function _isIpad() {
  var isIpad = navigator.userAgent.toLowerCase().indexOf('ipad') !== -1;

  if (!isIpad && navigator.userAgent.match(/Mac/) && navigator.maxTouchPoints && navigator.maxTouchPoints > 2) {
    return true;
  }

  return isIpad;
}

// 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.

/**
 * T-Rex runner.
 * @param {string} outerContainerId Outer containing element id.
 * @param {!Object=} opt_config
 * @constructor
 * @implements {EventListener}
 * @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;
  // A div to intercept touch events. Only set while (playing && useTouch).
  this.touchController = null;

  this.config = opt_config || Object.assign(Runner.config, Runner.normalConfig);
  // Logical dimensions of the container.
  this.dimensions = Runner.defaultDimensions;

  this.gameType = null;
  Runner.spriteDefinition = Runner.spriteDefinitionByType['original'];

  this.altGameImageSprite = null;
  this.altGameModeActive = false;
  this.altGameModeFlashTimer = null;
  this.fadeInTimer = 0;

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

  this.tRex = null;

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

  this.highestScore = window.localStorage.getItem("chrome-dino");
  this.syncHighestScore = false;

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

  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;

  /** @type {Object} */
  this.soundFx = {};
  this.generatedSoundFx = null;

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

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

  // Gamepad state.
  this.pollingGamepads = false;
  this.gamepadIndex = undefined;
  this.previousGamepad = null;

  if (this.isDisabled()) {
    this.setupDisabledRunner();
  } else {
    if (Runner.isAltGameModeEnabled()) {
      this.initAltGameType();
      Runner.gameType = this.gameType;
    }
    this.loadImages();

    window['initializeEasterEggHighScore'] =
        this.initializeHighScore.bind(this);
  }
}

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

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

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

/** @const */
const IS_IOS = !!window.navigator.userAgent.match(/iP(hone|ad|od)/i) && !!window.navigator.userAgent.match(/Safari/i) ||
  _isIpad() || /CriOS/.test(window.navigator.userAgent) || /FxiOS/.test(window.navigator.userAgent);

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

/** @const */
const IS_RTL = document.querySelector('html').dir == 'rtl';

/** @const */
const ARCADE_MODE_URL = 'chrome://dino/';

/** @const */
const RESOURCE_POSTFIX = 'offline-resources-';

/** @const */
const A11Y_STRINGS = {
  ariaLabel: 'dinoGameA11yAriaLabel',
  description: 'dinoGameA11yDescription',
  gameOver: 'dinoGameA11yGameOver',
  highScore: 'dinoGameA11yHighScore',
  jump: 'dinoGameA11yJump',
  started: 'dinoGameA11yStartGame',
  speedLabel: 'dinoGameA11ySpeedToggle',
};

/**
 * Default game configuration.
 * Shared config for all  versions of the game. Additional parameters are
 * defined in Runner.normalConfig and Runner.slowConfig.
 */
Runner.config = {
  AUDIOCUE_PROXIMITY_THRESHOLD: 190,
  AUDIOCUE_PROXIMITY_THRESHOLD_MOBILE_A11Y: 250,
  BG_CLOUD_SPEED: 0.2,
  BOTTOM_PAD: 10,
  // Scroll Y threshold at which the game can be activated.
  CANVAS_IN_VIEW_OFFSET: -10,
  CLEAR_TIME: 3000,
  CLOUD_FREQUENCY: 0.5,
  FADE_DURATION: 1,
  FLASH_DURATION: 1000,
  GAMEOVER_CLEAR_TIME: 1200,
  INITIAL_JUMP_VELOCITY: 12,
  INVERT_FADE_DURATION: 12000,
  MAX_BLINK_COUNT: 3,
  MAX_CLOUDS: 6,
  MAX_OBSTACLE_LENGTH: 3,
  MAX_OBSTACLE_DUPLICATION: 2,
  RESOURCE_TEMPLATE_ID: 'audio-resources',
  SPEED: 6,
  SPEED_DROP_COEFFICIENT: 3,
  ARCADE_MODE_INITIAL_TOP_POSITION: 35,
  ARCADE_MODE_TOP_POSITION_PERCENT: 0.1,
};

Runner.normalConfig = {
  ACCELERATION: 0.001,
  AUDIOCUE_PROXIMITY_THRESHOLD: 190,
  AUDIOCUE_PROXIMITY_THRESHOLD_MOBILE_A11Y: 250,
  GAP_COEFFICIENT: 0.6,
  INVERT_DISTANCE: 700,
  MAX_SPEED: 13,
  MOBILE_SPEED_COEFFICIENT: 1.2,
  SPEED: 6,
};


Runner.slowConfig = {
  ACCELERATION: 0.0005,
  AUDIOCUE_PROXIMITY_THRESHOLD: 170,
  AUDIOCUE_PROXIMITY_THRESHOLD_MOBILE_A11Y: 220,
  GAP_COEFFICIENT: 0.3,
  INVERT_DISTANCE: 350,
  MAX_SPEED: 9,
  MOBILE_SPEED_COEFFICIENT: 1.5,
  SPEED: 4.2,
};


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


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


/**
 * 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',
  POINTERDOWN: 'pointerdown',
  POINTERUP: 'pointerup',
  RESIZE: 'resize',
  TOUCHEND: 'touchend',
  TOUCHSTART: 'touchstart',
  VISIBILITY: 'visibilitychange',
  BLUR: 'blur',
  FOCUS: 'focus',
  LOAD: 'load',
  GAMEPADCONNECTED: 'gamepadconnected',
};

Runner.prototype = {
  /**
   * Initialize alternative game type.
   */
  initAltGameType() {
    if (GAME_TYPE.length > 0) {
      this.gameType = loadTimeData && loadTimeData.valueExists('altGameType') ?
          GAME_TYPE[parseInt(loadTimeData.getValue('altGameType'), 10) - 1] :
          '';
    }
  },

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

  /**
   * For disabled instances, set up a snackbar with the disabled message.
   */
  setupDisabledRunner() {
    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 {number|string} value
   */
  updateConfigSetting(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(/** @type {number} */ (value));
          break;
      }
    }
  },

  /**
   * Creates an on page image element from the base 64 encoded string source.
   * @param {string} resourceName Name in data object,
   * @return {HTMLImageElement} The created element.
   */
  createImageElement(resourceName) {
    const imgSrc = loadTimeData && loadTimeData.valueExists(resourceName) ?
        loadTimeData.getString(resourceName) :
        null;

    if (imgSrc) {
      const el =
          /** @type {HTMLImageElement} */ (document.createElement('img'));
      el.id = resourceName;
      el.src = imgSrc;
      document.getElementById('offline-resources').appendChild(el);
      return el;
    }
    return null;
  },

  /**
   * Cache the appropriate image sprite from the page and get the sprite sheet
   * definition.
   */
  loadImages() {
    let scale = '1x';
    this.spriteDef = Runner.spriteDefinition.LDPI;
    if (IS_HIDPI) {
      scale = '2x';
      this.spriteDef = Runner.spriteDefinition.HDPI;
    }

    Runner.imageSprite = /** @type {HTMLImageElement} */
        (document.getElementById(RESOURCE_POSTFIX + scale));

    if (this.gameType) {
      Runner.altGameImageSprite = /** @type {HTMLImageElement} */
          (this.createImageElement('altGameSpecificImage' + scale));
      Runner.altCommonImageSprite = /** @type {HTMLImageElement} */
          (this.createImageElement('altGameCommonImage' + scale));
    }
    Runner.origImageSprite = Runner.imageSprite;

