dino.js

// Copyright (c) 2014 The Chromium Authors. All rights reserved.
        // Use of this source code is governed by a BSD-style license that can be
        // found in the LICENSE file.
        // extract from chromium source code by @liuwayong
        (function () {
            'use strict';
            /**
             * T-Rex runner.
             * @param {string} outerContainerId Outer containing element id.
             * @param {Object} opt_config
             * @constructor
             * @export
             */
            function Runner(outerContainerId, opt_config) {
                // Singleton
                if (Runner.instance_) {
                    return Runner.instance_;
                }
                Runner.instance_ = this;

                this.outerContainerEl = document.querySelector(outerContainerId);
                this.containerEl = null;
                this.snackbarEl = null;
                this.detailsButton = this.outerContainerEl.querySelector('#details-button');

                this.config = opt_config || Runner.config;

                this.dimensions = Runner.defaultDimensions;

                this.canvas = null;
                this.canvasCtx = null;

                this.tRex = null;

                this.distanceMeter = null;
                this.distanceRan = 0;

                this.highestScore = 0;

                this.time = 0;
                this.runningTime = 0;
                this.msPerFrame = 1000 / FPS;
                this.currentSpeed = this.config.SPEED;

                this.obstacles = [];

                this.activated = false; // Whether the easter egg has been activated.
                this.playing = false; // Whether the game is currently in play state.
                this.crashed = false;
                this.paused = false;
                this.inverted = false;
                this.invertTimer = 0;
                this.resizeTimerId_ = null;

                this.playCount = 0;

                // Sound FX.
                this.audioBuffer = null;
                this.soundFx = {};

                // Global web audio context for playing sounds.
                this.audioContext = null;

                // Images.
                this.images = {};
                this.imagesLoaded = 0;

                if (this.isDisabled()) {
                    this.setupDisabledRunner();
                } else {
                    this.loadImages();
                }
            }
            window['Runner'] = Runner;


            /**
             * Default game width.
             * @const
             */
            var DEFAULT_WIDTH = 600;

            /**
             * Frames per second.
             * @const
             */
            var FPS = 60;

            /** @const */
            var IS_HIDPI = window.devicePixelRatio > 1;

            /** @const */
            var IS_IOS = /iPad|iPhone|iPod/.test(window.navigator.platform);

            /** @const */
            var IS_MOBILE = /Android/.test(window.navigator.userAgent) || IS_IOS;

            /** @const */
            var IS_TOUCH_ENABLED = 'ontouchstart' in window;

            /**
             * Default game configuration.
             * @enum {number}
             */
            Runner.config = {
                ACCELERATION: 0.001,
                BG_CLOUD_SPEED: 0.2,
                BOTTOM_PAD: 10,
                CLEAR_TIME: 3000,
                CLOUD_FREQUENCY: 0.5,
                GAMEOVER_CLEAR_TIME: 750,
                GAP_COEFFICIENT: 0.6,
                GRAVITY: 0.6,
                INITIAL_JUMP_VELOCITY: 12,
                INVERT_FADE_DURATION: 12000,
                INVERT_DISTANCE: 700,
                MAX_BLINK_COUNT: 3,
                MAX_CLOUDS: 6,
                MAX_OBSTACLE_LENGTH: 3,
                MAX_OBSTACLE_DUPLICATION: 2,
                MAX_SPEED: 13,
                MIN_JUMP_HEIGHT: 35,
                MOBILE_SPEED_COEFFICIENT: 1.2,
                RESOURCE_TEMPLATE_ID: 'audio-resources',
                SPEED: 6,
                SPEED_DROP_COEFFICIENT: 3
            };


            /**
             * Default dimensions.
             * @enum {string}
             */
            Runner.defaultDimensions = {
                WIDTH: DEFAULT_WIDTH,
                HEIGHT: 150
            };


            /**
             * CSS class names.
             * @enum {string}
             */
            Runner.classes = {
                CANVAS: 'runner-canvas',
                CONTAINER: 'runner-container',
                CRASHED: 'crashed',
                ICON: 'icon-offline',
                INVERTED: 'inverted',
                SNACKBAR: 'snackbar',
                SNACKBAR_SHOW: 'snackbar-show',
                TOUCH_CONTROLLER: 'controller'
            };


            /**
             * Sprite definition layout of the spritesheet.
             * @enum {Object}
             */
            Runner.spriteDefinition = {
                LDPI: {
                    CACTUS_LARGE: {
                        x: 332,
                        y: 2
                    },
                    CACTUS_SMALL: {
                        x: 228,
                        y: 2
                    },
                    CLOUD: {
                        x: 86,
                        y: 2
                    },
                    HORIZON: {
                        x: 2,
                        y: 54
                    },
                    MOON: {
                        x: 484,
                        y: 2
                    },
                    PTERODACTYL: {
                        x: 134,
                        y: 2
                    },
                    RESTART: {
                        x: 2,
                        y: 2
                    },
                    TEXT_SPRITE: {
                        x: 655,
                        y: 2
                    },
                    TREX: {
                        x: 848,
                        y: 2
                    },
                    STAR: {
                        x: 645,
                        y: 2
                    }
                },
                HDPI: {
                    CACTUS_LARGE: {
                        x: 652,
                        y: 2
                    },
                    CACTUS_SMALL: {
                        x: 446,
                        y: 2
                    },
                    CLOUD: {
                        x: 166,
                        y: 2
                    },
                    HORIZON: {
                        x: 2,
                        y: 104
                    },
                    MOON: {
                        x: 954,
                        y: 2
                    },
                    PTERODACTYL: {
                        x: 260,
                        y: 2
                    },
                    RESTART: {
                        x: 2,
                        y: 2
                    },
                    TEXT_SPRITE: {
                        x: 1294,
                        y: 2
                    },
                    TREX: {
                        x: 1678,
                        y: 2
                    },
                    STAR: {
                        x: 1276,
                        y: 2
                    }
                }
            };


            /**
             * Sound FX. Reference to the ID of the audio tag on interstitial page.
             * @enum {string}
             */
            Runner.sounds = {
                BUTTON_PRESS: 'offline-sound-press',
                HIT: 'offline-sound-hit',
                SCORE: 'offline-sound-reached'
            };


            /**
             * Key code mapping.
             * @enum {Object}
             */
            Runner.keycodes = {
                JUMP: {
                    '38': 1,
                    '32': 1
                }, // Up, spacebar
                DUCK: {
                    '40': 1
                }, // Down
                RESTART: {
                    '13': 1
                } // Enter
            };


            /**
             * Runner event names.
             * @enum {string}
             */
            Runner.events = {
                ANIM_END: 'webkitAnimationEnd',
                CLICK: 'click',
                KEYDOWN: 'keydown',
                KEYUP: 'keyup',
                MOUSEDOWN: 'mousedown',
                MOUSEUP: 'mouseup',
                RESIZE: 'resize',
                TOUCHEND: 'touchend',
                TOUCHSTART: 'touchstart',
                VISIBILITY: 'visibilitychange',
                BLUR: 'blur',
                FOCUS: 'focus',
                LOAD: 'load'
            };


            Runner.prototype = {
                /**
                 * Whether the easter egg has been disabled. CrOS enterprise enrolled devices.
                 * @return {boolean}
                 */
                isDisabled: function () {
                    // return loadTimeData && loadTimeData.valueExists('disabledEasterEgg');
                    return false;
                },

                /**
                 * For disabled instances, set up a snackbar with the disabled message.
                 */
                setupDisabledRunner: function () {
                    this.containerEl = document.createElement('div');
                    this.containerEl.className = Runner.classes.SNACKBAR;
                    this.containerEl.textContent = loadTimeData.getValue('disabledEasterEgg');
                    this.outerContainerEl.appendChild(this.containerEl);

                    // Show notification when the activation key is pressed.
                    document.addEventListener(Runner.events.KEYDOWN, function (e) {
                        if (Runner.keycodes.JUMP[e.keyCode]) {
                            this.containerEl.classList.add(Runner.classes.SNACKBAR_SHOW);
                            document.querySelector('.icon').classList.add('icon-disabled');
                        }
                    }.bind(this));
                },

                /**
                 * Setting individual settings for debugging.
                 * @param {string} setting
                 * @param {*} value
                 */
                updateConfigSetting: function (setting, value) {
                    if (setting in this.config && value != undefined) {
                        this.config[setting] = value;

                        switch (setting) {
                            case 'GRAVITY':
                            case 'MIN_JUMP_HEIGHT':
                            case 'SPEED_DROP_COEFFICIENT':
                                this.tRex.config[setting] = value;
                                break;
                            case 'INITIAL_JUMP_VELOCITY':
                                this.tRex.setJumpVelocity(value);
                                break;
                            case 'SPEED':
                                this.setSpeed(value);
                                break;
                        }
                    }
                },

                /**
                 * Cache the appropriate image sprite from the page and get the sprite sheet
                 * definition.
                 */
                loadImages: function () {
                    if (IS_HIDPI) {
                        Runner.imageSprite = document.getElementById('offline-resources-2x');
                        this.spriteDef = Runner.spriteDefinition.HDPI;
                    } else {
                        Runner.imageSprite = document.getElementById('offline-resources-1x');
                        this.spriteDef = Runner.spriteDefinition.LDPI;
                    }

                    if (Runner.imageSprite.complete) {
                        this.init();
                    } else {
                        // If the images are not yet loaded, add a listener.
                        Runner.imageSprite.addEventListener(Runner.events.LOAD,
                            this.init.bind(this));
                    }
                },

                /**
                 * Load and decode base 64 encoded sounds.
                 */
                loadSounds: function () {
                    if (!IS_IOS) {
                        this.audioContext = new AudioContext();

                        var resourceTemplate =
                            document.getElementById(this.config.RESOURCE_TEMPLATE_ID).content;

                        for (var sound in Runner.sounds) {
                            var soundSrc =
                                resourceTemplate.getElementById(Runner.sounds[sound]).src;
                            soundSrc = soundSrc.substr(soundSrc.indexOf(',') + 1);
                            var buffer = decodeBase64ToArrayBuffer(soundSrc);

                            // Async, so no guarantee of order in array.
                            this.audioContext.decodeAudioData(buffer, function (index, audioData) {
                                this.soundFx[index] = audioData;
                            }.bind(this, sound));
                        }
                    }
                },

                /**
                 * Sets the game speed. Adjust the speed accordingly if on a smaller screen.
                 * @param {number} opt_speed
                 */
                setSpeed: function (opt_speed) {
                    var speed = opt_speed || this.currentSpeed;

