reveal.js

				if( config.center ) {
					// Vertical stacks are not centred since their section
					// children will be
					if( slide.classList.contains( 'stack' ) ) {
						slide.style.top = 0;
					}
					else {
						slide.style.top = Math.max( - ( getAbsoluteHeight( slide ) / 2 ) - 20, -slideHeight / 2 ) + 'px';
					}
				}
				else {
					slide.style.top = '';
				}

			}

			updateProgress();

		}

	}

	/**
	 * Stores the vertical index of a stack so that the same
	 * vertical slide can be selected when navigating to and
	 * from the stack.
	 *
	 * @param {HTMLElement} stack The vertical stack element
	 * @param {int} v Index to memorize
	 */
	function setPreviousVerticalIndex( stack, v ) {

		if( typeof stack === 'object' && typeof stack.setAttribute === 'function' ) {
			stack.setAttribute( 'data-previous-indexv', v || 0 );
		}

	}

	/**
	 * Retrieves the vertical index which was stored using
	 * #setPreviousVerticalIndex() or 0 if no previous index
	 * exists.
	 *
	 * @param {HTMLElement} stack The vertical stack element
	 */
	function getPreviousVerticalIndex( stack ) {

		if( typeof stack === 'object' && typeof stack.setAttribute === 'function' && stack.classList.contains( 'stack' ) ) {
			// Prefer manually defined start-indexv
			var attributeName = stack.hasAttribute( 'data-start-indexv' ) ? 'data-start-indexv' : 'data-previous-indexv';

			return parseInt( stack.getAttribute( attributeName ) || 0, 10 );
		}

		return 0;

	}

	/**
	 * Displays the overview of slides (quick nav) by
	 * scaling down and arranging all slide elements.
	 *
	 * Experimental feature, might be dropped if perf
	 * can't be improved.
	 */
	function activateOverview() {

		// Only proceed if enabled in config
		if( config.overview ) {

			// Don't auto-slide while in overview mode
			cancelAutoSlide();

			var wasActive = dom.wrapper.classList.contains( 'overview' );

			// Vary the depth of the overview based on screen size
			var depth = window.innerWidth < 400 ? 1000 : 2500;

			dom.wrapper.classList.add( 'overview' );
			dom.wrapper.classList.remove( 'exit-overview' );

			clearTimeout( activateOverviewTimeout );
			clearTimeout( deactivateOverviewTimeout );

			// Not the pretties solution, but need to let the overview
			// class apply first so that slides are measured accurately
			// before we can position them
			activateOverviewTimeout = setTimeout( function(){

				var horizontalSlides = document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR );

				for( var i = 0, len1 = horizontalSlides.length; i < len1; i++ ) {
					var hslide = horizontalSlides[i],
						hoffset = config.rtl ? -105 : 105;

					hslide.setAttribute( 'data-index-h', i );

					// Apply CSS transform
					transformElement( hslide, 'translateZ(-'+ depth +'px) translate(' + ( ( i - indexh ) * hoffset ) + '%, 0%)' );

					if( hslide.classList.contains( 'stack' ) ) {

						var verticalSlides = hslide.querySelectorAll( 'section' );

						for( var j = 0, len2 = verticalSlides.length; j < len2; j++ ) {
							var verticalIndex = i === indexh ? indexv : getPreviousVerticalIndex( hslide );

							var vslide = verticalSlides[j];

							vslide.setAttribute( 'data-index-h', i );
							vslide.setAttribute( 'data-index-v', j );

							// Apply CSS transform
							transformElement( vslide, 'translate(0%, ' + ( ( j - verticalIndex ) * 105 ) + '%)' );

							// Navigate to this slide on click
							vslide.addEventListener( 'click', onOverviewSlideClicked, true );
						}

					}
					else {

						// Navigate to this slide on click
						hslide.addEventListener( 'click', onOverviewSlideClicked, true );