    // Disable the alt game mode if the sprites can't be loaded.
    if (!Runner.altGameImageSprite || !Runner.altCommonImageSprite) {
      Runner.isAltGameModeEnabled = () => false;
      this.altGameModeActive = false;
    }

    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() {
    if (!IS_IOS) {
      this.audioContext = new AudioContext();

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

      for (const sound in Runner.sounds) {
        let soundSrc =
            resourceTemplate.getElementById(Runner.sounds[sound]).src;
        soundSrc = soundSrc.substr(soundSrc.indexOf(',') + 1);
        const 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(opt_speed) {
    const speed = opt_speed || this.currentSpeed;

    // Reduce the speed on smaller mobile screens.
    if (this.dimensions.WIDTH < DEFAULT_WIDTH) {
      const mobileSpeed = Runner.slowDown ? speed :
                                            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() {
    // Hide the static icon.
    document.querySelector('.' + Runner.classes.ICON).style.visibility =
        'hidden';

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

    const ariaLabel = getA11yString(A11Y_STRINGS.ariaLabel);
    this.containerEl = document.createElement('div');
    this.containerEl.setAttribute('role', IS_MOBILE ? 'button' : 'application');
    this.containerEl.setAttribute('tabindex', '0');
    this.containerEl.setAttribute('title', ariaLabel);

    this.containerEl.className = Runner.classes.CONTAINER;

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

    // Live region for game status updates.
    this.a11yStatusEl = document.createElement('span');
    this.a11yStatusEl.className = 'offline-runner-live-region';
    this.a11yStatusEl.setAttribute('aria-live', 'assertive');
    this.a11yStatusEl.textContent = '';
    Runner.a11yStatusEl = this.a11yStatusEl;

    // Add checkbox to slow down the game.
    this.slowSpeedCheckboxLabel = document.createElement('label');
    this.slowSpeedCheckboxLabel.className = 'slow-speed-option hidden';
    this.slowSpeedCheckboxLabel.textContent =
        getA11yString(A11Y_STRINGS.speedLabel);

    this.slowSpeedCheckbox = document.createElement('input');
    this.slowSpeedCheckbox.setAttribute('type', 'checkbox');
    this.slowSpeedCheckbox.setAttribute(
        'title', getA11yString(A11Y_STRINGS.speedLabel));
    this.slowSpeedCheckbox.setAttribute('tabindex', '0');
    this.slowSpeedCheckbox.setAttribute('checked', 'checked');

    this.slowSpeedToggleEl = document.createElement('span');
    this.slowSpeedToggleEl.className = 'slow-speed-toggle';

    this.slowSpeedCheckboxLabel.appendChild(this.slowSpeedCheckbox);
    this.slowSpeedCheckboxLabel.appendChild(this.slowSpeedToggleEl);

    if (IS_IOS) {
      this.outerContainerEl.appendChild(this.a11yStatusEl);
    } else {
      this.containerEl.appendChild(this.a11yStatusEl);
    }

    announcePhrase(getA11yString(A11Y_STRINGS.description));

    this.generatedSoundFx = new GeneratedSoundFx();

    this.canvasCtx =
        /** @type {CanvasRenderingContext2D} */ (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);
    this.outerContainerEl.appendChild(this.slowSpeedCheckboxLabel);

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

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

    // Handle dark mode
    const darkModeMediaQuery =
        window.matchMedia('(prefers-color-scheme: dark)');
    this.isDarkMode = darkModeMediaQuery && darkModeMediaQuery.matches;
    darkModeMediaQuery.addListener((e) => {
      this.isDarkMode = e.matches;
    });
  },

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

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

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

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

    this.dimensions.WIDTH = this.outerContainerEl.offsetWidth - padding * 2;
    if (this.isArcadeMode()) {
      this.dimensions.WIDTH = Math.min(DEFAULT_WIDTH, this.dimensions.WIDTH);
      if (this.activated) {
        this.setArcadeModeContainerScale();
      }
    }

    // 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(this.altGameModeActive, this.tRex);
      }
    }
  },

  /**
   * Play the game intro.
   * Canvas container width expands out to the full width.
   */
  playIntro() {
    if (!this.activated && !this.crashed) {
      this.playingIntro = true;
      this.tRex.playingIntro = true;
      if (window.localStorage.getItem("chrome-dino") !== null) {
          this.distanceMeter.setHighScore(window.localStorage.getItem("chrome-dino"));
      }

      // CSS animation definition.
      const keyframes = '@-webkit-keyframes intro { ' +
            'from { width:' + Trex.config.WIDTH + 'px }' +
            'to { width: ' + this.dimensions.WIDTH + 'px }' +
          '}';
      // document.styleSheets[0].insertRule(keyframes, 0);
      
      // 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';

      this.setPlayStatus(true);
      this.activated = true;
    } else if (this.crashed) {
      this.restart();
    }
  },


  /**
   * Update the game status to started.
   */
  startGame() {
    if (this.isArcadeMode()) {
      this.setArcadeMode();
    }
    this.toggleSpeed();
    this.runningTime = 0;
    this.playingIntro = false;
    this.tRex.playingIntro = false;
    this.containerEl.style.webkitAnimation = '';
    this.playCount++;
    this.generatedSoundFx.background();
    announcePhrase(getA11yString(A11Y_STRINGS.started));

    if (Runner.audioCues) {
      this.containerEl.setAttribute('title', getA11yString(A11Y_STRINGS.jump));
    }

    // 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() {
    this.canvasCtx.clearRect(0, 0, this.dimensions.WIDTH,
        this.dimensions.HEIGHT);
  },

  /**
   * Checks whether the canvas area is in the viewport of the browser
   * through the current scroll position.
   * @return boolean.
   */
  isCanvasInView() {
    return this.containerEl.getBoundingClientRect().top >
        Runner.config.CANVAS_IN_VIEW_OFFSET;
  },

  /**
   * Enable the alt game mode. Switching out the sprites.
   */
  enableAltGameMode() {
    Runner.imageSprite = Runner.altGameImageSprite;
    Runner.spriteDefinition = Runner.spriteDefinitionByType[Runner.gameType];

    if (IS_HIDPI) {
      this.spriteDef = Runner.spriteDefinition.HDPI;
    } else {
      this.spriteDef = Runner.spriteDefinition.LDPI;
    }

    this.altGameModeActive = true;
    this.tRex.enableAltGameMode(this.spriteDef.TREX);
    this.horizon.enableAltGameMode(this.spriteDef);
    this.generatedSoundFx.background();
  },

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

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

    // Flashing when switching game modes.
    if (this.altGameModeFlashTimer < 0 || this.altGameModeFlashTimer === 0) {
      this.altGameModeFlashTimer = null;
      this.tRex.setFlashing(false);
      this.enableAltGameMode();
    } else if (this.altGameModeFlashTimer > 0) {
      this.altGameModeFlashTimer -= deltaTime;
      this.tRex.update(deltaTime);
      deltaTime = 0;
    }

    this.time = now;

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

      // Additional fade in - Prevents jump when switching sprites
      if (this.altGameModeActive &&
          this.fadeInTimer <= this.config.FADE_DURATION) {
        this.fadeInTimer += deltaTime / 1000;
        this.canvasCtx.globalAlpha = this.fadeInTimer;
      } else {
        this.canvasCtx.globalAlpha = 1;
      }