                    // Reduce the speed on smaller mobile screens.
                    if (this.dimensions.WIDTH < DEFAULT_WIDTH) {
                        var mobileSpeed = speed * this.dimensions.WIDTH / DEFAULT_WIDTH *
                            this.config.MOBILE_SPEED_COEFFICIENT;
                        this.currentSpeed = mobileSpeed > speed ? speed : mobileSpeed;
                    } else if (opt_speed) {
                        this.currentSpeed = opt_speed;
                    }
                },

                /**
                 * Game initialiser.
                 */
                init: function () {
                    // Hide the static icon.
                    document.querySelector('.' + Runner.classes.ICON).style.visibility =
                        'hidden';

                    this.adjustDimensions();
                    this.setSpeed();

                    this.containerEl = document.createElement('div');
                    this.containerEl.className = Runner.classes.CONTAINER;

                    // Player canvas container.
                    this.canvas = createCanvas(this.containerEl, this.dimensions.WIDTH,
                        this.dimensions.HEIGHT, Runner.classes.PLAYER);

                    this.canvasCtx = this.canvas.getContext('2d');
                    this.canvasCtx.fillStyle = '#f7f7f7';
                    this.canvasCtx.fill();
                    Runner.updateCanvasScaling(this.canvas);

                    // Horizon contains clouds, obstacles and the ground.
                    this.horizon = new Horizon(this.canvas, this.spriteDef, this.dimensions,
                        this.config.GAP_COEFFICIENT);

                    // Distance meter
                    this.distanceMeter = new DistanceMeter(this.canvas,
                        this.spriteDef.TEXT_SPRITE, this.dimensions.WIDTH);

                    // Draw t-rex
                    this.tRex = new Trex(this.canvas, this.spriteDef.TREX);

                    this.outerContainerEl.appendChild(this.containerEl);

                    if (IS_MOBILE) {
                        this.createTouchController();
                    }

                    this.startListening();
                    this.update();

                    window.addEventListener(Runner.events.RESIZE,
                        this.debounceResize.bind(this));
                },

                /**
                 * Create the touch controller. A div that covers whole screen.
                 */
                createTouchController: function () {
                    this.touchController = document.createElement('div');
                    this.touchController.className = Runner.classes.TOUCH_CONTROLLER;
                    this.outerContainerEl.appendChild(this.touchController);
                },

                /**
                 * Debounce the resize event.
                 */
                debounceResize: function () {
                    if (!this.resizeTimerId_) {
                        this.resizeTimerId_ =
                            setInterval(this.adjustDimensions.bind(this), 250);
                    }
                },

                /**
                 * Adjust game space dimensions on resize.
                 */
                adjustDimensions: function () {
                    clearInterval(this.resizeTimerId_);
                    this.resizeTimerId_ = null;

                    var boxStyles = window.getComputedStyle(this.outerContainerEl);
                    var padding = Number(boxStyles.paddingLeft.substr(0,
                        boxStyles.paddingLeft.length - 2));

                    this.dimensions.WIDTH = this.outerContainerEl.offsetWidth - padding * 2;

                    // Redraw the elements back onto the canvas.
                    if (this.canvas) {
                        this.canvas.width = this.dimensions.WIDTH;
                        this.canvas.height = this.dimensions.HEIGHT;

                        Runner.updateCanvasScaling(this.canvas);

                        this.distanceMeter.calcXPos(this.dimensions.WIDTH);
                        this.clearCanvas();
                        this.horizon.update(0, 0, true);
                        this.tRex.update(0);

                        // Outer container and distance meter.
                        if (this.playing || this.crashed || this.paused) {
                            this.containerEl.style.width = this.dimensions.WIDTH + 'px';
                            this.containerEl.style.height = this.dimensions.HEIGHT + 'px';
                            this.distanceMeter.update(0, Math.ceil(this.distanceRan));
                            this.stop();
                        } else {
                            this.tRex.draw(0, 0);
                        }

                        // Game over panel.
                        if (this.crashed && this.gameOverPanel) {
                            this.gameOverPanel.updateDimensions(this.dimensions.WIDTH);
                            this.gameOverPanel.draw();
                        }
                    }
                },

                /**
                 * Play the game intro.
                 * Canvas container width expands out to the full width.
                 */
                playIntro: function () {
                    if (!this.activated && !this.crashed) {
                        this.playingIntro = true;
                        this.tRex.playingIntro = true;

                        // CSS animation definition.
                        var keyframes = '@-webkit-keyframes intro { ' +
                            'from { width:' + Trex.config.WIDTH + 'px }' +
                            'to { width: ' + this.dimensions.WIDTH + 'px }' +
                            '}';

                        // create a style sheet to put the keyframe rule in 
                        // and then place the style sheet in the html head    
                        var sheet = document.createElement('style');
                        sheet.innerHTML = keyframes;
                        document.head.appendChild(sheet);

                        this.containerEl.addEventListener(Runner.events.ANIM_END,
                            this.startGame.bind(this));

                        this.containerEl.style.webkitAnimation = 'intro .4s ease-out 1 both';
                        this.containerEl.style.width = this.dimensions.WIDTH + 'px';

                        // if (this.touchController) {
                        //     this.outerContainerEl.appendChild(this.touchController);
                        // }
                        this.playing = true;
                        this.activated = true;
                    } else if (this.crashed) {
                        this.restart();
                    }
                },


                /**
                 * Update the game status to started.
                 */
                startGame: function () {
                    this.runningTime = 0;
                    this.playingIntro = false;
                    this.tRex.playingIntro = false;
                    this.containerEl.style.webkitAnimation = '';
                    this.playCount++;

                    // Handle tabbing off the page. Pause the current game.
                    document.addEventListener(Runner.events.VISIBILITY,
                        this.onVisibilityChange.bind(this));

                    window.addEventListener(Runner.events.BLUR,
                        this.onVisibilityChange.bind(this));

                    window.addEventListener(Runner.events.FOCUS,
                        this.onVisibilityChange.bind(this));
                },

                clearCanvas: function () {
                    this.canvasCtx.clearRect(0, 0, this.dimensions.WIDTH,
                        this.dimensions.HEIGHT);
                },

                /**
                 * Update the game frame and schedules the next one.
                 */
                update: function () {
                    this.updatePending = false;

                    var now = getTimeStamp();
                    var deltaTime = now - (this.time || now);
                    this.time = now;

                    if (this.playing) {
                        this.clearCanvas();

                        if (this.tRex.jumping) {
                            this.tRex.updateJump(deltaTime);
                        }

                        this.runningTime += deltaTime;
                        var hasObstacles = this.runningTime > this.config.CLEAR_TIME;

                        // First jump triggers the intro.
                        if (this.tRex.jumpCount == 1 && !this.playingIntro) {
                            this.playIntro();
                        }

                        // The horizon doesn't move until the intro is over.
                        if (this.playingIntro) {
                            this.horizon.update(0, this.currentSpeed, hasObstacles);
                        } else {
                            deltaTime = !this.activated ? 0 : deltaTime;
                            this.horizon.update(deltaTime, this.currentSpeed, hasObstacles,
                                this.inverted);
                        }

                        // Check for collisions.
                        var collision = hasObstacles &&
                            checkForCollision(this.horizon.obstacles[0], this.tRex);

                        if (!collision) {
                            this.distanceRan += this.currentSpeed * deltaTime / this.msPerFrame;

                            if (this.currentSpeed < this.config.MAX_SPEED) {
                                this.currentSpeed += this.config.ACCELERATION;
                            }
                        } else {
                            this.gameOver();
                        }

                        var playAchievementSound = this.distanceMeter.update(deltaTime,
                            Math.ceil(this.distanceRan));

                        if (playAchievementSound) {
                            this.playSound(this.soundFx.SCORE);
                        }

                        // Night mode.
                        if (this.invertTimer > this.config.INVERT_FADE_DURATION) {
                            this.invertTimer = 0;
                            this.invertTrigger = false;
                            this.invert();
                        } else if (this.invertTimer) {
                            this.invertTimer += deltaTime;
                        } else {
                            var actualDistance =
                                this.distanceMeter.getActualDistance(Math.ceil(this.distanceRan));

                            if (actualDistance > 0) {
                                this.invertTrigger = !(actualDistance %
                                    this.config.INVERT_DISTANCE);

                                if (this.invertTrigger && this.invertTimer === 0) {
                                    this.invertTimer += deltaTime;
                                    this.invert();
                                }
                            }
                        }
                    }

                    if (this.playing || (!this.activated &&
                            this.tRex.blinkCount < Runner.config.MAX_BLINK_COUNT)) {
                        this.tRex.update(deltaTime);
                        this.scheduleNextUpdate();
                    }
                },

                /**
                 * Event handler.
                 */
                handleEvent: function (e) {
                    return (function (evtType, events) {
                        switch (evtType) {
                            case events.KEYDOWN:
                            case events.TOUCHSTART:
                            case events.MOUSEDOWN:
                                this.onKeyDown(e);
                                break;
                            case events.KEYUP:
                            case events.TOUCHEND:
                            case events.MOUSEUP:
                                this.onKeyUp(e);
                                break;
                        }
                    }.bind(this))(e.type, Runner.events);
                },

                /**
                 * Bind relevant key / mouse / touch listeners.
                 */
                startListening: function () {
                    // Keys.
                    document.addEventListener(Runner.events.KEYDOWN, this);
                    document.addEventListener(Runner.events.KEYUP, this);

                    if (IS_MOBILE) {
                        // Mobile only touch devices.
                        this.touchController.addEventListener(Runner.events.TOUCHSTART, this);
                        this.touchController.addEventListener(Runner.events.TOUCHEND, this);
                        this.containerEl.addEventListener(Runner.events.TOUCHSTART, this);
                    } else {
                        // Mouse.
                        document.addEventListener(Runner.events.MOUSEDOWN, this);
                        document.addEventListener(Runner.events.MOUSEUP, this);
                    }
                },

                /**
                 * Remove all listeners.
                 */
                stopListening: function () {
                    document.removeEventListener(Runner.events.KEYDOWN, this);
                    document.removeEventListener(Runner.events.KEYUP, this);

                    if (IS_MOBILE) {
                        this.touchController.removeEventListener(Runner.events.TOUCHSTART, this);
                        this.touchController.removeEventListener(Runner.events.TOUCHEND, this);
                        this.containerEl.removeEventListener(Runner.events.TOUCHSTART, this);
                    } else {
                        document.removeEventListener(Runner.events.MOUSEDOWN, this);
                        document.removeEventListener(Runner.events.MOUSEUP, this);
                    }
                },