					}
				}

				updateSlidesVisibility();

				layout();

				if( !wasActive ) {
					// Notify observers of the overview showing
					dispatchEvent( 'overviewshown', {
						'indexh': indexh,
						'indexv': indexv,
						'currentSlide': currentSlide
					} );
				}

			}, 10 );

		}

	}

	/**
	 * Exits the slide overview and enters the currently
	 * active slide.
	 */
	function deactivateOverview() {

		// Only proceed if enabled in config
		if( config.overview ) {

			clearTimeout( activateOverviewTimeout );
			clearTimeout( deactivateOverviewTimeout );

			dom.wrapper.classList.remove( 'overview' );

			// Temporarily add a class so that transitions can do different things
			// depending on whether they are exiting/entering overview, or just
			// moving from slide to slide
			dom.wrapper.classList.add( 'exit-overview' );

			deactivateOverviewTimeout = setTimeout( function () {
				dom.wrapper.classList.remove( 'exit-overview' );
			}, 10);

			// Select all slides
			var slides = toArray( document.querySelectorAll( SLIDES_SELECTOR ) );

			for( var i = 0, len = slides.length; i < len; i++ ) {
				var element = slides[i];

				element.style.display = '';

				// Resets all transforms to use the external styles
				transformElement( element, '' );

				element.removeEventListener( 'click', onOverviewSlideClicked, true );
			}

			slide( indexh, indexv );

			cueAutoSlide();

			// Notify observers of the overview hiding
			dispatchEvent( 'overviewhidden', {
				'indexh': indexh,
				'indexv': indexv,
				'currentSlide': currentSlide
			} );

		}
	}

	/**
	 * Toggles the slide overview mode on and off.
	 *
	 * @param {Boolean} override Optional flag which overrides the
	 * toggle logic and forcibly sets the desired state. True means
	 * overview is open, false means it's closed.
	 */
	function toggleOverview( override ) {

		if( typeof override === 'boolean' ) {
			override ? activateOverview() : deactivateOverview();
		}
		else {
			isOverview() ? deactivateOverview() : activateOverview();
		}

	}

	/**
	 * Checks if the overview is currently active.
	 *
	 * @return {Boolean} true if the overview is active,
	 * false otherwise
	 */
	function isOverview() {

		return dom.wrapper.classList.contains( 'overview' );

	}

	/**
	 * Checks if the current or specified slide is vertical
	 * (nested within another slide).
	 *
	 * @param {HTMLElement} slide [optional] The slide to check
	 * orientation of
	 */
	function isVerticalSlide( slide ) {

		// Prefer slide argument, otherwise use current slide
		slide = slide ? slide : currentSlide;

		return slide && !!slide.parentNode.nodeName.match( /section/i );

	}

	/**
	 * Handling the fullscreen functionality via the fullscreen API
	 *
	 * @see http://fullscreen.spec.whatwg.org/
	 * @see https://developer.mozilla.org/en-US/docs/DOM/Using_fullscreen_mode
	 */
	function enterFullscreen() {

		var element = document.body;

		// Check which implementation is available
		var requestMethod = element.requestFullScreen ||
							element.webkitRequestFullscreen ||
							element.webkitRequestFullScreen ||
							element.mozRequestFullScreen ||
							element.msRequestFullScreen;

		if( requestMethod ) {
			requestMethod.apply( element );
		}

	}

	/**
	 * Enters the paused mode which fades everything on screen to
	 * black.
	 */
	function pause() {

		var wasPaused = dom.wrapper.classList.contains( 'paused' );

		cancelAutoSlide();
		dom.wrapper.classList.add( 'paused' );

		if( wasPaused === false ) {
			dispatchEvent( 'paused' );
		}

	}

	/**
	 * Exits from the paused mode.
	 */
	function resume() {

		var wasPaused = dom.wrapper.classList.contains( 'paused' );
		dom.wrapper.classList.remove( 'paused' );

		cueAutoSlide();

		if( wasPaused ) {
			dispatchEvent( 'resumed' );
		}

	}

	/**
	 * Toggles the paused mode on and off.
	 */
	function togglePause() {

		if( isPaused() ) {
			resume();
		}
		else {
			pause();
		}

	}

	/**
	 * Checks if we are currently in the paused mode.
	 */
	function isPaused() {

		return dom.wrapper.classList.contains( 'paused' );