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

      this.runningTime += deltaTime;
      const 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 if (!this.crashed) {
        const showNightMode = this.isDarkMode ^ this.inverted;
        deltaTime = !this.activated ? 0 : deltaTime;
        this.horizon.update(
            deltaTime, this.currentSpeed, hasObstacles, showNightMode);
      }

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

      // For a11y, audio cues.
      if (Runner.audioCues && hasObstacles) {
        const jumpObstacle =
            this.horizon.obstacles[0].typeConfig.type != 'COLLECTABLE';

        if (!this.horizon.obstacles[0].jumpAlerted) {
          const threshold = Runner.isMobileMouseInput ?
              Runner.config.AUDIOCUE_PROXIMITY_THRESHOLD_MOBILE_A11Y :
              Runner.config.AUDIOCUE_PROXIMITY_THRESHOLD;
          const adjProximityThreshold = threshold +
              (threshold * Math.log10(this.currentSpeed / Runner.config.SPEED));

          if (this.horizon.obstacles[0].xPos < adjProximityThreshold) {
            if (jumpObstacle) {
              this.generatedSoundFx.jump();
            }
            this.horizon.obstacles[0].jumpAlerted = true;
          }
        }
      }

      // Activated alt game mode.
      if (Runner.isAltGameModeEnabled() && collision &&
          this.horizon.obstacles[0].typeConfig.type == 'COLLECTABLE') {
        this.horizon.removeFirstObstacle();
        this.tRex.setFlashing(true);
        collision = false;
        this.altGameModeFlashTimer = this.config.FLASH_DURATION;
        this.runningTime = 0;
        this.generatedSoundFx.collect();
      }

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

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

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

      if (!Runner.audioCues && playAchievementSound) {
        this.playSound(this.soundFx.SCORE);
      }

      // Night mode.
      if (!Runner.isAltGameModeEnabled()) {
        if (this.invertTimer > this.config.INVERT_FADE_DURATION) {
          this.invertTimer = 0;
          this.invertTrigger = false;
          this.invert(false);
        } else if (this.invertTimer) {
          this.invertTimer += deltaTime;
        } else {
          const 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(false);
            }
          }
        }
      }
    }

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

  /**
   * Event handler.
   * @param {Event} e
   */
  handleEvent(e) {
    return (function(evtType, events) {
      switch (evtType) {
        case events.KEYDOWN:
        case events.TOUCHSTART:
        case events.POINTERDOWN:
          this.onKeyDown(e);
          break;
        case events.KEYUP:
        case events.TOUCHEND:
        case events.POINTERUP:
          this.onKeyUp(e);
          break;
        case events.GAMEPADCONNECTED:
          this.onGamepadConnected(e);
          break;
      }
    }.bind(this))(e.type, Runner.events);
  },

  /**
   * Initialize audio cues if activated by focus on the canvas element.
   * @param {Event} e
   */
  handleCanvasKeyPress(e) {
    if (!this.activated && !Runner.audioCues) {
      this.toggleSpeed();
      Runner.audioCues = true;
      this.generatedSoundFx.init();
      Runner.generatedSoundFx = this.generatedSoundFx;
      Runner.config.CLEAR_TIME *= 1.2;
    } else if (e.keyCode && Runner.keycodes.JUMP[e.keyCode]) {
      this.onKeyDown(e);
    }
  },

  /**
   * Prevent space key press from scrolling.
   * @param {Event} e
   */
  preventScrolling(e) {
    if (e.keyCode === 32) {
      e.preventDefault();
    }
  },

  /**
   * Toggle speed setting if toggle is shown.
   */
  toggleSpeed() {
    if (Runner.audioCues) {
      const speedChange = Runner.slowDown != this.slowSpeedCheckbox.checked;

      if (speedChange) {
        Runner.slowDown = this.slowSpeedCheckbox.checked;
        const updatedConfig =
            Runner.slowDown ? Runner.slowConfig : Runner.normalConfig;

        Runner.config = Object.assign(Runner.config, updatedConfig);
        this.currentSpeed = updatedConfig.SPEED;
        this.tRex.enableSlowConfig();
        this.horizon.adjustObstacleSpeed();
      }
      if (this.playing) {
        this.disableSpeedToggle(true);
      }
    }
  },

  /**
   * Show the speed toggle.
   * From focus event or when audio cues are activated.
   * @param {Event=} e
   */
  showSpeedToggle(e) {
    const isFocusEvent = e && e.type == 'focus';
    if (Runner.audioCues || isFocusEvent) {
      this.slowSpeedCheckboxLabel.classList.toggle(
          HIDDEN_CLASS, isFocusEvent ? false : !this.crashed);
    }
  },

  /**
   * Disable the speed toggle.
   * @param {boolean} disable
   */
  disableSpeedToggle(disable) {
    if (disable) {
      this.slowSpeedCheckbox.setAttribute('disabled', 'disabled');
    } else {
      this.slowSpeedCheckbox.removeAttribute('disabled');
    }
  },

  /**
   * Bind relevant key / mouse / touch listeners.
   */
  startListening() {
    // A11y keyboard / screen reader activation.
    this.containerEl.addEventListener(
        Runner.events.KEYDOWN, this.handleCanvasKeyPress.bind(this));
    if (!IS_MOBILE) {
      this.containerEl.addEventListener(
          Runner.events.FOCUS, this.showSpeedToggle.bind(this));
    }
    this.canvas.addEventListener(
        Runner.events.KEYDOWN, this.preventScrolling.bind(this));
    this.canvas.addEventListener(
        Runner.events.KEYUP, this.preventScrolling.bind(this));

    // Keys.
    document.addEventListener(Runner.events.KEYDOWN, this);
    document.addEventListener(Runner.events.KEYUP, this);

    // Touch / pointer.
    this.containerEl.addEventListener(Runner.events.TOUCHSTART, this);
    document.addEventListener(Runner.events.POINTERDOWN, this);
    document.addEventListener(Runner.events.POINTERUP, this);

    if (this.isArcadeMode()) {
      // Gamepad
      window.addEventListener(Runner.events.GAMEPADCONNECTED, this);
    }
  },

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

    if (this.touchController) {
      this.touchController.removeEventListener(Runner.events.TOUCHSTART, this);
      this.touchController.removeEventListener(Runner.events.TOUCHEND, this);
    }

    this.containerEl.removeEventListener(Runner.events.TOUCHSTART, this);
    document.removeEventListener(Runner.events.POINTERDOWN, this);
    document.removeEventListener(Runner.events.POINTERUP, this);

    if (this.isArcadeMode()) {
      window.removeEventListener(Runner.events.GAMEPADCONNECTED, this);
    }
  },

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

    if (this.isCanvasInView()) {
      // Allow toggling of speed toggle.
      if (Runner.keycodes.JUMP[e.keyCode] &&
          e.target == this.slowSpeedCheckbox) {
        return;
      }

      if (!this.crashed && !this.paused) {
        // For a11y, screen reader activation.
        const isMobileMouseInput = IS_MOBILE &&
                e.type === Runner.events.POINTERDOWN &&
                e.pointerType == 'mouse' && e.target == this.containerEl ||
            (IS_IOS && e.pointerType == 'touch' &&
             document.activeElement == this.containerEl);

        if (Runner.keycodes.JUMP[e.keyCode] ||
            e.type === Runner.events.TOUCHSTART || isMobileMouseInput ||
            (Runner.keycodes.DUCK[e.keyCode] && this.altGameModeActive)) {
          e.preventDefault();
          // Starting the game for the first time.
          if (!this.playing) {
            // Started by touch so create a touch controller.
            if (!this.touchController && e.type === Runner.events.TOUCHSTART) {
              this.createTouchController();
            }

            if (isMobileMouseInput) {
              this.handleCanvasKeyPress(e);
            }
            this.loadSounds();
            this.setPlayStatus(true);
            this.update();
            if (window.errorPageController) {
              errorPageController.trackEasterEgg();
            }
          }
          // Start jump.
          if (!this.tRex.jumping && !this.tRex.ducking) {
            if (Runner.audioCues) {
              this.generatedSoundFx.cancelFootSteps();
            } else {
              this.playSound(this.soundFx.BUTTON_PRESS);
            }
            this.tRex.startJump(this.currentSpeed);
          }
          // Ducking is disabled on alt game modes.
        } else if (
            !this.altGameModeActive && this.playing &&
            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(e) {
    const keyCode = String(e.keyCode);
    const isjumpKey = Runner.keycodes.JUMP[keyCode] ||
        e.type === Runner.events.TOUCHEND || e.type === Runner.events.POINTERUP;