                /**
                 * Process keydown.
                 * @param {Event} e
                 */
                onKeyDown: function (e) {
                    // Prevent native page scrolling whilst tapping on mobile.
                    if (IS_MOBILE && this.playing) {
                        e.preventDefault();
                    }

                    if (e.target != this.detailsButton) {
                        if (!this.crashed && (Runner.keycodes.JUMP[e.keyCode] ||
                                e.type == Runner.events.TOUCHSTART)) {
                            if (!this.playing) {
                                this.loadSounds();
                                this.playing = true;
                                this.update();
                                if (window.errorPageController) {
                                    errorPageController.trackEasterEgg();
                                }
                            }
                            //  Play sound effect and jump on starting the game for the first time.
                            if (!this.tRex.jumping && !this.tRex.ducking) {
                                this.playSound(this.soundFx.BUTTON_PRESS);
                                this.tRex.startJump(this.currentSpeed);
                            }
                        }

                        if (this.crashed && e.type == Runner.events.TOUCHSTART &&
                            e.currentTarget == this.containerEl) {
                            this.restart();
                        }
                    }

                    if (this.playing && !this.crashed && Runner.keycodes.DUCK[e.keyCode]) {
                        e.preventDefault();
                        if (this.tRex.jumping) {
                            // Speed drop, activated only when jump key is not pressed.
                            this.tRex.setSpeedDrop();
                        } else if (!this.tRex.jumping && !this.tRex.ducking) {
                            // Duck.
                            this.tRex.setDuck(true);
                        }
                    }
                },


                /**
                 * Process key up.
                 * @param {Event} e
                 */
                onKeyUp: function (e) {
                    var keyCode = String(e.keyCode);
                    var isjumpKey = Runner.keycodes.JUMP[keyCode] ||
                        e.type == Runner.events.TOUCHEND ||
                        e.type == Runner.events.MOUSEDOWN;

                    if (this.isRunning() && isjumpKey) {
                        this.tRex.endJump();
                    } else if (Runner.keycodes.DUCK[keyCode]) {
                        this.tRex.speedDrop = false;
                        this.tRex.setDuck(false);
                    } else if (this.crashed) {
                        // Check that enough time has elapsed before allowing jump key to restart.
                        var deltaTime = getTimeStamp() - this.time;

                        if (Runner.keycodes.RESTART[keyCode] || this.isLeftClickOnCanvas(e) ||
                            (deltaTime >= this.config.GAMEOVER_CLEAR_TIME &&
                                Runner.keycodes.JUMP[keyCode])) {
                            this.restart();
                        }
                    } else if (this.paused && isjumpKey) {
                        // Reset the jump state
                        this.tRex.reset();
                        this.play();
                    }
                },

                /**
                 * Returns whether the event was a left click on canvas.
                 * On Windows right click is registered as a click.
                 * @param {Event} e
                 * @return {boolean}
                 */
                isLeftClickOnCanvas: function (e) {
                    return e.button != null && e.button < 2 &&
                        e.type == Runner.events.MOUSEUP && e.target == this.canvas;
                },

                /**
                 * RequestAnimationFrame wrapper.
                 */
                scheduleNextUpdate: function () {
                    if (!this.updatePending) {
                        this.updatePending = true;
                        this.raqId = requestAnimationFrame(this.update.bind(this));
                    }
                },

                /**
                 * Whether the game is running.
                 * @return {boolean}
                 */
                isRunning: function () {
                    return !!this.raqId;
                },

                /**
                 * Game over state.
                 */
                gameOver: function () {
                    this.playSound(this.soundFx.HIT);
                    vibrate(200);

                    this.stop();
                    this.crashed = true;
                    this.distanceMeter.acheivement = false;

                    this.tRex.update(100, Trex.status.CRASHED);

                    // Game over panel.
                    if (!this.gameOverPanel) {
                        this.gameOverPanel = new GameOverPanel(this.canvas,
                            this.spriteDef.TEXT_SPRITE, this.spriteDef.RESTART,
                            this.dimensions);
                    } else {
                        this.gameOverPanel.draw();
                    }

                    // Update the high score.
                    if (this.distanceRan > this.highestScore) {
                        this.highestScore = Math.ceil(this.distanceRan);
                        this.distanceMeter.setHighScore(this.highestScore);
                    }

                    // Reset the time clock.
                    this.time = getTimeStamp();
                },

                stop: function () {
                    this.playing = false;
                    this.paused = true;
                    cancelAnimationFrame(this.raqId);
                    this.raqId = 0;
                },

                play: function () {
                    if (!this.crashed) {
                        this.playing = true;
                        this.paused = false;
                        this.tRex.update(0, Trex.status.RUNNING);
                        this.time = getTimeStamp();
                        this.update();
                    }
                },

                restart: function () {
                    if (!this.raqId) {
                        this.playCount++;
                        this.runningTime = 0;
                        this.playing = true;
                        this.crashed = false;
                        this.distanceRan = 0;
                        this.setSpeed(this.config.SPEED);
                        this.time = getTimeStamp();
                        this.containerEl.classList.remove(Runner.classes.CRASHED);
                        this.clearCanvas();
                        this.distanceMeter.reset(this.highestScore);
                        this.horizon.reset();
                        this.tRex.reset();
                        this.playSound(this.soundFx.BUTTON_PRESS);
                        this.invert(true);
                        this.update();
                    }
                },

                /**
                 * Pause the game if the tab is not in focus.
                 */
                onVisibilityChange: function (e) {
                    if (document.hidden || document.webkitHidden || e.type == 'blur' ||
                        document.visibilityState != 'visible') {
                        this.stop();
                    } else if (!this.crashed) {
                        this.tRex.reset();
                        this.play();
                    }
                },

                /**
                 * Play a sound.
                 * @param {SoundBuffer} soundBuffer
                 */
                playSound: function (soundBuffer) {
                    if (soundBuffer) {
                        var sourceNode = this.audioContext.createBufferSource();
                        sourceNode.buffer = soundBuffer;
                        sourceNode.connect(this.audioContext.destination);
                        sourceNode.start(0);
                    }
                },

                /**
                 * Inverts the current page / canvas colors.
                 * @param {boolean} Whether to reset colors.
                 */
                invert: function (reset) {
                    if (reset) {
                        document.body.classList.toggle(Runner.classes.INVERTED, false);
                        this.invertTimer = 0;
                        this.inverted = false;
                    } else {
                        this.inverted = document.body.classList.toggle(Runner.classes.INVERTED,
                            this.invertTrigger);
                    }
                }
            };


            /**
             * Updates the canvas size taking into
             * account the backing store pixel ratio and
             * the device pixel ratio.
             *
             * See article by Paul Lewis:
             * http://www.html5rocks.com/en/tutorials/canvas/hidpi/
             *
             * @param {HTMLCanvasElement} canvas
             * @param {number} opt_width
             * @param {number} opt_height
             * @return {boolean} Whether the canvas was scaled.
             */
            Runner.updateCanvasScaling = function (canvas, opt_width, opt_height) {
                var context = canvas.getContext('2d');

                // Query the various pixel ratios
                var devicePixelRatio = Math.floor(window.devicePixelRatio) || 1;
                var backingStoreRatio = Math.floor(context.webkitBackingStorePixelRatio) || 1;
                var ratio = devicePixelRatio / backingStoreRatio;

                // Upscale the canvas if the two ratios don't match
                if (devicePixelRatio !== backingStoreRatio) {
                    var oldWidth = opt_width || canvas.width;
                    var oldHeight = opt_height || canvas.height;

                    canvas.width = oldWidth * ratio;
                    canvas.height = oldHeight * ratio;

                    canvas.style.width = oldWidth + 'px';
                    canvas.style.height = oldHeight + 'px';

                    // Scale the context to counter the fact that we've manually scaled
                    // our canvas element.
                    context.scale(ratio, ratio);
                    return true;
                } else if (devicePixelRatio == 1) {
                    // Reset the canvas width / height. Fixes scaling bug when the page is
                    // zoomed and the devicePixelRatio changes accordingly.
                    canvas.style.width = canvas.width + 'px';
                    canvas.style.height = canvas.height + 'px';
                }
                return false;
            };


            /**
             * Get random number.
             * @param {number} min
             * @param {number} max
             * @param {number}
             */
            function getRandomNum(min, max) {
                return Math.floor(Math.random() * (max - min + 1)) + min;
            }


            /**
             * Vibrate on mobile devices.
             * @param {number} duration Duration of the vibration in milliseconds.
             */
            function vibrate(duration) {
                if (IS_MOBILE && window.navigator.vibrate) {
                    window.navigator.vibrate(duration);
                }
            }


            /**
             * Create canvas element.
             * @param {HTMLElement} container Element to append canvas to.
             * @param {number} width
             * @param {number} height
             * @param {string} opt_classname
             * @return {HTMLCanvasElement}
             */
            function createCanvas(container, width, height, opt_classname) {
                var canvas = document.createElement('canvas');
                canvas.className = opt_classname ? Runner.classes.CANVAS + ' ' +
                    opt_classname : Runner.classes.CANVAS;
                canvas.width = width;
                canvas.height = height;
                container.appendChild(canvas);

                return canvas;
            }


            /**
             * Decodes the base 64 audio to ArrayBuffer used by Web Audio.
             * @param {string} base64String
             */
            function decodeBase64ToArrayBuffer(base64String) {
                var len = (base64String.length / 4) * 3;
                var str = atob(base64String);
                var arrayBuffer = new ArrayBuffer(len);
                var bytes = new Uint8Array(arrayBuffer);

                for (var i = 0; i < len; i++) {
                    bytes[i] = str.charCodeAt(i);
                }
                return bytes.buffer;
            }


            /**
             * Return the current timestamp.
             * @return {number}
             */
            function getTimeStamp() {
                return IS_IOS ? new Date().getTime() : performance.now();
            }


            //******************************************************************************


            /**
             * Game over panel.
             * @param {!HTMLCanvasElement} canvas
             * @param {Object} textImgPos
             * @param {Object} restartImgPos
             * @param {!Object} dimensions Canvas dimensions.
             * @constructor
             */
            function GameOverPanel(canvas, textImgPos, restartImgPos, dimensions) {
                this.canvas = canvas;
                this.canvasCtx = canvas.getContext('2d');
                this.canvasDimensions = dimensions;
                this.textImgPos = textImgPos;
                this.restartImgPos = restartImgPos;
                this.draw();
            };