	}

	/**
	 * Steps from the current point in the presentation to the
	 * slide which matches the specified horizontal and vertical
	 * indices.
	 *
	 * @param {int} h Horizontal index of the target slide
	 * @param {int} v Vertical index of the target slide
	 * @param {int} f Optional index of a fragment within the
	 * target slide to activate
	 * @param {int} o Optional origin for use in multimaster environments
	 */
	function slide( h, v, f, o ) {

		// Remember where we were at before
		previousSlide = currentSlide;

		// Query all horizontal slides in the deck
		var horizontalSlides = document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR );

		// If no vertical index is specified and the upcoming slide is a
		// stack, resume at its previous vertical index
		if( v === undefined ) {
			v = getPreviousVerticalIndex( horizontalSlides[ h ] );
		}

		// If we were on a vertical stack, remember what vertical index
		// it was on so we can resume at the same position when returning
		if( previousSlide && previousSlide.parentNode && previousSlide.parentNode.classList.contains( 'stack' ) ) {
			setPreviousVerticalIndex( previousSlide.parentNode, indexv );
		}

		// Remember the state before this slide
		var stateBefore = state.concat();

		// Reset the state array
		state.length = 0;

		var indexhBefore = indexh,
			indexvBefore = indexv;

		// Activate and transition to the new slide
		indexh = updateSlides( HORIZONTAL_SLIDES_SELECTOR, h === undefined ? indexh : h );
		indexv = updateSlides( VERTICAL_SLIDES_SELECTOR, v === undefined ? indexv : v );

		// Update the visibility of slides now that the indices have changed
		updateSlidesVisibility();

		layout();

		// Apply the new state
		stateLoop: for( var i = 0, len = state.length; i < len; i++ ) {
			// Check if this state existed on the previous slide. If it
			// did, we will avoid adding it repeatedly
			for( var j = 0; j < stateBefore.length; j++ ) {
				if( stateBefore[j] === state[i] ) {
					stateBefore.splice( j, 1 );
					continue stateLoop;
				}
			}

			document.documentElement.classList.add( state[i] );

			// Dispatch custom event matching the state's name
			dispatchEvent( state[i] );
		}

		// Clean up the remains of the previous state
		while( stateBefore.length ) {
			document.documentElement.classList.remove( stateBefore.pop() );
		}

		// If the overview is active, re-activate it to update positions
		if( isOverview() ) {
			activateOverview();
		}

		// Find the current horizontal slide and any possible vertical slides
		// within it
		var currentHorizontalSlide = horizontalSlides[ indexh ],
			currentVerticalSlides = currentHorizontalSlide.querySelectorAll( 'section' );

		// Store references to the previous and current slides
		currentSlide = currentVerticalSlides[ indexv ] || currentHorizontalSlide;


		// Show fragment, if specified
		if( typeof f !== 'undefined' ) {
			var fragments = sortFragments( currentSlide.querySelectorAll( '.fragment' ) );

			toArray( fragments ).forEach( function( fragment, indexf ) {
				if( indexf < f ) {
					fragment.classList.add( 'visible' );
				}
				else {
					fragment.classList.remove( 'visible' );
				}
			} );
		}

		// Dispatch an event if the slide changed
		var slideChanged = ( indexh !== indexhBefore || indexv !== indexvBefore );
		if( slideChanged ) {
			dispatchEvent( 'slidechanged', {
				'indexh': indexh,
				'indexv': indexv,
				'previousSlide': previousSlide,
				'currentSlide': currentSlide,
				'origin': o
			} );
		}
		else {
			// Ensure that the previous slide is never the same as the current
			previousSlide = null;
		}