    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.
      const deltaTime = getTimeStamp() - this.time;

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

  /**
   * Process gamepad connected event.
   * @param {Event} e
   */
  onGamepadConnected(e) {
    if (!this.pollingGamepads) {
      this.pollGamepadState();
    }
  },

  /**
   * rAF loop for gamepad polling.
   */
  pollGamepadState() {
    const gamepads = navigator.getGamepads();
    this.pollActiveGamepad(gamepads);

    this.pollingGamepads = true;
    requestAnimationFrame(this.pollGamepadState.bind(this));
  },

  /**
   * Polls for a gamepad with the jump button pressed. If one is found this
   * becomes the "active" gamepad and all others are ignored.
   * @param {!Array<Gamepad>} gamepads
   */
  pollForActiveGamepad(gamepads) {
    for (let i = 0; i < gamepads.length; ++i) {
      if (gamepads[i] && gamepads[i].buttons.length > 0 &&
          gamepads[i].buttons[0].pressed) {
        this.gamepadIndex = i;
        this.pollActiveGamepad(gamepads);
        return;
      }
    }
  },

  /**
   * Polls the chosen gamepad for button presses and generates KeyboardEvents
   * to integrate with the rest of the game logic.
   * @param {!Array<Gamepad>} gamepads
   */
  pollActiveGamepad(gamepads) {
    if (this.gamepadIndex === undefined) {
      this.pollForActiveGamepad(gamepads);
      return;
    }

    const gamepad = gamepads[this.gamepadIndex];
    if (!gamepad) {
      this.gamepadIndex = undefined;
      this.pollForActiveGamepad(gamepads);
      return;
    }

    // The gamepad specification defines the typical mapping of physical buttons
    // to button indicies: https://w3c.github.io/gamepad/#remapping
    this.pollGamepadButton(gamepad, 0, 38);  // Jump
    if (gamepad.buttons.length >= 2) {
      this.pollGamepadButton(gamepad, 1, 40);  // Duck
    }
    if (gamepad.buttons.length >= 10) {
      this.pollGamepadButton(gamepad, 9, 13);  // Restart
    }

    this.previousGamepad = gamepad;
  },

  /**
   * Generates a key event based on a gamepad button.
   * @param {!Gamepad} gamepad
   * @param {number} buttonIndex
   * @param {number} keyCode
   */
  pollGamepadButton(gamepad, buttonIndex, keyCode) {
    const state = gamepad.buttons[buttonIndex].pressed;
    let previousState = false;
    if (this.previousGamepad) {
      previousState = this.previousGamepad.buttons[buttonIndex].pressed;
    }
    // Generate key events on the rising and falling edge of a button press.
    if (state !== previousState) {
      const e = new KeyboardEvent(state ? Runner.events.KEYDOWN
                                      : Runner.events.KEYUP,
                                { keyCode: keyCode });
      document.dispatchEvent(e);
    }
  },

  /**
   * Handle interactions on the game over screen state.
   * A user is able to tap the high score twice to reset it.
   * @param {Event} e
   */
  handleGameOverClicks(e) {
    if (e.target != this.slowSpeedCheckbox) {
      e.preventDefault();
      if (this.distanceMeter.hasClickedOnHighScore(e) && this.highestScore) {
        if (this.distanceMeter.isHighScoreFlashing()) {
          // Subsequent click, reset the high score.
          this.saveHighScore(0, true);
          this.distanceMeter.resetHighScore();
          this.distanceMeter.setHighScore(window.localStorage.removeItem('chrome-dino'));
        } else {
          // First click, flash the high score.
          this.distanceMeter.startHighScoreFlashing();
        }
      } else {
        this.distanceMeter.cancelHighScoreFlashing();
        this.restart();
      }
    }
  },

  /**
   * 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(e) {
    return e.button != null && e.button < 2 &&
        e.type === Runner.events.POINTERUP &&
        (e.target === this.canvas ||
         (IS_MOBILE && Runner.audioCues && e.target === this.containerEl));
  },

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

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

  /**
   * Set the initial high score as stored in the user's profile.
   * @param {number} highScore
   */
  initializeHighScore(highScore) {
    this.syncHighestScore = true;
    highScore = Math.ceil(highScore);
    if (highScore < this.highestScore) {
      if (window.errorPageController) {
        errorPageController.updateEasterEggHighScore(this.highestScore);
      }
      return;
    }
    this.highestScore = highScore;
    this.distanceMeter.setHighScore(this.highestScore);
  },

  /**
   * Sets the current high score and saves to the profile if available.
   * @param {number} distanceRan Total distance ran.
   * @param {boolean=} opt_resetScore Whether to reset the score.
   */
  saveHighScore(distanceRan, opt_resetScore) {
    this.highestScore = Math.ceil(distanceRan);
    this.distanceMeter.setHighScore(this.highestScore);

    // Store the new high score in the profile.
    if (this.syncHighestScore && window.errorPageController) {
      if (opt_resetScore) {
        errorPageController.resetEasterEggHighScore();
      } else {
        errorPageController.updateEasterEggHighScore(this.highestScore);
      }
    }
  },

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

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

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

    // Game over panel.
    if (!this.gameOverPanel) {
      const origSpriteDef = IS_HIDPI ?
          Runner.spriteDefinitionByType.original.HDPI :
          Runner.spriteDefinitionByType.original.LDPI;

      if (this.canvas) {
        if (Runner.isAltGameModeEnabled) {
          this.gameOverPanel = new GameOverPanel(
              this.canvas, origSpriteDef.TEXT_SPRITE, origSpriteDef.RESTART,
              this.dimensions, origSpriteDef.ALT_GAME_END,
              this.altGameModeActive);
        } else {
          this.gameOverPanel = new GameOverPanel(
              this.canvas, origSpriteDef.TEXT_SPRITE, origSpriteDef.RESTART,
              this.dimensions);
        }
      }
    }

    this.gameOverPanel.draw(this.altGameModeActive, this.tRex);

    // Update the high score.
    if (this.distanceRan > this.highestScore) {
      this.saveHighScore(this.distanceRan);
      window.localStorage.setItem('chrome-dino', this.distanceRan);
    }

