Touching DOM as less as possible

scripts/script.js

 $(function() {
 
-	var slides = $( 'section.slide' );
+	var domSlides = $( 'section.slide' ),
+        location = document.location,
+        slides = [],
+        backhash = {},
+        linkScreen = $( 'link[title=screen]' ),
+        linkProjection = $( 'link[title=projection]' );
+
+    for (var i = 0, len = domSlides.length; i < len; i++) {
+        var id = domSlides[i].id;
+        slides[i] = '#' + id;
+        backhash['#' + id] = i;
+    }
 
 	function fullScreen() {
-		return (
-			window.screenX == 0 &&
+		return window.screenX == 0 &&
 			window.screenY == 0 &&
 			screen.width == window.outerWidth &&
-			screen.height == window.outerHeight
-		) ? true : false;
+            screen.height == window.outerHeight;
 	}
 	
 	function turnSlide( e ) {
 		if( !fullScreen() ) return;
-		var current = $( document.location.hash );
+		var current = backhash[location.hash],
+            target,
+            command;
 		switch ( e.which ) {
 			case 33 : // PgUp
 			case 38 : // Up
 			case 37 : // Left
-				var target = current.prev( slides );
+                current--;
 				break;
 			case 34 : // PgDown
 			case 40 : // Down
 			case 39 : // Right
-				var target = current.next( slides );
+                current++;
 				break;
 			case 32 : // Space
-				var target = current[ e.shiftKey ? 'prev' : 'next' ]( slides );
+                current += e.shiftKey ? -1 : 1;
 				break;
 			default:
 				return;
 		}
+        target = slides[current];
 		e.preventDefault();
-		if( !target.length ) return;
-		else document.location.hash = '#' + target.attr( 'id' );
+		if( target ) location.hash = target;
 	}
 
 	function updateView() {
-		var screen = $( 'link[title=screen]' ),
-			projection = $( 'link[title=projection]' ),
-			fullscreen = fullScreen();
-		screen.attr( 'disabled', fullscreen );
-		projection.attr( 'disabled', !fullscreen );
-		if( !fullscreen || $( document.location.hash ).length ) return;
-		else document.location.hash = '#' + slides.first().attr( 'id' );
+		var fullscreen = fullScreen();
+		linkScreen.attr( 'disabled', fullscreen );
+		linkProjection.attr( 'disabled', !fullscreen );
+        if( fullscreen && !backhash[location.hash]) location.hash = slides[0];
 	}
 
 	$( document ).