		// Solves an edge case where the previous slide maintains the
		// 'present' class when navigating between adjacent vertical
		// stacks
		if( previousSlide ) {
			previousSlide.classList.remove( 'present' );

			// Reset all slides upon navigate to home
			// Issue: #285
			if ( document.querySelector( HOME_SLIDE_SELECTOR ).classList.contains( 'present' ) ) {
				// Launch async task
				setTimeout( function () {
					var slides = toArray( document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR + '.stack') ), i;
					for( i in slides ) {
						if( slides[i] ) {
							// Reset stack
							setPreviousVerticalIndex( slides[i], 0 );
						}
					}
				}, 0 );
			}
		}

		// Handle embedded content
		if( slideChanged ) {
			stopEmbeddedContent( previousSlide );
			startEmbeddedContent( currentSlide );
		}

		updateControls();
		updateProgress();
		updateBackground();

		// Update the URL hash
		writeURL();

	}

	/**
	 * Syncs the presentation with the current DOM. Useful
	 * when new slides or control elements are added or when
	 * the configuration has changed.
	 */
	function sync() {

		// Subscribe to input
		removeEventListeners();
		addEventListeners();

		// Force a layout to make sure the current config is accounted for
		layout();

		// Reflect the current autoSlide value
		autoSlide = config.autoSlide;

		// Start auto-sliding if it's enabled
		cueAutoSlide();

		// Re-create the slide backgrounds
		createBackgrounds();

		updateControls();
		updateProgress();
		updateBackground();

	}

	/**
	 * Updates one dimension of slides by showing the slide
	 * with the specified index.
	 *
	 * @param {String} selector A CSS selector that will fetch
	 * the group of slides we are working with
	 * @param {Number} index The index of the slide that should be
	 * shown
	 *
	 * @return {Number} The index of the slide that is now shown,
	 * might differ from the passed in index if it was out of
	 * bounds.
	 */
	function updateSlides( selector, index ) {

		// Select all slides and convert the NodeList result to
		// an array
		var slides = toArray( document.querySelectorAll( selector ) ),
			slidesLength = slides.length;

		if( slidesLength ) {

			// Should the index loop?
			if( config.loop ) {
				index %= slidesLength;

				if( index < 0 ) {
					index = slidesLength + index;
				}
			}

			// Enforce max and minimum index bounds
			index = Math.max( Math.min( index, slidesLength - 1 ), 0 );

			for( var i = 0; i < slidesLength; i++ ) {
				var element = slides[i];

				var reverse = config.rtl && !isVerticalSlide( element );

				element.classList.remove( 'past' );
				element.classList.remove( 'present' );
				element.classList.remove( 'future' );

				// http://www.w3.org/html/wg/drafts/html/master/editing.html#the-hidden-attribute
				element.setAttribute( 'hidden', '' );

				if( i < index ) {
					// Any element previous to index is given the 'past' class
					element.classList.add( reverse ? 'future' : 'past' );
				}
				else if( i > index ) {
					// Any element subsequent to index is given the 'future' class
					element.classList.add( reverse ? 'past' : 'future' );

					var fragments = toArray( element.querySelectorAll( '.fragment.visible' ) );

					// No fragments in future slides should be visible ahead of time
					while( fragments.length ) {
						fragments.pop().classList.remove( 'visible' );
					}
				}

				// If this element contains vertical slides
				if( element.querySelector( 'section' ) ) {
					element.classList.add( 'stack' );
				}
			}

			// Mark the current slide as present
			slides[index].classList.add( 'present' );
			slides[index].removeAttribute( 'hidden' );

			// If this slide has a state associated with it, add it
			// onto the current state of the deck
			var slideState = slides[index].getAttribute( 'data-state' );
			if( slideState ) {
				state = state.concat( slideState.split( ' ' ) );
			}

			// If this slide has a data-autoslide attribute associated use this as
			// autoSlide value otherwise use the global configured time
			var slideAutoSlide = slides[index].getAttribute( 'data-autoslide' );
			if( slideAutoSlide ) {
				autoSlide = parseInt( slideAutoSlide, 10 );
			}
			else {
				autoSlide = config.autoSlide;
			}

			cueAutoSlide();