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

    if (Runner.audioCues) {
      this.generatedSoundFx.stopAll();
      announcePhrase(
          getA11yString(A11Y_STRINGS.gameOver)
              .replace(
                  '$1',
                  this.distanceMeter.getActualDistance(this.distanceRan)
                      .toString()) +
          ' ' +
          getA11yString(A11Y_STRINGS.highScore)
              .replace(
                  '$1',

                  this.distanceMeter.getActualDistance(this.highestScore)
                      .toString()));
      this.containerEl.setAttribute(
          'title', getA11yString(A11Y_STRINGS.ariaLabel));
    }
    this.showSpeedToggle();
    this.disableSpeedToggle(false);
  },

  stop() {
    this.setPlayStatus(false);
    this.paused = true;
    cancelAnimationFrame(this.raqId);
    this.raqId = 0;
    this.generatedSoundFx.stopAll();
  },

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

  restart() {
    if (!this.raqId) {
      this.playCount++;
      this.runningTime = 0;
      this.setPlayStatus(true);
      this.toggleSpeed();
      this.paused = false;
      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.horizon.reset();
      this.tRex.reset();
      this.playSound(this.soundFx.BUTTON_PRESS);
      this.invert(true);
      this.flashTimer = null;
      this.update();
      this.gameOverPanel.reset();
      this.generatedSoundFx.background();
      this.containerEl.setAttribute('title', getA11yString(A11Y_STRINGS.jump));
      announcePhrase(getA11yString(A11Y_STRINGS.started));
    }
  },

  setPlayStatus(isPlaying) {
    if (this.touchController) {
      this.touchController.classList.toggle(HIDDEN_CLASS, !isPlaying);
    }
    this.playing = isPlaying;
  },

  /**
   * Whether the game should go into arcade mode.
   * @return {boolean}
   */
  isArcadeMode() {
    // In RTL languages the title is wrapped with the left to right mark
    // control characters &#x202A; and &#x202C but are invisible.
    /* return IS_RTL ? document.title.indexOf(ARCADE_MODE_URL) == 1 :
                    document.title === ARCADE_MODE_URL; */
    return true;
  },

  /**
   * Hides offline messaging for a fullscreen game only experience.
   */
  setArcadeMode() {
    document.body.classList.add(Runner.classes.ARCADE_MODE);
    this.setArcadeModeContainerScale();
  },

  /**
   * Sets the scaling for arcade mode.
   */
  setArcadeModeContainerScale() {
    const windowHeight = window.innerHeight;
    const scaleHeight = windowHeight / this.dimensions.HEIGHT;
    const scaleWidth = window.innerWidth / this.dimensions.WIDTH;
    const scale = Math.max(1, Math.min(scaleHeight, scaleWidth));
    const scaledCanvasHeight = this.dimensions.HEIGHT * scale;
    // Positions the game container at 10% of the available vertical window
    // height minus the game container height.
    const translateY = Math.ceil(Math.max(0, (windowHeight - scaledCanvasHeight -
        Runner.config.ARCADE_MODE_INITIAL_TOP_POSITION) *
        Runner.config.ARCADE_MODE_TOP_POSITION_PERCENT)) *
        window.devicePixelRatio;

    const cssScale = IS_RTL ? -scale + ',' + scale : scale;
    this.containerEl.style.transform =
        'scale(' + cssScale + ') translateY(' + translateY + 'px)';
  },

  /**
   * Pause the game if the tab is not in focus.
   */
  onVisibilityChange(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 {AudioBuffer} soundBuffer
   */
  playSound(soundBuffer) {
    if (soundBuffer) {
      const sourceNode = this.audioContext.createBufferSource();
      sourceNode.buffer = soundBuffer;
      sourceNode.connect(this.audioContext.destination);
      sourceNode.start(0);
    }
  },

  /**
   * Inverts the current page / canvas colors.
   * @param {boolean} reset Whether to reset colors.
   */
  invert(reset) {
    const htmlEl = document.firstElementChild;

    if (reset) {
      htmlEl.classList.toggle(Runner.classes.INVERTED,
          false);
      this.invertTimer = 0;
      this.inverted = false;
    } else {
      this.inverted = htmlEl.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) {
  const context =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));

  // Query the various pixel ratios
  const devicePixelRatio = Math.floor(window.devicePixelRatio) || 1;
  /** @suppress {missingProperties} */
  const backingStoreRatio =
      Math.floor(context.webkitBackingStorePixelRatio) || 1;
  const ratio = devicePixelRatio / backingStoreRatio;

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


/**
 * Whether events are enabled.
 * @return {boolean}
 */
Runner.isAltGameModeEnabled = function() {
  return loadTimeData && loadTimeData.valueExists('enableAltGameMode');
};


/**
 * Generated sound FX class for audio cues.
 * @constructor
 */
function GeneratedSoundFx() {
  this.audioCues = false;
  this.context = null;
  this.panner = null;
}

GeneratedSoundFx.prototype = {
  init() {
    this.audioCues = true;
    if (!this.context) {
      // iOS only supports the webkit version.
      this.context = window.webkitAudioContext ? new webkitAudioContext() :
                                                 new AudioContext();
      if (IS_IOS) {
        this.context.onstatechange = (function() {
                                       if (this.context.state != 'running') {
                                         this.context.resume();
                                       }
                                     }).bind(this);
        this.context.resume();
      }
      this.panner = this.context.createStereoPanner ?
          this.context.createStereoPanner() :
          null;
    }
  },

  stopAll() {
    this.cancelFootSteps();
  },

  /**
   * Play oscillators at certain frequency and for a certain time.
   * @param {number} frequency
   * @param {number} startTime
   * @param {number} duration
   * @param {?number=} opt_vol
   * @param {number=} opt_pan
   */
  playNote(frequency, startTime, duration, opt_vol, opt_pan) {
    const osc1 = this.context.createOscillator();
    const osc2 = this.context.createOscillator();
    const volume = this.context.createGain();

    // Set oscillator wave type
    osc1.type = 'triangle';
    osc2.type = 'triangle';
    volume.gain.value = 0.1;

    // Set up node routing
    if (this.panner) {
      this.panner.pan.value = opt_pan || 0;
      osc1.connect(volume).connect(this.panner);
      osc2.connect(volume).connect(this.panner);
      this.panner.connect(this.context.destination);
    } else {
      osc1.connect(volume);
      osc2.connect(volume);
      volume.connect(this.context.destination);
    }

    // Detune oscillators for chorus effect
    osc1.frequency.value = frequency + 1;
    osc2.frequency.value = frequency - 2;

    // Fade out
    volume.gain.setValueAtTime(opt_vol || 0.01, startTime + duration - 0.05);
    volume.gain.linearRampToValueAtTime(0.00001, startTime + duration);

    // Start oscillators
    osc1.start(startTime);
    osc2.start(startTime);
    // Stop oscillators
    osc1.stop(startTime + duration);
    osc2.stop(startTime + duration);
  },

  background() {
    if (this.audioCues) {
      const now = this.context.currentTime;
      this.playNote(493.883, now, 0.116);
      this.playNote(659.255, now + 0.116, 0.232);
      this.loopFootSteps();
    }
  },

  loopFootSteps() {
    if (this.audioCues && !this.bgSoundIntervalId) {
      this.bgSoundIntervalId = setInterval(function() {
        this.playNote(73.42, this.context.currentTime, 0.05, 0.16);
        this.playNote(69.30, this.context.currentTime + 0.116, 0.116, 0.16);
      }.bind(this), 280);
    }
  },

  cancelFootSteps() {
    if (this.audioCues && this.bgSoundIntervalId) {
      clearInterval(this.bgSoundIntervalId);
      this.bgSoundIntervalId = null;
      this.playNote(103.83, this.context.currentTime, 0.232, 0.02);
      this.playNote(116.54, this.context.currentTime + 0.116, 0.232, 0.02);
    }
  },

  collect() {
    if (this.audioCues) {
      this.cancelFootSteps();
      const now = this.context.currentTime;
      this.playNote(830.61, now, 0.116);
      this.playNote(1318.51, now + 0.116, 0.232);
    }
  },

  jump() {
    if (this.audioCues) {
      const now = this.context.currentTime;
      this.playNote(659.25, now, 0.116, 0.3, -0.6);
      this.playNote(880, now + 0.116, 0.232, 0.3, -0.6);
    }
  },
};


