTriggerHeight * page.scrollTriggers.length;

			// In the compact layout, only slides with scroll triggers cover the
			// full viewport height. This helps avoid empty gaps before or after
			// a sticky slide.
			if( readerLayout === 'compact' && page.scrollTriggers.length > 0 ) {
				page.pageHeight = viewportHeight;
				page.pageElement.style.setProperty( '--page-height', viewportHeight + 'px' );
			}
			else {
				page.pageHeight = pageHeight;
				page.pageElement.style.removeProperty( '--page-height' );
			}

			page.pageElement.style.scrollSnapAlign = page.pageHeight < viewportHeight ? 'center' : 'start';

			// This variable is used to pad the height of our page in CSS
			page.pageElement.style.setProperty( '--page-scroll-padding', page.scrollPadding + 'px' );

			// The total height including scrollable space
			page.totalHeight = page.pageHeight + page.scrollPadding;

			page.bottom = page.top + page.totalHeight;

			// If this is a sticky page, stick it to the vertical center
			if( page.scrollTriggers.length > 0 ) {
				page.stickyElement.style.position = 'sticky';
				page.stickyElement.style.top = Math.max( ( viewportHeight - page.pageHeight ) / 2, 0 ) + 'px';

				// Make this page freeze at the vertical center of the viewport
				page.top -= ( viewportHeight - page.pageHeight ) / 2;
			}
			else {
				page.stickyElement.style.position = 'relative';
			}

			return page;
		} );

	}

	layout() {

		this.sync();
		this.onScroll();

	}

	scrollToSlide( slideElement ) {

		if( !this.active ) {
			this.activatedCallbacks.push( () => this.scrollToSlide( slideElement ) );
		}
		else {
			slideElement.parentNode.scrollIntoView();
		}

	}

	onScroll() {

		const viewportElement = this.Reveal.getViewportElement();
		const viewportHeight = viewportElement.offsetHeight;

		const scrollTop = viewportElement.scrollTop;

		// Find the page closest to the center of the viewport, this
		// is the page we want to focus and activate
		const activePage = this.pages.reduce( ( closestPage, page ) => {
			const distance = Math.abs( ( page.top + page.pageHeight / 2 ) - scrollTop - viewportHeight / 2 );
			return distance < closestPage.distance ? { page, distance } : closestPage;
		}, { page: this.pages[0], distance: Infinity } ).page;

		this.pages.forEach( ( page, pageIndex ) => {
			const isWithinPreloadRange = scrollTop + viewportHeight >= page.top - viewportHeight && scrollTop < page.top + page.bottom + viewportHeight;
			const isPartiallyVisible = scrollTop + viewportHeight >= page.top && scrollTop < page.top + page.bottom;

			// Preload content when it appears within range
			if( isWithinPreloadRange ) {
				if( !page.preloaded ) {
					page.preloaded = true;
					this.Reveal.slideContent.load( page.slideElement );
				}
			}
			else if( page.preloaded ) {
				page.preloaded = false;
				this.Reveal.slideContent.unload( page.slideElement );
			}

			// Activate the current page — there can only be one active page at
			// a time.
			if( page === activePage ) {
				if( !page.active ) {
					page.active = true;
					page.pageElement.classList.add( 'present' );
					page.slideElement.classList.add( 'present' );

					this.Reveal.setCurrentReaderPage( pageIndex, page.pageElement );
					this.Reveal.slideContent.startEmbeddedContent( page.slideElement );

					if( page.backgroundElement ) {
						this.Reveal.slideContent.startEmbeddedContent( page.backgroundElement );
					}
				}
			}
			// Deactivate previously active pages
			else if( page.active ) {
				page.active = false;
				page.pageElement.classList.remove( 'present' );
				page.slideElement.classList.remove( 'present' );
				this.Reveal.slideContent.stopEmbeddedContent( page.slideElement );

				if( page.backgroundElement ) {
					this.Reveal.slideContent.stopEmbeddedContent( page.backgroundElement );
				}
			}

			// Handle scroll freezing and triggers for slides in view
			if( isPartiallyVisible && page.totalHeight > page.pageHeight ) {
				let scrollProgress = ( scrollTop - page.top ) / page.scrollPadding;
				scrollProgress = Math.max( Math.min( scrollProgress, 1 ), 0 );

				page.scrollTriggers.forEach( trigger => {
					if( scrollProgress >= trigger.range[0] && scrollProgress < trigger.range[1] ) {
						if( !trigger.active ) {
							trigger.active = true;
							this.Reveal.fragments.update( trigger.fragmentIndex, page.fragments, page.slideElement );
						}
					}
					else {
						trigger.active = false;
					}
				} );
			}
		} );

	}

}