		}
		else {
			// Since there are no slides we can't be anywhere beyond the
			// zeroth index
			index = 0;
		}

		return index;

	}

	/**
	 * Optimization method; hide all slides that are far away
	 * from the present slide.
	 */
	function updateSlidesVisibility() {

		// Select all slides and convert the NodeList result to
		// an array
		var horizontalSlides = toArray( document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ) ),
			horizontalSlidesLength = horizontalSlides.length,
			distanceX,
			distanceY;

		if( horizontalSlidesLength ) {

			// The number of steps away from the present slide that will
			// be visible
			var viewDistance = isOverview() ? 10 : config.viewDistance;

			// Limit view distance on weaker devices
			if( isMobileDevice ) {
				viewDistance = isOverview() ? 6 : 1;
			}

			for( var x = 0; x < horizontalSlidesLength; x++ ) {
				var horizontalSlide = horizontalSlides[x];

				var verticalSlides = toArray( horizontalSlide.querySelectorAll( 'section' ) ),
					verticalSlidesLength = verticalSlides.length;

				// Loops so that it measures 1 between the first and last slides
				distanceX = Math.abs( ( indexh - x ) % ( horizontalSlidesLength - viewDistance ) ) || 0;

				if( verticalSlidesLength ) {

					// Always show the vertical stack itself, even if its child
					// slides are invisible
					horizontalSlide.style.display = 'block';

					var oy = getPreviousVerticalIndex( horizontalSlide );

					for( var y = 0; y < verticalSlidesLength; y++ ) {
						var verticalSlide = verticalSlides[y];

						distanceY = x === indexh ? Math.abs( indexv - y ) : Math.abs( y - oy );

						verticalSlide.style.display = ( distanceX + distanceY ) > viewDistance ? 'none' : 'block';
					}

				}
				else {

					horizontalSlide.style.display = distanceX > viewDistance ? 'none' : 'block';

				}
			}

		}

	}

	/**
	 * Updates the progress bar to reflect the current slide.
	 */
	function updateProgress() {

		// Update progress if enabled
		if( config.progress && dom.progress ) {

			var horizontalSlides = toArray( document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ) );

			// The number of past and total slides
			var totalCount = document.querySelectorAll( SLIDES_SELECTOR + ':not(.stack)' ).length;
			var pastCount = 0;

			// Step through all slides and count the past ones
			mainLoop: for( var i = 0; i < horizontalSlides.length; i++ ) {

				var horizontalSlide = horizontalSlides[i];
				var verticalSlides = toArray( horizontalSlide.querySelectorAll( 'section' ) );

				for( var j = 0; j < verticalSlides.length; j++ ) {

					// Stop as soon as we arrive at the present
					if( verticalSlides[j].classList.contains( 'present' ) ) {
						break mainLoop;
					}

					pastCount++;

				}

				// Stop as soon as we arrive at the present
				if( horizontalSlide.classList.contains( 'present' ) ) {
					break;
				}

				// Don't count the wrapping section for vertical slides
				if( horizontalSlide.classList.contains( 'stack' ) === false ) {
					pastCount++;
				}

			}

			dom.progressbar.style.width = ( pastCount / ( totalCount - 1 ) ) * window.innerWidth + 'px';

		}

	}

	/**
	 * Updates the state of all control/navigation arrows.
	 */
	function updateControls() {

		if ( config.controls && dom.controls ) {

			var routes = availableRoutes();
			var fragments = availableFragments();

			// Remove the 'enabled' class from all directions
			dom.controlsLeft.concat( dom.controlsRight )
							.concat( dom.controlsUp )
							.concat( dom.controlsDown )
							.concat( dom.controlsPrev )
							.concat( dom.controlsNext ).forEach( function( node ) {
				node.classList.remove( 'enabled' );
				node.classList.remove( 'fragmented' );
			} );