/**
 * Speak a phrase using Speech Synthesis API for a11y.
 * @param {string} phrase Sentence to speak.
 */
function speakPhrase(phrase) {
  if ('speechSynthesis' in window) {
    const msg = new SpeechSynthesisUtterance(phrase);
    const voices = window.speechSynthesis.getVoices();
    msg.text = phrase;
    speechSynthesis.speak(msg);
  }
}


/**
 * For screen readers make an announcement to the live region.
 * @param {string} phrase Sentence to speak.
 */
function announcePhrase(phrase) {
  if (Runner.a11yStatusEl) {
    Runner.a11yStatusEl.textContent = '';
    Runner.a11yStatusEl.textContent = phrase;
  }
}


/**
 * Returns a string from loadTimeData data object.
 * @param {string} stringName
 * @return {string}
 */
function getA11yString(stringName) {
  return loadTimeData && loadTimeData.valueExists(stringName) ?
      loadTimeData.getString(stringName) :
      '';
}


/**
 * Get random number.
 * @param {number} min
 * @param {number} max
 */
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 {Element} 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) {
  const canvas =
      /** @type {!HTMLCanvasElement} */ (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) {
  const len = (base64String.length / 4) * 3;
  const str = atob(base64String);
  const arrayBuffer = new ArrayBuffer(len);
  const bytes = new Uint8Array(arrayBuffer);

  for (let 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.
 * @param {Object=} opt_altGameEndImgPos
 * @param {boolean=} opt_altGameActive
 * @constructor
 */
function GameOverPanel(
    canvas, textImgPos, restartImgPos, dimensions, opt_altGameEndImgPos,
    opt_altGameActive) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  this.canvasDimensions = dimensions;
  this.textImgPos = textImgPos;
  this.restartImgPos = restartImgPos;
  this.altGameEndImgPos = opt_altGameEndImgPos;
  this.altGameModeActive = opt_altGameActive;

  // Retry animation.
  this.frameTimeStamp = 0;
  this.animTimer = 0;
  this.currentFrame = 0;

  this.gameOverRafId = null;

  this.flashTimer = 0;
  this.flashCounter = 0;
  this.originalText = true;
}

GameOverPanel.RESTART_ANIM_DURATION = 875;
GameOverPanel.LOGO_PAUSE_DURATION = 875;
GameOverPanel.FLASH_ITERATIONS = 5;

/**
 * Animation frames spec.
 */
GameOverPanel.animConfig = {
  frames: [0, 36, 72, 108, 144, 180, 216, 252],
  msPerFrame: GameOverPanel.RESTART_ANIM_DURATION / 8,
};

/**
 * 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(width, opt_height) {
    this.canvasDimensions.WIDTH = width;
    if (opt_height) {
      this.canvasDimensions.HEIGHT = opt_height;
    }
    this.currentFrame = GameOverPanel.animConfig.frames.length - 1;
  },

  drawGameOverText(dimensions, opt_useAltText) {
    const centerX = this.canvasDimensions.WIDTH / 2;
    let textSourceX = dimensions.TEXT_X;
    let textSourceY = dimensions.TEXT_Y;
    let textSourceWidth = dimensions.TEXT_WIDTH;
    let textSourceHeight = dimensions.TEXT_HEIGHT;

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

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

    if (!opt_useAltText) {
      textSourceX += this.textImgPos.x;
      textSourceY += this.textImgPos.y;
    }

    const spriteSource =
        opt_useAltText ? Runner.altCommonImageSprite : Runner.origImageSprite;

    this.canvasCtx.save();

    if (IS_RTL) {
      this.canvasCtx.translate(this.canvasDimensions.WIDTH, 0);
      this.canvasCtx.scale(-1, 1);
    }

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

    this.canvasCtx.restore();
  },

  /**
   * Draw additional adornments for alternative game types.
   */
  drawAltGameElements(tRex) {
    // Additional adornments.
    if (this.altGameModeActive && Runner.spriteDefinition.ALT_GAME_END_CONFIG) {
      const altGameEndConfig = Runner.spriteDefinition.ALT_GAME_END_CONFIG;

      let altGameEndSourceWidth = altGameEndConfig.WIDTH;
      let altGameEndSourceHeight = altGameEndConfig.HEIGHT;
      const altGameEndTargetX = tRex.xPos + altGameEndConfig.X_OFFSET;
      const altGameEndTargetY = tRex.yPos + altGameEndConfig.Y_OFFSET;

      if (IS_HIDPI) {
        altGameEndSourceWidth *= 2;
        altGameEndSourceHeight *= 2;
      }

      this.canvasCtx.drawImage(
          Runner.altCommonImageSprite, this.altGameEndImgPos.x,
          this.altGameEndImgPos.y, altGameEndSourceWidth,
          altGameEndSourceHeight, altGameEndTargetX, altGameEndTargetY,
          altGameEndConfig.WIDTH, altGameEndConfig.HEIGHT);
    }
  },

  /**
   * Draw restart button.
   */
  drawRestartButton() {
    const dimensions = GameOverPanel.dimensions;
    let framePosX = GameOverPanel.animConfig.frames[this.currentFrame];
    let restartSourceWidth = dimensions.RESTART_WIDTH;
    let restartSourceHeight = dimensions.RESTART_HEIGHT;
    const restartTargetX =
        (this.canvasDimensions.WIDTH / 2) - (dimensions.RESTART_WIDTH / 2);
    const restartTargetY = this.canvasDimensions.HEIGHT / 2;

    if (IS_HIDPI) {
      restartSourceWidth *= 2;
      restartSourceHeight *= 2;
      framePosX *= 2;
    }

    this.canvasCtx.save();

    if (IS_RTL) {
      this.canvasCtx.translate(this.canvasDimensions.WIDTH, 0);
      this.canvasCtx.scale(-1, 1);
    }

    this.canvasCtx.drawImage(
        Runner.origImageSprite, this.restartImgPos.x + framePosX,
        this.restartImgPos.y, restartSourceWidth, restartSourceHeight,
        restartTargetX, restartTargetY, dimensions.RESTART_WIDTH,
        dimensions.RESTART_HEIGHT);
    this.canvasCtx.restore();
  },


  /**
   * Draw the panel.
   * @param {boolean} opt_altGameModeActive
   * @param {!Trex} opt_tRex
   */
  draw(opt_altGameModeActive, opt_tRex) {
    if (opt_altGameModeActive) {
      this.altGameModeActive = opt_altGameModeActive;
    }

    this.drawGameOverText(GameOverPanel.dimensions, false);
    this.drawRestartButton();
    this.drawAltGameElements(opt_tRex);
    this.update();
  },

  /**
   * Update animation frames.
   */
  update() {
    const now = getTimeStamp();
    const deltaTime = now - (this.frameTimeStamp || now);

    this.frameTimeStamp = now;
    this.animTimer += deltaTime;
    this.flashTimer += deltaTime;