            /**
             * Dimensions used in the panel.
             * @enum {number}
             */
            GameOverPanel.dimensions = {
                TEXT_X: 0,
                TEXT_Y: 13,
                TEXT_WIDTH: 191,
                TEXT_HEIGHT: 11,
                RESTART_WIDTH: 36,
                RESTART_HEIGHT: 32
            };


            GameOverPanel.prototype = {
                /**
                 * Update the panel dimensions.
                 * @param {number} width New canvas width.
                 * @param {number} opt_height Optional new canvas height.
                 */
                updateDimensions: function (width, opt_height) {
                    this.canvasDimensions.WIDTH = width;
                    if (opt_height) {
                        this.canvasDimensions.HEIGHT = opt_height;
                    }
                },

                /**
                 * Draw the panel.
                 */
                draw: function () {
                    var dimensions = GameOverPanel.dimensions;

                    var centerX = this.canvasDimensions.WIDTH / 2;

                    // Game over text.
                    var textSourceX = dimensions.TEXT_X;
                    var textSourceY = dimensions.TEXT_Y;
                    var textSourceWidth = dimensions.TEXT_WIDTH;
                    var textSourceHeight = dimensions.TEXT_HEIGHT;

                    var textTargetX = Math.round(centerX - (dimensions.TEXT_WIDTH / 2));
                    var textTargetY = Math.round((this.canvasDimensions.HEIGHT - 25) / 3);
                    var textTargetWidth = dimensions.TEXT_WIDTH;
                    var textTargetHeight = dimensions.TEXT_HEIGHT;

                    var restartSourceWidth = dimensions.RESTART_WIDTH;
                    var restartSourceHeight = dimensions.RESTART_HEIGHT;
                    var restartTargetX = centerX - (dimensions.RESTART_WIDTH / 2);
                    var restartTargetY = this.canvasDimensions.HEIGHT / 2;

                    if (IS_HIDPI) {
                        textSourceY *= 2;
                        textSourceX *= 2;
                        textSourceWidth *= 2;
                        textSourceHeight *= 2;
                        restartSourceWidth *= 2;
                        restartSourceHeight *= 2;
                    }

                    textSourceX += this.textImgPos.x;
                    textSourceY += this.textImgPos.y;

                    // Game over text from sprite.
                    this.canvasCtx.drawImage(Runner.imageSprite,
                        textSourceX, textSourceY, textSourceWidth, textSourceHeight,
                        textTargetX, textTargetY, textTargetWidth, textTargetHeight);

                    // Restart button.
                    this.canvasCtx.drawImage(Runner.imageSprite,
                        this.restartImgPos.x, this.restartImgPos.y,
                        restartSourceWidth, restartSourceHeight,
                        restartTargetX, restartTargetY, dimensions.RESTART_WIDTH,
                        dimensions.RESTART_HEIGHT);
                }
            };


            //******************************************************************************

            /**
             * Check for a collision.
             * @param {!Obstacle} obstacle
             * @param {!Trex} tRex T-rex object.
             * @param {HTMLCanvasContext} opt_canvasCtx Optional canvas context for drawing
             *    collision boxes.
             * @return {Array<CollisionBox>}
             */
            function checkForCollision(obstacle, tRex, opt_canvasCtx) {
                var obstacleBoxXPos = Runner.defaultDimensions.WIDTH + obstacle.xPos;

                // Adjustments are made to the bounding box as there is a 1 pixel white
                // border around the t-rex and obstacles.
                var tRexBox = new CollisionBox(
                    tRex.xPos + 1,
                    tRex.yPos + 1,
                    tRex.config.WIDTH - 2,
                    tRex.config.HEIGHT - 2);

                var obstacleBox = new CollisionBox(
                    obstacle.xPos + 1,
                    obstacle.yPos + 1,
                    obstacle.typeConfig.width * obstacle.size - 2,
                    obstacle.typeConfig.height - 2);

                // Debug outer box
                if (opt_canvasCtx) {
                    drawCollisionBoxes(opt_canvasCtx, tRexBox, obstacleBox);
                }

                // Simple outer bounds check.
                if (boxCompare(tRexBox, obstacleBox)) {
                    var collisionBoxes = obstacle.collisionBoxes;
                    var tRexCollisionBoxes = tRex.ducking ?
                        Trex.collisionBoxes.DUCKING : Trex.collisionBoxes.RUNNING;

                    // Detailed axis aligned box check.
                    for (var t = 0; t < tRexCollisionBoxes.length; t++) {
                        for (var i = 0; i < collisionBoxes.length; i++) {
                            // Adjust the box to actual positions.
                            var adjTrexBox =
                                createAdjustedCollisionBox(tRexCollisionBoxes[t], tRexBox);
                            var adjObstacleBox =
                                createAdjustedCollisionBox(collisionBoxes[i], obstacleBox);
                            var crashed = boxCompare(adjTrexBox, adjObstacleBox);

                            // Draw boxes for debug.
                            if (opt_canvasCtx) {
                                drawCollisionBoxes(opt_canvasCtx, adjTrexBox, adjObstacleBox);
                            }

                            if (crashed) {
                                return [adjTrexBox, adjObstacleBox];
                            }
                        }
                    }
                }
                return false;
            };


            /**
             * Adjust the collision box.
             * @param {!CollisionBox} box The original box.
             * @param {!CollisionBox} adjustment Adjustment box.
             * @return {CollisionBox} The adjusted collision box object.
             */
            function createAdjustedCollisionBox(box, adjustment) {
                return new CollisionBox(
                    box.x + adjustment.x,
                    box.y + adjustment.y,
                    box.width,
                    box.height);
            };


            /**
             * Draw the collision boxes for debug.
             */
            function drawCollisionBoxes(canvasCtx, tRexBox, obstacleBox) {
                canvasCtx.save();
                canvasCtx.strokeStyle = '#f00';
                canvasCtx.strokeRect(tRexBox.x, tRexBox.y, tRexBox.width, tRexBox.height);

                canvasCtx.strokeStyle = '#0f0';
                canvasCtx.strokeRect(obstacleBox.x, obstacleBox.y,
                    obstacleBox.width, obstacleBox.height);
                canvasCtx.restore();
            };


            /**
             * Compare two collision boxes for a collision.
             * @param {CollisionBox} tRexBox
             * @param {CollisionBox} obstacleBox
             * @return {boolean} Whether the boxes intersected.
             */
            function boxCompare(tRexBox, obstacleBox) {
                var crashed = false;
                var tRexBoxX = tRexBox.x;
                var tRexBoxY = tRexBox.y;

                var obstacleBoxX = obstacleBox.x;
                var obstacleBoxY = obstacleBox.y;

                // Axis-Aligned Bounding Box method.
                if (tRexBox.x < obstacleBoxX + obstacleBox.width &&
                    tRexBox.x + tRexBox.width > obstacleBoxX &&
                    tRexBox.y < obstacleBox.y + obstacleBox.height &&
                    tRexBox.height + tRexBox.y > obstacleBox.y) {
                    crashed = true;
                }

                return crashed;
            };


            //******************************************************************************

            /**
             * Collision box object.
             * @param {number} x X position.
             * @param {number} y Y Position.
             * @param {number} w Width.
             * @param {number} h Height.
             */
            function CollisionBox(x, y, w, h) {
                this.x = x;
                this.y = y;
                this.width = w;
                this.height = h;
            };


            //******************************************************************************

            /**
             * Obstacle.
             * @param {HTMLCanvasCtx} canvasCtx
             * @param {Obstacle.type} type
             * @param {Object} spritePos Obstacle position in sprite.
             * @param {Object} dimensions
             * @param {number} gapCoefficient Mutipler in determining the gap.
             * @param {number} speed
             * @param {number} opt_xOffset
             */
            function Obstacle(canvasCtx, type, spriteImgPos, dimensions,
                gapCoefficient, speed, opt_xOffset) {

                this.canvasCtx = canvasCtx;
                this.spritePos = spriteImgPos;
                this.typeConfig = type;
                this.gapCoefficient = gapCoefficient;
                this.size = getRandomNum(1, Obstacle.MAX_OBSTACLE_LENGTH);
                this.dimensions = dimensions;
                this.remove = false;
                this.xPos = dimensions.WIDTH + (opt_xOffset || 0);
                this.yPos = 0;
                this.width = 0;
                this.collisionBoxes = [];
                this.gap = 0;
                this.speedOffset = 0;

                // For animated obstacles.
                this.currentFrame = 0;
                this.timer = 0;

                this.init(speed);
            };

            /**
             * Coefficient for calculating the maximum gap.
             * @const
             */
            Obstacle.MAX_GAP_COEFFICIENT = 1.5;

            /**
             * Maximum obstacle grouping count.
             * @const
             */
            Obstacle.MAX_OBSTACLE_LENGTH = 3,


                Obstacle.prototype = {
                    /**
                     * Initialise the DOM for the obstacle.
                     * @param {number} speed
                     */
                    init: function (speed) {
                        this.cloneCollisionBoxes();

                        // Only allow sizing if we're at the right speed.
                        if (this.size > 1 && this.typeConfig.multipleSpeed > speed) {
                            this.size = 1;
                        }

                        this.width = this.typeConfig.width * this.size;

                        // Check if obstacle can be positioned at various heights.
                        if (Array.isArray(this.typeConfig.yPos)) {
                            var yPosConfig = IS_MOBILE ? this.typeConfig.yPosMobile :
                                this.typeConfig.yPos;
                            this.yPos = yPosConfig[getRandomNum(0, yPosConfig.length - 1)];
                        } else {
                            this.yPos = this.typeConfig.yPos;
                        }

                        this.draw();

                        // Make collision box adjustments,
                        // Central box is adjusted to the size as one box.
                        //      ____        ______        ________
                        //    _|   |-|    _|     |-|    _|       |-|
                        //   | |<->| |   | |<--->| |   | |<----->| |
                        //   | | 1 | |   | |  2  | |   | |   3   | |
                        //   |_|___|_|   |_|_____|_|   |_|_______|_|
                        //
                        if (this.size > 1) {
                            this.collisionBoxes[1].width = this.width - this.collisionBoxes[0].width -
                                this.collisionBoxes[2].width;
                            this.collisionBoxes[2].x = this.width - this.collisionBoxes[2].width;
                        }

                        // For obstacles that go at a different speed from the horizon.
                        if (this.typeConfig.speedOffset) {
                            this.speedOffset = Math.random() > 0.5 ? this.typeConfig.speedOffset :
                                -this.typeConfig.speedOffset;
                        }