			// Add the 'enabled' class to the available routes
			if( routes.left ) dom.controlsLeft.forEach( function( el ) { el.classList.add( 'enabled' );	} );
			if( routes.right ) dom.controlsRight.forEach( function( el ) { el.classList.add( 'enabled' ); } );
			if( routes.up ) dom.controlsUp.forEach( function( el ) { el.classList.add( 'enabled' );	} );
			if( routes.down ) dom.controlsDown.forEach( function( el ) { el.classList.add( 'enabled' ); } );

			// Prev/next buttons
			if( routes.left || routes.up ) dom.controlsPrev.forEach( function( el ) { el.classList.add( 'enabled' ); } );
			if( routes.right || routes.down ) dom.controlsNext.forEach( function( el ) { el.classList.add( 'enabled' ); } );

			// Highlight fragment directions
			if( currentSlide ) {

				// Always apply fragment decorator to prev/next buttons
				if( fragments.prev ) dom.controlsPrev.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );
				if( fragments.next ) dom.controlsNext.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );

				// Apply fragment decorators to directional buttons based on
				// what slide axis they are in
				if( isVerticalSlide( currentSlide ) ) {
					if( fragments.prev ) dom.controlsUp.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );
					if( fragments.next ) dom.controlsDown.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );
				}
				else {
					if( fragments.prev ) dom.controlsLeft.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );
					if( fragments.next ) dom.controlsRight.forEach( function( el ) { el.classList.add( 'fragmented', 'enabled' ); } );
				}
			}

		}

	}

	/**
	 * Updates the background elements to reflect the current 
	 * slide.
	 */
	function updateBackground() {

		// Update the classes of all backgrounds to match the 
		// states of their slides (past/present/future)
		toArray( dom.background.childNodes ).forEach( function( backgroundh, h ) {

			// Reverse past/future classes when in RTL mode
			var horizontalPast = config.rtl ? 'future' : 'past',
				horizontalFuture = config.rtl ? 'past' : 'future';

			backgroundh.className = 'slide-background ' + ( h < indexh ? horizontalPast : h > indexh ? horizontalFuture : 'present' );

			toArray( backgroundh.childNodes ).forEach( function( backgroundv, v ) {

				backgroundv.className = 'slide-background ' + ( v < indexv ? 'past' : v > indexv ? 'future' : 'present' );

			} );

		} );

		// Allow the first background to apply without transition
		setTimeout( function() {
			dom.background.classList.remove( 'no-transition' );
		}, 1 );

	}

	/**
	 * Determine what available routes there are for navigation.
	 *
	 * @return {Object} containing four booleans: left/right/up/down
	 */
	function availableRoutes() {

		var horizontalSlides = document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ),
			verticalSlides = document.querySelectorAll( VERTICAL_SLIDES_SELECTOR );

		var routes = {
			left: indexh > 0 || config.loop,
			right: indexh < horizontalSlides.length - 1 || config.loop,
			up: indexv > 0,
			down: indexv < verticalSlides.length - 1
		};

		// reverse horizontal controls for rtl
		if( config.rtl ) {
			var left = routes.left;
			routes.left = routes.right;
			routes.right = left;
		}

		return routes;

	}

	/**
	 * Returns an object describing the available fragment
	 * directions.
	 *
	 * @return {Object} two boolean properties: prev/next
	 */
	function availableFragments() {

		if( currentSlide && config.fragments ) {
			var fragments = currentSlide.querySelectorAll( '.fragment' );
			var hiddenFragments = currentSlide.querySelectorAll( '.fragment:not(.visible)' );

			return {
				prev: fragments.length - hiddenFragments.length > 0,
				next: !!hiddenFragments.length
			};
		}
		else {
			return { prev: false, next: false };
		}

	}

	/**
	 * Start playback of any embedded content inside of
	 * the targeted slide.
	 */
	function startEmbeddedContent( slide ) {

		if( slide ) {
			// HTML5 media elements
			toArray( slide.querySelectorAll( 'video, audio' ) ).forEach( function( el ) {
				if( el.hasAttribute( 'data-autoplay' ) ) {
					el.play();
				}
			} );