    // Restart Button
    if (this.currentFrame == 0 &&
        this.animTimer > GameOverPanel.LOGO_PAUSE_DURATION) {
      this.animTimer = 0;
      this.currentFrame++;
      this.drawRestartButton();
    } else if (
        this.currentFrame > 0 &&
        this.currentFrame < GameOverPanel.animConfig.frames.length) {
      if (this.animTimer >= GameOverPanel.animConfig.msPerFrame) {
        this.currentFrame++;
        this.drawRestartButton();
      }
    } else if (
        !this.altGameModeActive &&
        this.currentFrame == GameOverPanel.animConfig.frames.length) {
      this.reset();
      return;
    }

    // Game over text
    if (this.altGameModeActive &&
        Runner.spriteDefinitionByType.original.ALT_GAME_OVER_TEXT_CONFIG) {
      const altTextConfig =
          Runner.spriteDefinitionByType.original.ALT_GAME_OVER_TEXT_CONFIG;

      if (this.flashCounter < GameOverPanel.FLASH_ITERATIONS &&
          this.flashTimer > altTextConfig.FLASH_DURATION) {
        this.flashTimer = 0;
        this.originalText = !this.originalText;

        this.clearGameOverTextBounds();
        if (this.originalText) {
          this.drawGameOverText(GameOverPanel.dimensions, false);
          this.flashCounter++;
        } else {
          this.drawGameOverText(altTextConfig, true);
        }
      } else if (this.flashCounter >= GameOverPanel.FLASH_ITERATIONS) {
        this.reset();
        return;
      }
    }

    this.gameOverRafId = requestAnimationFrame(this.update.bind(this));
  },

  /**
   * Clear game over text.
   */
  clearGameOverTextBounds() {
    this.canvasCtx.save();

    this.canvasCtx.clearRect(
        Math.round(
            this.canvasDimensions.WIDTH / 2 -
            (GameOverPanel.dimensions.TEXT_WIDTH / 2)),
        Math.round((this.canvasDimensions.HEIGHT - 25) / 3),
        GameOverPanel.dimensions.TEXT_WIDTH,
        GameOverPanel.dimensions.TEXT_HEIGHT + 4);
    this.canvasCtx.restore();
  },

  reset() {
    if (this.gameOverRafId) {
      cancelAnimationFrame(this.gameOverRafId);
      this.gameOverRafId = null;
    }
    this.animTimer = 0;
    this.frameTimeStamp = 0;
    this.currentFrame = 0;
    this.flashTimer = 0;
    this.flashCounter = 0;
    this.originalText = true;
  },
};


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

/**
 * Check for a collision.
 * @param {!Obstacle} obstacle
 * @param {!Trex} tRex T-rex object.
 * @param {CanvasRenderingContext2D=} opt_canvasCtx Optional canvas context for
 *    drawing collision boxes.
 * @return {Array<CollisionBox>|undefined}
 */
function checkForCollision(obstacle, tRex, opt_canvasCtx) {
  const 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.
  const tRexBox = new CollisionBox(
      tRex.xPos + 1,
      tRex.yPos + 1,
      tRex.config.WIDTH - 2,
      tRex.config.HEIGHT - 2);

  const 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)) {
    const collisionBoxes = obstacle.collisionBoxes;
    let tRexCollisionBoxes = [];

    if (Runner.isAltGameModeEnabled()) {
      tRexCollisionBoxes = Runner.spriteDefinition.TREX.COLLISION_BOXES;
    } else {
      tRexCollisionBoxes = tRex.ducking ? Trex.collisionBoxes.DUCKING :
                                          Trex.collisionBoxes.RUNNING;
    }

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

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

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


/**
 * 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) {
  let crashed = false;
  const tRexBoxX = tRexBox.x;
  const tRexBoxY = tRexBox.y;

  const obstacleBoxX = obstacleBox.x;
  const 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.
 * @constructor
 */
function CollisionBox(x, y, w, h) {
  this.x = x;
  this.y = y;
  this.width = w;
  this.height = h;
}


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

/**
 * Obstacle.
 * @param {CanvasRenderingContext2D} canvasCtx
 * @param {ObstacleType} type
 * @param {Object} spriteImgPos Obstacle position in sprite.
 * @param {Object} dimensions
 * @param {number} gapCoefficient Mutipler in determining the gap.
 * @param {number} speed
 * @param {number=} opt_xOffset
 * @param {boolean=} opt_isAltGameMode
 * @constructor
 */
function Obstacle(
    canvasCtx, type, spriteImgPos, dimensions, gapCoefficient, speed,
    opt_xOffset, opt_isAltGameMode) {
  this.canvasCtx = canvasCtx;
  this.spritePos = spriteImgPos;
  this.typeConfig = type;
  this.gapCoefficient = Runner.slowDown ? gapCoefficient * 2 : 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;
  this.altGameModeActive = opt_isAltGameMode;
  this.imageSprite = this.typeConfig.type == 'COLLECTABLE' ?
      Runner.altCommonImageSprite :
      this.altGameModeActive ? Runner.altGameImageSprite : Runner.imageSprite;

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

  this.init(speed);
}

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

/**
 * Maximum obstacle grouping count.
 */
Obstacle.MAX_OBSTACLE_LENGTH = 3;


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

    // Increase gap for audio cues enabled.
    if (Runner.audioCues) {
      this.gap *= 2;
    }
  },

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

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

    // X position in sprite.
    let 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(
        this.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(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(gapCoefficient, speed) {
    const minGap = Math.round(
        this.width * speed + this.typeConfig.minGap * gapCoefficient);
    const 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() {
    return this.xPos + this.width > 0;
  },

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

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


//******************************************************************************
/**
 * T-rex game character.
 * @param {HTMLCanvasElement} canvas
 * @param {Object} spritePos Positioning within image sprite.
 * @constructor
 */
function Trex(canvas, spritePos) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  this.spritePos = spritePos;
  this.xPos = 0;
  this.yPos = 0;
  this.xInitialPos = 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 = Object.assign(Trex.config, Trex.normalJumpConfig);
  // 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.altGameModeEnabled = false;
  this.flashing = false;

  this.init();
}


/**
 * T-rex player config.
 */
Trex.config = {
  DROP_VELOCITY: -5,
  FLASH_OFF: 175,
  FLASH_ON: 100,
  HEIGHT: 47,
  HEIGHT_DUCK: 25,
  INTRO_DURATION: 1500,
  SPEED_DROP_COEFFICIENT: 3,
  SPRITE_WIDTH: 262,
  START_X_POS: 50,
  WIDTH: 44,
  WIDTH_DUCK: 59,
};

Trex.slowJumpConfig = {
  GRAVITY: 0.25,
  MAX_JUMP_HEIGHT: 50,
  MIN_JUMP_HEIGHT: 45,
  INITIAL_JUMP_VELOCITY: -20,
};

Trex.normalJumpConfig = {
  GRAVITY: 0.6,
  MAX_JUMP_HEIGHT: 30,
  MIN_JUMP_HEIGHT: 30,
  INITIAL_JUMP_VELOCITY: -10,
};

/**
 * Used in collision detection.
 * @enum {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() {
    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);
  },

  /**
   * Assign the appropriate jump parameters based on the game speed.
   */
  enableSlowConfig: function() {
    const jumpConfig =
        Runner.slowDown ? Trex.slowJumpConfig : Trex.normalJumpConfig;
    Trex.config = Object.assign(Trex.config, jumpConfig);

    this.adjustAltGameConfigForSlowSpeed();
  },

  /**
   * Enables the alternative game. Redefines the dino config.
   * @param {Object} spritePos New positioning within image sprite.
   */
  enableAltGameMode: function(spritePos) {
    this.altGameModeEnabled = true;
    this.spritePos = spritePos;
    const spriteDefinition = Runner.spriteDefinition['TREX'];