                        this.gap = this.getGap(this.gapCoefficient, speed);
                    },

                    /**
                     * Draw and crop based on size.
                     */
                    draw: function () {
                        var sourceWidth = this.typeConfig.width;
                        var sourceHeight = this.typeConfig.height;

                        if (IS_HIDPI) {
                            sourceWidth = sourceWidth * 2;
                            sourceHeight = sourceHeight * 2;
                        }

                        // X position in sprite.
                        var sourceX = (sourceWidth * this.size) * (0.5 * (this.size - 1)) +
                            this.spritePos.x;

                        // Animation frames.
                        if (this.currentFrame > 0) {
                            sourceX += sourceWidth * this.currentFrame;
                        }

                        this.canvasCtx.drawImage(Runner.imageSprite,
                            sourceX, this.spritePos.y,
                            sourceWidth * this.size, sourceHeight,
                            this.xPos, this.yPos,
                            this.typeConfig.width * this.size, this.typeConfig.height);
                    },

                    /**
                     * Obstacle frame update.
                     * @param {number} deltaTime
                     * @param {number} speed
                     */
                    update: function (deltaTime, speed) {
                        if (!this.remove) {
                            if (this.typeConfig.speedOffset) {
                                speed += this.speedOffset;
                            }
                            this.xPos -= Math.floor((speed * FPS / 1000) * deltaTime);

                            // Update frame
                            if (this.typeConfig.numFrames) {
                                this.timer += deltaTime;
                                if (this.timer >= this.typeConfig.frameRate) {
                                    this.currentFrame =
                                        this.currentFrame == this.typeConfig.numFrames - 1 ?
                                        0 : this.currentFrame + 1;
                                    this.timer = 0;
                                }
                            }
                            this.draw();

                            if (!this.isVisible()) {
                                this.remove = true;
                            }
                        }
                    },

                    /**
                     * Calculate a random gap size.
                     * - Minimum gap gets wider as speed increses
                     * @param {number} gapCoefficient
                     * @param {number} speed
                     * @return {number} The gap size.
                     */
                    getGap: function (gapCoefficient, speed) {
                        var minGap = Math.round(this.width * speed +
                            this.typeConfig.minGap * gapCoefficient);
                        var maxGap = Math.round(minGap * Obstacle.MAX_GAP_COEFFICIENT);
                        return getRandomNum(minGap, maxGap);
                    },

                    /**
                     * Check if obstacle is visible.
                     * @return {boolean} Whether the obstacle is in the game area.
                     */
                    isVisible: function () {
                        return this.xPos + this.width > 0;
                    },

                    /**
                     * Make a copy of the collision boxes, since these will change based on
                     * obstacle type and size.
                     */
                    cloneCollisionBoxes: function () {
                        var collisionBoxes = this.typeConfig.collisionBoxes;

                        for (var i = collisionBoxes.length - 1; i >= 0; i--) {
                            this.collisionBoxes[i] = new CollisionBox(collisionBoxes[i].x,
                                collisionBoxes[i].y, collisionBoxes[i].width,
                                collisionBoxes[i].height);
                        }
                    }
                };


            /**
             * Obstacle definitions.
             * minGap: minimum pixel space betweeen obstacles.
             * multipleSpeed: Speed at which multiples are allowed.
             * speedOffset: speed faster / slower than the horizon.
             * minSpeed: Minimum speed which the obstacle can make an appearance.
             */
            Obstacle.types = [{
                    type: 'CACTUS_SMALL',
                    width: 17,
                    height: 35,
                    yPos: 105,
                    multipleSpeed: 4,
                    minGap: 120,
                    minSpeed: 0,
                    collisionBoxes: [
                        new CollisionBox(0, 7, 5, 27),
                        new CollisionBox(4, 0, 6, 34),
                        new CollisionBox(10, 4, 7, 14)
                    ]
                },
                {
                    type: 'CACTUS_LARGE',
                    width: 25,
                    height: 50,
                    yPos: 90,
                    multipleSpeed: 7,
                    minGap: 120,
                    minSpeed: 0,
                    collisionBoxes: [
                        new CollisionBox(0, 12, 7, 38),
                        new CollisionBox(8, 0, 7, 49),
                        new CollisionBox(13, 10, 10, 38)
                    ]
                },
                {
                    type: 'PTERODACTYL',
                    width: 46,
                    height: 40,
                    yPos: [100, 75, 50], // Variable height.
                    yPosMobile: [100, 50], // Variable height mobile.
                    multipleSpeed: 999,
                    minSpeed: 8.5,
                    minGap: 150,
                    collisionBoxes: [
                        new CollisionBox(15, 15, 16, 5),
                        new CollisionBox(18, 21, 24, 6),
                        new CollisionBox(2, 14, 4, 3),
                        new CollisionBox(6, 10, 4, 7),
                        new CollisionBox(10, 8, 6, 9)
                    ],
                    numFrames: 2,
                    frameRate: 1000 / 6,
                    speedOffset: .8
                }
            ];


            //******************************************************************************
            /**
             * T-rex game character.
             * @param {HTMLCanvas} canvas
             * @param {Object} spritePos Positioning within image sprite.
             * @constructor
             */
            function Trex(canvas, spritePos) {
                this.canvas = canvas;
                this.canvasCtx = canvas.getContext('2d');
                this.spritePos = spritePos;
                this.xPos = 0;
                this.yPos = 0;
                // Position when on the ground.
                this.groundYPos = 0;
                this.currentFrame = 0;
                this.currentAnimFrames = [];
                this.blinkDelay = 0;
                this.blinkCount = 0;
                this.animStartTime = 0;
                this.timer = 0;
                this.msPerFrame = 1000 / FPS;
                this.config = Trex.config;
                // Current status.
                this.status = Trex.status.WAITING;

                this.jumping = false;
                this.ducking = false;
                this.jumpVelocity = 0;
                this.reachedMinHeight = false;
                this.speedDrop = false;
                this.jumpCount = 0;
                this.jumpspotX = 0;

                this.init();
            };


            /**
             * T-rex player config.
             * @enum {number}
             */
            Trex.config = {
                DROP_VELOCITY: -5,
                GRAVITY: 0.6,
                HEIGHT: 47,
                HEIGHT_DUCK: 25,
                INIITAL_JUMP_VELOCITY: -10,
                INTRO_DURATION: 1500,
                MAX_JUMP_HEIGHT: 30,
                MIN_JUMP_HEIGHT: 30,
                SPEED_DROP_COEFFICIENT: 3,
                SPRITE_WIDTH: 262,
                START_X_POS: 50,
                WIDTH: 44,
                WIDTH_DUCK: 59
            };


            /**
             * Used in collision detection.
             * @type {Array<CollisionBox>}
             */
            Trex.collisionBoxes = {
                DUCKING: [
                    new CollisionBox(1, 18, 55, 25)
                ],
                RUNNING: [
                    new CollisionBox(22, 0, 17, 16),
                    new CollisionBox(1, 18, 30, 9),
                    new CollisionBox(10, 35, 14, 8),
                    new CollisionBox(1, 24, 29, 5),
                    new CollisionBox(5, 30, 21, 4),
                    new CollisionBox(9, 34, 15, 4)
                ]
            };


            /**
             * Animation states.
             * @enum {string}
             */
            Trex.status = {
                CRASHED: 'CRASHED',
                DUCKING: 'DUCKING',
                JUMPING: 'JUMPING',
                RUNNING: 'RUNNING',
                WAITING: 'WAITING'
            };

            /**
             * Blinking coefficient.
             * @const
             */
            Trex.BLINK_TIMING = 7000;


            /**
             * Animation config for different states.
             * @enum {Object}
             */
            Trex.animFrames = {
                WAITING: {
                    frames: [44, 0],
                    msPerFrame: 1000 / 3
                },
                RUNNING: {
                    frames: [88, 132],
                    msPerFrame: 1000 / 12
                },
                CRASHED: {
                    frames: [220],
                    msPerFrame: 1000 / 60
                },
                JUMPING: {
                    frames: [0],
                    msPerFrame: 1000 / 60
                },
                DUCKING: {
                    frames: [264, 323],
                    msPerFrame: 1000 / 8
                }
            };


            Trex.prototype = {
                /**
                 * T-rex player initaliser.
                 * Sets the t-rex to blink at random intervals.
                 */
                init: function () {
                    this.groundYPos = Runner.defaultDimensions.HEIGHT - this.config.HEIGHT -
                        Runner.config.BOTTOM_PAD;
                    this.yPos = this.groundYPos;
                    this.minJumpHeight = this.groundYPos - this.config.MIN_JUMP_HEIGHT;

                    this.draw(0, 0);
                    this.update(0, Trex.status.WAITING);
                },

                /**
                 * Setter for the jump velocity.
                 * The approriate drop velocity is also set.
                 */
                setJumpVelocity: function (setting) {
                    this.config.INIITAL_JUMP_VELOCITY = -setting;
                    this.config.DROP_VELOCITY = -setting / 2;
                },

                /**
                 * Set the animation status.
                 * @param {!number} deltaTime
                 * @param {Trex.status} status Optional status to switch to.
                 */
                update: function (deltaTime, opt_status) {
                    this.timer += deltaTime;

                    // Update the status.
                    if (opt_status) {
                        this.status = opt_status;
                        this.currentFrame = 0;
                        this.msPerFrame = Trex.animFrames[opt_status].msPerFrame;
                        this.currentAnimFrames = Trex.animFrames[opt_status].frames;

                        if (opt_status == Trex.status.WAITING) {
                            this.animStartTime = getTimeStamp();
                            this.setBlinkDelay();
                        }
                    }

                    // Game intro animation, T-rex moves in from the left.
                    if (this.playingIntro && this.xPos < this.config.START_X_POS) {
                        this.xPos += Math.round((this.config.START_X_POS /
                            this.config.INTRO_DURATION) * deltaTime);
                    }

                    if (this.status == Trex.status.WAITING) {
                        this.blink(getTimeStamp());
                    } else {
                        this.draw(this.currentAnimFrames[this.currentFrame], 0);
                    }

                    // Update the frame position.
                    if (this.timer >= this.msPerFrame) {
                        this.currentFrame = this.currentFrame ==
                            this.currentAnimFrames.length - 1 ? 0 : this.currentFrame + 1;
                        this.timer = 0;
                    }

                    // Speed drop becomes duck if the down key is still being pressed.
                    if (this.speedDrop && this.yPos == this.groundYPos) {
                        this.speedDrop = false;
                        this.setDuck(true);
                    }
                },