			// YouTube embeds
			toArray( slide.querySelectorAll( 'iframe[src*="youtube.com/embed/"]' ) ).forEach( function( el ) {
				if( el.hasAttribute( 'data-autoplay' ) ) {
					el.contentWindow.postMessage('{"event":"command","func":"playVideo","args":""}', '*');
				}
			});
		}

	}

	/**
	 * Stop playback of any embedded content inside of
	 * the targeted slide.
	 */
	function stopEmbeddedContent( slide ) {

		if( slide ) {
			// HTML5 media elements
			toArray( slide.querySelectorAll( 'video, audio' ) ).forEach( function( el ) {
				if( !el.hasAttribute( 'data-ignore' ) ) {
					el.pause();
				}
			} );

			// YouTube embeds
			toArray( slide.querySelectorAll( 'iframe[src*="youtube.com/embed/"]' ) ).forEach( function( el ) {
				if( !el.hasAttribute( 'data-ignore' ) && typeof el.contentWindow.postMessage === 'function' ) {
					el.contentWindow.postMessage('{"event":"command","func":"pauseVideo","args":""}', '*');
				}
			});
		}

	}

	/**
	 * Reads the current URL (hash) and navigates accordingly.
	 */
	function readURL() {

		var hash = window.location.hash;

		// Attempt to parse the hash as either an index or name
		var bits = hash.slice( 2 ).split( '/' ),
			name = hash.replace( /#|\//gi, '' );

		// If the first bit is invalid and there is a name we can
		// assume that this is a named link
		if( isNaN( parseInt( bits[0], 10 ) ) && name.length ) {
			// Find the slide with the specified name
			var element = document.querySelector( '#' + name );

			if( element ) {
				// Find the position of the named slide and navigate to it
				var indices = Reveal.getIndices( element );
				slide( indices.h, indices.v );
			}
			// If the slide doesn't exist, navigate to the current slide
			else {
				slide( indexh || 0, indexv || 0 );
			}
		}
		else {
			// Read the index components of the hash
			var h = parseInt( bits[0], 10 ) || 0,
				v = parseInt( bits[1], 10 ) || 0;

			if( h !== indexh || v !== indexv ) {
				slide( h, v );
			}
		}

	}

	/**
	 * Updates the page URL (hash) to reflect the current
	 * state.
	 *
	 * @param {Number} delay The time in ms to wait before
	 * writing the hash
	 */
	function writeURL( delay ) {

		if( config.history ) {

			// Make sure there's never more than one timeout running
			clearTimeout( writeURLTimeout );

			// If a delay is specified, timeout this call
			if( typeof delay === 'number' ) {
				writeURLTimeout = setTimeout( writeURL, delay );
			}
			else {
				var url = '/';

				// If the current slide has an ID, use that as a named link
				if( currentSlide && typeof currentSlide.getAttribute( 'id' ) === 'string' ) {
					url = '/' + currentSlide.getAttribute( 'id' );
				}
				// Otherwise use the /h/v index
				else {
					if( indexh > 0 || indexv > 0 ) url += indexh;
					if( indexv > 0 ) url += '/' + indexv;
				}

				window.location.hash = url;
			}
		}

	}

	/**
	 * Retrieves the h/v location of the current, or specified,
	 * slide.
	 *
	 * @param {HTMLElement} slide If specified, the returned
	 * index will be for this slide rather than the currently
	 * active one
	 *
	 * @return {Object} { h: <int>, v: <int>, f: <int> }
	 */
	function getIndices( slide ) {

		// By default, return the current indices
		var h = indexh,
			v = indexv,
			f;

		// If a slide is specified, return the indices of that slide
		if( slide ) {
			var isVertical = isVerticalSlide( slide );
			var slideh = isVertical ? slide.parentNode : slide;

			// Select all horizontal slides
			var horizontalSlides = toArray( document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ) );