                /**
                 * Draw the t-rex to a particular position.
                 * @param {number} x
                 * @param {number} y
                 */
                draw: function (x, y) {
                    var sourceX = x;
                    var sourceY = y;
                    var sourceWidth = this.ducking && this.status != Trex.status.CRASHED ?
                        this.config.WIDTH_DUCK : this.config.WIDTH;
                    var sourceHeight = this.config.HEIGHT;

                    if (IS_HIDPI) {
                        sourceX *= 2;
                        sourceY *= 2;
                        sourceWidth *= 2;
                        sourceHeight *= 2;
                    }

                    // Adjustments for sprite sheet position.
                    sourceX += this.spritePos.x;
                    sourceY += this.spritePos.y;

                    // Ducking.
                    if (this.ducking && this.status != Trex.status.CRASHED) {
                        this.canvasCtx.drawImage(Runner.imageSprite, sourceX, sourceY,
                            sourceWidth, sourceHeight,
                            this.xPos, this.yPos,
                            this.config.WIDTH_DUCK, this.config.HEIGHT);
                    } else {
                        // Crashed whilst ducking. Trex is standing up so needs adjustment.
                        if (this.ducking && this.status == Trex.status.CRASHED) {
                            this.xPos++;
                        }
                        // Standing / running
                        this.canvasCtx.drawImage(Runner.imageSprite, sourceX, sourceY,
                            sourceWidth, sourceHeight,
                            this.xPos, this.yPos,
                            this.config.WIDTH, this.config.HEIGHT);
                    }
                },

                /**
                 * Sets a random time for the blink to happen.
                 */
                setBlinkDelay: function () {
                    this.blinkDelay = Math.ceil(Math.random() * Trex.BLINK_TIMING);
                },

                /**
                 * Make t-rex blink at random intervals.
                 * @param {number} time Current time in milliseconds.
                 */
                blink: function (time) {
                    var deltaTime = time - this.animStartTime;

                    if (deltaTime >= this.blinkDelay) {
                        this.draw(this.currentAnimFrames[this.currentFrame], 0);

                        if (this.currentFrame == 1) {
                            // Set new random delay to blink.
                            this.setBlinkDelay();
                            this.animStartTime = time;
                            this.blinkCount++;
                        }
                    }
                },

                /**
                 * Initialise a jump.
                 * @param {number} speed
                 */
                startJump: function (speed) {
                    if (!this.jumping) {
                        this.update(0, Trex.status.JUMPING);
                        // Tweak the jump velocity based on the speed.
                        this.jumpVelocity = this.config.INIITAL_JUMP_VELOCITY - (speed / 10);
                        this.jumping = true;
                        this.reachedMinHeight = false;
                        this.speedDrop = false;
                    }
                },

                /**
                 * Jump is complete, falling down.
                 */
                endJump: function () {
                    if (this.reachedMinHeight &&
                        this.jumpVelocity < this.config.DROP_VELOCITY) {
                        this.jumpVelocity = this.config.DROP_VELOCITY;
                    }
                },

                /**
                 * Update frame for a jump.
                 * @param {number} deltaTime
                 * @param {number} speed
                 */
                updateJump: function (deltaTime, speed) {
                    var msPerFrame = Trex.animFrames[this.status].msPerFrame;
                    var framesElapsed = deltaTime / msPerFrame;

                    // Speed drop makes Trex fall faster.
                    if (this.speedDrop) {
                        this.yPos += Math.round(this.jumpVelocity *
                            this.config.SPEED_DROP_COEFFICIENT * framesElapsed);
                    } else {
                        this.yPos += Math.round(this.jumpVelocity * framesElapsed);
                    }

                    this.jumpVelocity += this.config.GRAVITY * framesElapsed;

                    // Minimum height has been reached.
                    if (this.yPos < this.minJumpHeight || this.speedDrop) {
                        this.reachedMinHeight = true;
                    }

                    // Reached max height
                    if (this.yPos < this.config.MAX_JUMP_HEIGHT || this.speedDrop) {
                        this.endJump();
                    }

                    // Back down at ground level. Jump completed.
                    if (this.yPos > this.groundYPos) {
                        this.reset();
                        this.jumpCount++;
                    }

                    this.update(deltaTime);
                },

                /**
                 * Set the speed drop. Immediately cancels the current jump.
                 */
                setSpeedDrop: function () {
                    this.speedDrop = true;
                    this.jumpVelocity = 1;
                },

                /**
                 * @param {boolean} isDucking.
                 */
                setDuck: function (isDucking) {
                    if (isDucking && this.status != Trex.status.DUCKING) {
                        this.update(0, Trex.status.DUCKING);
                        this.ducking = true;
                    } else if (this.status == Trex.status.DUCKING) {
                        this.update(0, Trex.status.RUNNING);
                        this.ducking = false;
                    }
                },

                /**
                 * Reset the t-rex to running at start of game.
                 */
                reset: function () {
                    this.yPos = this.groundYPos;
                    this.jumpVelocity = 0;
                    this.jumping = false;
                    this.ducking = false;
                    this.update(0, Trex.status.RUNNING);
                    this.midair = false;
                    this.speedDrop = false;
                    this.jumpCount = 0;
                }
            };


            //******************************************************************************

            /**
             * Handles displaying the distance meter.
             * @param {!HTMLCanvasElement} canvas
             * @param {Object} spritePos Image position in sprite.
             * @param {number} canvasWidth
             * @constructor
             */
            function DistanceMeter(canvas, spritePos, canvasWidth) {
                this.canvas = canvas;
                this.canvasCtx = canvas.getContext('2d');
                this.image = Runner.imageSprite;
                this.spritePos = spritePos;
                this.x = 0;
                this.y = 5;

                this.currentDistance = 0;
                this.maxScore = 0;
                this.highScore = 0;
                this.container = null;

                this.digits = [];
                this.acheivement = false;
                this.defaultString = '';
                this.flashTimer = 0;
                this.flashIterations = 0;
                this.invertTrigger = false;

                this.config = DistanceMeter.config;
                this.maxScoreUnits = this.config.MAX_DISTANCE_UNITS;
                this.init(canvasWidth);
            };


            /**
             * @enum {number}
             */
            DistanceMeter.dimensions = {
                WIDTH: 10,
                HEIGHT: 13,
                DEST_WIDTH: 11
            };


            /**
             * Y positioning of the digits in the sprite sheet.
             * X position is always 0.
             * @type {Array<number>}
             */
            DistanceMeter.yPos = [0, 13, 27, 40, 53, 67, 80, 93, 107, 120];


            /**
             * Distance meter config.
             * @enum {number}
             */
            DistanceMeter.config = {
                // Number of digits.
                MAX_DISTANCE_UNITS: 5,

                // Distance that causes achievement animation.
                ACHIEVEMENT_DISTANCE: 100,

                // Used for conversion from pixel distance to a scaled unit.
                COEFFICIENT: 0.025,

                // Flash duration in milliseconds.
                FLASH_DURATION: 1000 / 4,

                // Flash iterations for achievement animation.
                FLASH_ITERATIONS: 3
            };


            DistanceMeter.prototype = {
                /**
                 * Initialise the distance meter to '00000'.
                 * @param {number} width Canvas width in px.
                 */
                init: function (width) {
                    var maxDistanceStr = '';

                    this.calcXPos(width);
                    this.maxScore = this.maxScoreUnits;
                    for (var i = 0; i < this.maxScoreUnits; i++) {
                        this.draw(i, 0);
                        this.defaultString += '0';
                        maxDistanceStr += '9';
                    }

                    this.maxScore = parseInt(maxDistanceStr);
                },

                /**
                 * Calculate the xPos in the canvas.
                 * @param {number} canvasWidth
                 */
                calcXPos: function (canvasWidth) {
                    this.x = canvasWidth - (DistanceMeter.dimensions.DEST_WIDTH *
                        (this.maxScoreUnits + 1));
                },

                /**
                 * Draw a digit to canvas.
                 * @param {number} digitPos Position of the digit.
                 * @param {number} value Digit value 0-9.
                 * @param {boolean} opt_highScore Whether drawing the high score.
                 */
                draw: function (digitPos, value, opt_highScore) {
                    var sourceWidth = DistanceMeter.dimensions.WIDTH;
                    var sourceHeight = DistanceMeter.dimensions.HEIGHT;
                    var sourceX = DistanceMeter.dimensions.WIDTH * value;
                    var sourceY = 0;

                    var targetX = digitPos * DistanceMeter.dimensions.DEST_WIDTH;
                    var targetY = this.y;
                    var targetWidth = DistanceMeter.dimensions.WIDTH;
                    var targetHeight = DistanceMeter.dimensions.HEIGHT;

                    // For high DPI we 2x source values.
                    if (IS_HIDPI) {
                        sourceWidth *= 2;
                        sourceHeight *= 2;
                        sourceX *= 2;
                    }

                    sourceX += this.spritePos.x;
                    sourceY += this.spritePos.y;

                    this.canvasCtx.save();

                    if (opt_highScore) {
                        // Left of the current score.
                        var highScoreX = this.x - (this.maxScoreUnits * 2) *
                            DistanceMeter.dimensions.WIDTH;
                        this.canvasCtx.translate(highScoreX, this.y);
                    } else {
                        this.canvasCtx.translate(this.x, this.y);
                    }

                    this.canvasCtx.drawImage(this.image, sourceX, sourceY,
                        sourceWidth, sourceHeight,
                        targetX, targetY,
                        targetWidth, targetHeight
                    );

                    this.canvasCtx.restore();
                },

                /**
                 * Covert pixel distance to a 'real' distance.
                 * @param {number} distance Pixel distance ran.
                 * @return {number} The 'real' distance ran.
                 */
                getActualDistance: function (distance) {
                    return distance ? Math.round(distance * this.config.COEFFICIENT) : 0;
                },

                /**
                 * Update the distance meter.
                 * @param {number} distance
                 * @param {number} deltaTime
                 * @return {boolean} Whether the acheivement sound fx should be played.
                 */
                update: function (deltaTime, distance) {
                    var paint = true;
                    var playSound = false;

                    if (!this.acheivement) {
                        distance = this.getActualDistance(distance);
                        // Score has gone beyond the initial digit count.
                        if (distance > this.maxScore && this.maxScoreUnits ==
                            this.config.MAX_DISTANCE_UNITS) {
                            this.maxScoreUnits++;
                            this.maxScore = parseInt(this.maxScore + '9');
                        } else {
                            this.distance = 0;
                        }

                        if (distance > 0) {
                            // Acheivement unlocked
                            if (distance % this.config.ACHIEVEMENT_DISTANCE == 0) {
                                // Flash score and play sound.
                                this.acheivement = true;
                                this.flashTimer = 0;
                                playSound = true;
                            }

                            // Create a string representation of the distance with leading 0.
                            var distanceStr = (this.defaultString +
                                distance).substr(-this.maxScoreUnits);
                            this.digits = distanceStr.split('');
                        } else {
                            this.digits = this.defaultString.split('');
                        }
                    } else {
                        // Control flashing of the score on reaching acheivement.
                        if (this.flashIterations <= this.config.FLASH_ITERATIONS) {
                            this.flashTimer += deltaTime;

                            if (this.flashTimer < this.config.FLASH_DURATION) {
                                paint = false;
                            } else if (this.flashTimer >
                                this.config.FLASH_DURATION * 2) {
                                this.flashTimer = 0;
                                this.flashIterations++;
                            }
                        } else {
                            this.acheivement = false;
                            this.flashIterations = 0;
                            this.flashTimer = 0;
                        }
                    }

                    // Draw the digits if not flashing.
                    if (paint) {
                        for (var i = this.digits.length - 1; i >= 0; i--) {
                            this.draw(i, parseInt(this.digits[i]));
                        }
                    }

                    this.drawHighScore();
                    return playSound;
                },

                /**
                 * Draw the high score.
                 */
                drawHighScore: function () {
                    this.canvasCtx.save();
                    this.canvasCtx.globalAlpha = .8;
                    for (var i = this.highScore.length - 1; i >= 0; i--) {
                        this.draw(i, parseInt(this.highScore[i], 10), true);
                    }
                    this.canvasCtx.restore();
                },

                /**
                 * Set the highscore as a array string.
                 * Position of char in the sprite: H - 10, I - 11.
                 * @param {number} distance Distance ran in pixels.
                 */
                setHighScore: function (distance) {
                    distance = this.getActualDistance(distance);
                    var highScoreStr = (this.defaultString +
                        distance).substr(-this.maxScoreUnits);

                    this.highScore = ['10', '11', ''].concat(highScoreStr.split(''));
                },

                /**
                 * Reset the distance meter back to '00000'.
                 */
                reset: function () {
                    this.update(0);
                    this.acheivement = false;
                }
            };


            //******************************************************************************

            /**
             * Cloud background item.
             * Similar to an obstacle object but without collision boxes.
             * @param {HTMLCanvasElement} canvas Canvas element.
             * @param {Object} spritePos Position of image in sprite.
             * @param {number} containerWidth
             */
            function Cloud(canvas, spritePos, containerWidth) {
                this.canvas = canvas;
                this.canvasCtx = this.canvas.getContext('2d');
                this.spritePos = spritePos;
                this.containerWidth = containerWidth;
                this.xPos = containerWidth;
                this.yPos = 0;
                this.remove = false;
                this.cloudGap = getRandomNum(Cloud.config.MIN_CLOUD_GAP,
                    Cloud.config.MAX_CLOUD_GAP);

                this.init();
            };


            /**
             * Cloud object config.
             * @enum {number}
             */
            Cloud.config = {
                HEIGHT: 14,
                MAX_CLOUD_GAP: 400,
                MAX_SKY_LEVEL: 30,
                MIN_CLOUD_GAP: 100,
                MIN_SKY_LEVEL: 71,
                WIDTH: 46
            };


            Cloud.prototype = {
                /**
                 * Initialise the cloud. Sets the Cloud height.
                 */
                init: function () {
                    this.yPos = getRandomNum(Cloud.config.MAX_SKY_LEVEL,
                        Cloud.config.MIN_SKY_LEVEL);
                    this.draw();
                },

                /**
                 * Draw the cloud.
                 */
                draw: function () {
                    this.canvasCtx.save();
                    var sourceWidth = Cloud.config.WIDTH;
                    var sourceHeight = Cloud.config.HEIGHT;

                    if (IS_HIDPI) {
                        sourceWidth = sourceWidth * 2;
                        sourceHeight = sourceHeight * 2;
                    }

                    this.canvasCtx.drawImage(Runner.imageSprite, this.spritePos.x,
                        this.spritePos.y,
                        sourceWidth, sourceHeight,
                        this.xPos, this.yPos,
                        Cloud.config.WIDTH, Cloud.config.HEIGHT);

                    this.canvasCtx.restore();
                },

                /**
                 * Update the cloud position.
                 * @param {number} speed
                 */
                update: function (speed) {
                    if (!this.remove) {
                        this.xPos -= Math.ceil(speed);
                        this.draw();

                        // Mark as removeable if no longer in the canvas.
                        if (!this.isVisible()) {
                            this.remove = true;
                        }
                    }
                },

                /**
                 * Check if the cloud is visible on the stage.
                 * @return {boolean}
                 */
                isVisible: function () {
                    return this.xPos + Cloud.config.WIDTH > 0;
                }
            };


            //******************************************************************************

            /**
             * Nightmode shows a moon and stars on the horizon.
             */
            function NightMode(canvas, spritePos, containerWidth) {
                this.spritePos = spritePos;
                this.canvas = canvas;
                this.canvasCtx = canvas.getContext('2d');
                this.xPos = containerWidth - 50;
                this.yPos = 30;
                this.currentPhase = 0;
                this.opacity = 0;
                this.containerWidth = containerWidth;
                this.stars = [];
                this.drawStars = false;
                this.placeStars();
            };

            /**
             * @enum {number}
             */
            NightMode.config = {
                FADE_SPEED: 0.035,
                HEIGHT: 40,
                MOON_SPEED: 0.25,
                NUM_STARS: 2,
                STAR_SIZE: 9,
                STAR_SPEED: 0.3,
                STAR_MAX_Y: 70,
                WIDTH: 20
            };

            NightMode.phases = [140, 120, 100, 60, 40, 20, 0];

            NightMode.prototype = {
                /**
                 * Update moving moon, changing phases.
                 * @param {boolean} activated Whether night mode is activated.
                 * @param {number} delta
                 */
                update: function (activated, delta) {
                    // Moon phase.
                    if (activated && this.opacity == 0) {
                        this.currentPhase++;

                        if (this.currentPhase >= NightMode.phases.length) {
                            this.currentPhase = 0;
                        }
                    }

                    // Fade in / out.
                    if (activated && (this.opacity < 1 || this.opacity == 0)) {
                        this.opacity += NightMode.config.FADE_SPEED;
                    } else if (this.opacity > 0) {
                        this.opacity -= NightMode.config.FADE_SPEED;
                    }

                    // Set moon positioning.
                    if (this.opacity > 0) {
                        this.xPos = this.updateXPos(this.xPos, NightMode.config.MOON_SPEED);

                        // Update stars.
                        if (this.drawStars) {
                            for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
                                this.stars[i].x = this.updateXPos(this.stars[i].x,
                                    NightMode.config.STAR_SPEED);
                            }
                        }
                        this.draw();
                    } else {
                        this.opacity = 0;
                        this.placeStars();
                    }
                    this.drawStars = true;
                },

                updateXPos: function (currentPos, speed) {
                    if (currentPos < -NightMode.config.WIDTH) {
                        currentPos = this.containerWidth;
                    } else {
                        currentPos -= speed;
                    }
                    return currentPos;
                },

                draw: function () {
                    var moonSourceWidth = this.currentPhase == 3 ? NightMode.config.WIDTH * 2 :
                        NightMode.config.WIDTH;
                    var moonSourceHeight = NightMode.config.HEIGHT;
                    var moonSourceX = this.spritePos.x + NightMode.phases[this.currentPhase];
                    var moonOutputWidth = moonSourceWidth;
                    var starSize = NightMode.config.STAR_SIZE;
                    var starSourceX = Runner.spriteDefinition.LDPI.STAR.x;

                    if (IS_HIDPI) {
                        moonSourceWidth *= 2;
                        moonSourceHeight *= 2;
                        moonSourceX = this.spritePos.x +
                            (NightMode.phases[this.currentPhase] * 2);
                        starSize *= 2;
                        starSourceX = Runner.spriteDefinition.HDPI.STAR.x;
                    }

                    this.canvasCtx.save();
                    this.canvasCtx.globalAlpha = this.opacity;

                    // Stars.
                    if (this.drawStars) {
                        for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
                            this.canvasCtx.drawImage(Runner.imageSprite,
                                starSourceX, this.stars[i].sourceY, starSize, starSize,
                                Math.round(this.stars[i].x), this.stars[i].y,
                                NightMode.config.STAR_SIZE, NightMode.config.STAR_SIZE);
                        }
                    }

                    // Moon.
                    this.canvasCtx.drawImage(Runner.imageSprite, moonSourceX,
                        this.spritePos.y, moonSourceWidth, moonSourceHeight,
                        Math.round(this.xPos), this.yPos,
                        moonOutputWidth, NightMode.config.HEIGHT);

                    this.canvasCtx.globalAlpha = 1;
                    this.canvasCtx.restore();
                },

                // Do star placement.
                placeStars: function () {
                    var segmentSize = Math.round(this.containerWidth /
                        NightMode.config.NUM_STARS);

                    for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
                        this.stars[i] = {};
                        this.stars[i].x = getRandomNum(segmentSize * i, segmentSize * (i + 1));
                        this.stars[i].y = getRandomNum(0, NightMode.config.STAR_MAX_Y);

                        if (IS_HIDPI) {
                            this.stars[i].sourceY = Runner.spriteDefinition.HDPI.STAR.y +
                                NightMode.config.STAR_SIZE * 2 * i;
                        } else {
                            this.stars[i].sourceY = Runner.spriteDefinition.LDPI.STAR.y +
                                NightMode.config.STAR_SIZE * i;
                        }
                    }
                },

                reset: function () {
                    this.currentPhase = 0;
                    this.opacity = 0;
                    this.update(false);
                }

            };


            //******************************************************************************

            /**
             * Horizon Line.
             * Consists of two connecting lines. Randomly assigns a flat / bumpy horizon.
             * @param {HTMLCanvasElement} canvas
             * @param {Object} spritePos Horizon position in sprite.
             * @constructor
             */
            function HorizonLine(canvas, spritePos) {
                this.spritePos = spritePos;
                this.canvas = canvas;
                this.canvasCtx = canvas.getContext('2d');
                this.sourceDimensions = {};
                this.dimensions = HorizonLine.dimensions;
                this.sourceXPos = [this.spritePos.x, this.spritePos.x +
                    this.dimensions.WIDTH
                ];
                this.xPos = [];
                this.yPos = 0;
                this.bumpThreshold = 0.5;

                this.setSourceDimensions();
                this.draw();
            };


            /**
             * Horizon line dimensions.
             * @enum {number}
             */
            HorizonLine.dimensions = {
                WIDTH: 600,
                HEIGHT: 12,
                YPOS: 127
            };


            HorizonLine.prototype = {
                /**
                 * Set the source dimensions of the horizon line.
                 */
                setSourceDimensions: function () {

                    for (var dimension in HorizonLine.dimensions) {
                        if (IS_HIDPI) {
                            if (dimension != 'YPOS') {
                                this.sourceDimensions[dimension] =
                                    HorizonLine.dimensions[dimension] * 2;
                            }
                        } else {
                            this.sourceDimensions[dimension] =
                                HorizonLine.dimensions[dimension];
                        }
                        this.dimensions[dimension] = HorizonLine.dimensions[dimension];
                    }

                    this.xPos = [0, HorizonLine.dimensions.WIDTH];
                    this.yPos = HorizonLine.dimensions.YPOS;
                },

                /**
                 * Return the crop x position of a type.
                 */
                getRandomType: function () {
                    return Math.random() > this.bumpThreshold ? this.dimensions.WIDTH : 0;
                },

                /**
                 * Draw the horizon line.
                 */
                draw: function () {
                    this.canvasCtx.drawImage(Runner.imageSprite, this.sourceXPos[0],
                        this.spritePos.y,
                        this.sourceDimensions.WIDTH, this.sourceDimensions.HEIGHT,
                        this.xPos[0], this.yPos,
                        this.dimensions.WIDTH, this.dimensions.HEIGHT);

                    this.canvasCtx.drawImage(Runner.imageSprite, this.sourceXPos[1],
                        this.spritePos.y,
                        this.sourceDimensions.WIDTH, this.sourceDimensions.HEIGHT,
                        this.xPos[1], this.yPos,
                        this.dimensions.WIDTH, this.dimensions.HEIGHT);
                },

                /**
                 * Update the x position of an indivdual piece of the line.
                 * @param {number} pos Line position.
                 * @param {number} increment
                 */
                updateXPos: function (pos, increment) {
                    var line1 = pos;
                    var line2 = pos == 0 ? 1 : 0;

                    this.xPos[line1] -= increment;
                    this.xPos[line2] = this.xPos[line1] + this.dimensions.WIDTH;

                    if (this.xPos[line1] <= -this.dimensions.WIDTH) {
                        this.xPos[line1] += this.dimensions.WIDTH * 2;
                        this.xPos[line2] = this.xPos[line1] - this.dimensions.WIDTH;
                        this.sourceXPos[line1] = this.getRandomType() + this.spritePos.x;
                    }
                },

                /**
                 * Update the horizon line.
                 * @param {number} deltaTime
                 * @param {number} speed
                 */
                update: function (deltaTime, speed) {
                    var increment = Math.floor(speed * (FPS / 1000) * deltaTime);

                    if (this.xPos[0] <= 0) {
                        this.updateXPos(0, increment);
                    } else {
                        this.updateXPos(1, increment);
                    }
                    this.draw();
                },

                /**
                 * Reset horizon to the starting position.
                 */
                reset: function () {
                    this.xPos[0] = 0;
                    this.xPos[1] = HorizonLine.dimensions.WIDTH;
                }
            };


            //******************************************************************************

            /**
             * Horizon background class.
             * @param {HTMLCanvasElement} canvas
             * @param {Object} spritePos Sprite positioning.
             * @param {Object} dimensions Canvas dimensions.
             * @param {number} gapCoefficient
             * @constructor
             */
            function Horizon(canvas, spritePos, dimensions, gapCoefficient) {
                this.canvas = canvas;
                this.canvasCtx = this.canvas.getContext('2d');
                this.config = Horizon.config;
                this.dimensions = dimensions;
                this.gapCoefficient = gapCoefficient;
                this.obstacles = [];
                this.obstacleHistory = [];
                this.horizonOffsets = [0, 0];
                this.cloudFrequency = this.config.CLOUD_FREQUENCY;
                this.spritePos = spritePos;
                this.nightMode = null;

                // Cloud
                this.clouds = [];
                this.cloudSpeed = this.config.BG_CLOUD_SPEED;

                // Horizon
                this.horizonLine = null;
                this.init();
            };


            /**
             * Horizon config.
             * @enum {number}
             */
            Horizon.config = {
                BG_CLOUD_SPEED: 0.2,
                BUMPY_THRESHOLD: .3,
                CLOUD_FREQUENCY: .5,
                HORIZON_HEIGHT: 16,
                MAX_CLOUDS: 6
            };


            Horizon.prototype = {
                /**
                 * Initialise the horizon. Just add the line and a cloud. No obstacles.
                 */
                init: function () {
                    this.addCloud();
                    this.horizonLine = new HorizonLine(this.canvas, this.spritePos.HORIZON);
                    this.nightMode = new NightMode(this.canvas, this.spritePos.MOON,
                        this.dimensions.WIDTH);
                },

                /**
                 * @param {number} deltaTime
                 * @param {number} currentSpeed
                 * @param {boolean} updateObstacles Used as an override to prevent
                 *     the obstacles from being updated / added. This happens in the
                 *     ease in section.
                 * @param {boolean} showNightMode Night mode activated.
                 */
                update: function (deltaTime, currentSpeed, updateObstacles, showNightMode) {
                    this.runningTime += deltaTime;
                    this.horizonLine.update(deltaTime, currentSpeed);
                    this.nightMode.update(showNightMode);
                    this.updateClouds(deltaTime, currentSpeed);

                    if (updateObstacles) {
                        this.updateObstacles(deltaTime, currentSpeed);
                    }
                },

                /**
                 * Update the cloud positions.
                 * @param {number} deltaTime
                 * @param {number} currentSpeed
                 */
                updateClouds: function (deltaTime, speed) {
                    var cloudSpeed = this.cloudSpeed / 1000 * deltaTime * speed;
                    var numClouds = this.clouds.length;

                    if (numClouds) {
                        for (var i = numClouds - 1; i >= 0; i--) {
                            this.clouds[i].update(cloudSpeed);
                        }

                        var lastCloud = this.clouds[numClouds - 1];

                        // Check for adding a new cloud.
                        if (numClouds < this.config.MAX_CLOUDS &&
                            (this.dimensions.WIDTH - lastCloud.xPos) > lastCloud.cloudGap &&
                            this.cloudFrequency > Math.random()) {
                            this.addCloud();
                        }

                        // Remove expired clouds.
                        this.clouds = this.clouds.filter(function (obj) {
                            return !obj.remove;
                        });
                    } else {
                        this.addCloud();
                    }
                },

                /**
                 * Update the obstacle positions.
                 * @param {number} deltaTime
                 * @param {number} currentSpeed
                 */
                updateObstacles: function (deltaTime, currentSpeed) {
                    // Obstacles, move to Horizon layer.
                    var updatedObstacles = this.obstacles.slice(0);

                    for (var i = 0; i < this.obstacles.length; i++) {
                        var obstacle = this.obstacles[i];
                        obstacle.update(deltaTime, currentSpeed);

                        // Clean up existing obstacles.
                        if (obstacle.remove) {
                            updatedObstacles.shift();
                        }
                    }
                    this.obstacles = updatedObstacles;

                    if (this.obstacles.length > 0) {
                        var lastObstacle = this.obstacles[this.obstacles.length - 1];

                        if (lastObstacle && !lastObstacle.followingObstacleCreated &&
                            lastObstacle.isVisible() &&
                            (lastObstacle.xPos + lastObstacle.width + lastObstacle.gap) <
                            this.dimensions.WIDTH) {
                            this.addNewObstacle(currentSpeed);
                            lastObstacle.followingObstacleCreated = true;
                        }
                    } else {
                        // Create new obstacles.
                        this.addNewObstacle(currentSpeed);
                    }
                },

                removeFirstObstacle: function () {
                    this.obstacles.shift();
                },

                /**
                 * Add a new obstacle.
                 * @param {number} currentSpeed
                 */
                addNewObstacle: function (currentSpeed) {
                    var obstacleTypeIndex = getRandomNum(0, Obstacle.types.length - 1);
                    var obstacleType = Obstacle.types[obstacleTypeIndex];

                    // Check for multiples of the same type of obstacle.
                    // Also check obstacle is available at current speed.
                    if (this.duplicateObstacleCheck(obstacleType.type) ||
                        currentSpeed < obstacleType.minSpeed) {
                        this.addNewObstacle(currentSpeed);
                    } else {
                        var obstacleSpritePos = this.spritePos[obstacleType.type];

                        this.obstacles.push(new Obstacle(this.canvasCtx, obstacleType,
                            obstacleSpritePos, this.dimensions,
                            this.gapCoefficient, currentSpeed, obstacleType.width));

                        this.obstacleHistory.unshift(obstacleType.type);

                        if (this.obstacleHistory.length > 1) {
                            this.obstacleHistory.splice(Runner.config.MAX_OBSTACLE_DUPLICATION);
                        }
                    }
                },

                /**
                 * Returns whether the previous two obstacles are the same as the next one.
                 * Maximum duplication is set in config value MAX_OBSTACLE_DUPLICATION.
                 * @return {boolean}
                 */
                duplicateObstacleCheck: function (nextObstacleType) {
                    var duplicateCount = 0;

                    for (var i = 0; i < this.obstacleHistory.length; i++) {
                        duplicateCount = this.obstacleHistory[i] == nextObstacleType ?
                            duplicateCount + 1 : 0;
                    }
                    return duplicateCount >= Runner.config.MAX_OBSTACLE_DUPLICATION;
                },

                /**
                 * Reset the horizon layer.
                 * Remove existing obstacles and reposition the horizon line.
                 */
                reset: function () {
                    this.obstacles = [];
                    this.horizonLine.reset();
                    this.nightMode.reset();
                },

                /**
                 * Update the canvas width and scaling.
                 * @param {number} width Canvas width.
                 * @param {number} height Canvas height.
                 */
                resize: function (width, height) {
                    this.canvas.width = width;
                    this.canvas.height = height;
                },

                /**
                 * Add a new cloud to the horizon.
                 */
                addCloud: function () {
                    this.clouds.push(new Cloud(this.canvas, this.spritePos.CLOUD,
                        this.dimensions.WIDTH));
                }
            };
        })();


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