    // Update animation frames.
    Trex.animFrames.RUNNING.frames =
        [spriteDefinition.RUNNING_1.x, spriteDefinition.RUNNING_2.x];
    Trex.animFrames.CRASHED.frames = [spriteDefinition.CRASHED.x];

    if (typeof spriteDefinition.JUMPING.x == 'object') {
      Trex.animFrames.JUMPING.frames = spriteDefinition.JUMPING.x;
    } else {
      Trex.animFrames.JUMPING.frames = [spriteDefinition.JUMPING.x];
    }

    Trex.animFrames.DUCKING.frames =
        [spriteDefinition.RUNNING_1.x, spriteDefinition.RUNNING_2.x];

    // Update Trex config
    Trex.config.GRAVITY = spriteDefinition.GRAVITY || Trex.config.GRAVITY;
    Trex.config.HEIGHT = spriteDefinition.RUNNING_1.h,
    Trex.config.INITIAL_JUMP_VELOCITY = spriteDefinition.INITIAL_JUMP_VELOCITY;
    Trex.config.MAX_JUMP_HEIGHT = spriteDefinition.MAX_JUMP_HEIGHT;
    Trex.config.MIN_JUMP_HEIGHT = spriteDefinition.MIN_JUMP_HEIGHT;
    Trex.config.WIDTH = spriteDefinition.RUNNING_1.w;
    Trex.config.WIDTH_JUMP = spriteDefinition.JUMPING.w;
    Trex.config.INVERT_JUMP = spriteDefinition.INVERT_JUMP;

    this.adjustAltGameConfigForSlowSpeed(spriteDefinition.GRAVITY);
    this.config = Trex.config;

    // Adjust bottom horizon placement.
    this.groundYPos = Runner.defaultDimensions.HEIGHT - this.config.HEIGHT -
        Runner.spriteDefinition['BOTTOM_PAD'];
    this.yPos = this.groundYPos;
    this.reset();
  },

  /**
   * Slow speeds adjustments for the alt game modes.
   * @param {number=} opt_gravityValue
   */
  adjustAltGameConfigForSlowSpeed: function(opt_gravityValue) {
    if (Runner.slowDown) {
      if (opt_gravityValue) {
        Trex.config.GRAVITY = opt_gravityValue / 1.5;
      }
      Trex.config.MIN_JUMP_HEIGHT *= 1.5;
      Trex.config.MAX_JUMP_HEIGHT *= 1.5;
      Trex.config.INITIAL_JUMP_VELOCITY =
          Trex.config.INITIAL_JUMP_VELOCITY * 1.5;
    }
  },

  /**
   * Setter whether dino is flashing.
   * @param {boolean} status
   */
  setFlashing: function(status) {
    this.flashing = status;
  },

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

  /**
   * Set the animation status.
   * @param {!number} deltaTime
   * @param {Trex.status=} opt_status Optional status to switch to.
   */
  update(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);
      this.xInitialPos = this.xPos;
    }

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

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

    if (!this.altGameModeEnabled) {
      // 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(x, y) {
    let sourceX = x;
    let sourceY = y;
    let sourceWidth = this.ducking && this.status !== Trex.status.CRASHED ?
        this.config.WIDTH_DUCK :
        this.config.WIDTH;
    let sourceHeight = this.config.HEIGHT;
    const outputHeight = sourceHeight;

    let jumpOffset = Runner.spriteDefinition.TREX.JUMPING.xOffset;

    // Width of sprite changes on jump.
    if (this.altGameModeEnabled && this.jumping &&
        this.status !== Trex.status.CRASHED) {
      sourceWidth = this.config.WIDTH_JUMP;
    }

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

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

    // Flashing.
    if (this.flashing) {
      if (this.timer < this.config.FLASH_ON) {
        this.canvasCtx.globalAlpha = 0.5;
      } else if (this.timer > this.config.FLASH_OFF) {
        this.timer = 0;
      }
    }

    // Ducking.
    if (!this.altGameModeEnabled && this.ducking &&
        this.status !== Trex.status.CRASHED) {
      this.canvasCtx.drawImage(Runner.imageSprite, sourceX, sourceY,
          sourceWidth, sourceHeight,
          this.xPos, this.yPos,
          this.config.WIDTH_DUCK, outputHeight);
    } else if (
        this.altGameModeEnabled && this.jumping &&
        this.status !== Trex.status.CRASHED) {
      // Jumping with adjustments.
      this.canvasCtx.drawImage(
          Runner.imageSprite, sourceX, sourceY, sourceWidth, sourceHeight,
          this.xPos - jumpOffset, this.yPos, this.config.WIDTH_JUMP,
          outputHeight);
    } 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, outputHeight);
    }
    this.canvasCtx.globalAlpha = 1;
  },

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

  /**
   * Make t-rex blink at random intervals.
   * @param {number} time Current time in milliseconds.
   */
  blink(time) {
    const 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(speed) {
    if (!this.jumping) {
      this.update(0, Trex.status.JUMPING);
      // Tweak the jump velocity based on the speed.
      this.jumpVelocity = this.config.INITIAL_JUMP_VELOCITY - (speed / 10);
      this.jumping = true;
      this.reachedMinHeight = false;
      this.speedDrop = false;

      if (this.config.INVERT_JUMP) {
        this.minJumpHeight = this.groundYPos + this.config.MIN_JUMP_HEIGHT;
      }
    }
  },

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

  /**
   * Update frame for a jump.
   * @param {number} deltaTime
   */
  updateJump(deltaTime) {
    const msPerFrame = Trex.animFrames[this.status].msPerFrame;
    const 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 if (this.config.INVERT_JUMP) {
      this.yPos -= Math.round(this.jumpVelocity * framesElapsed);
    } else {
      this.yPos += Math.round(this.jumpVelocity * framesElapsed);
    }

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

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

    // Reached max height.
    if (this.config.INVERT_JUMP && (this.yPos > -this.config.MAX_JUMP_HEIGHT) ||
        !this.config.INVERT_JUMP && (this.yPos < this.config.MAX_JUMP_HEIGHT) ||
        this.speedDrop) {
      this.endJump();
    }

    // Back down at ground level. Jump completed.
    if ((this.config.INVERT_JUMP && this.yPos) < this.groundYPos ||
        (!this.config.INVERT_JUMP && this.yPos) > this.groundYPos) {
      this.reset();
      this.jumpCount++;

      if (Runner.audioCues) {
        Runner.generatedSoundFx.loopFootSteps();
      }
    }
  },

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

  /**
   * @param {boolean} isDucking
   */
  setDuck(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() {
    this.xPos = this.xInitialPos;
    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 =
      /** @type {CanvasRenderingContext2D} */ (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.achievement = false;
  this.defaultString = '';
  this.flashTimer = 0;
  this.flashIterations = 0;
  this.invertTrigger = false;
  this.flashingRafId = null;
  this.highScoreBounds = {};
  this.highScoreFlashing = false;

  this.config = DistanceMeter.config;
  this.maxScoreUnits = this.config.MAX_DISTANCE_UNITS;
  this.canvasWidth = canvasWidth;
  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,

  // Padding around the high score hit area.
  HIGH_SCORE_HIT_AREA_PADDING: 4,
};


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

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

    this.maxScore = parseInt(maxDistanceStr, 10);
  },

  /**
   * Calculate the xPos in the canvas.
   * @param {number} canvasWidth
   */
  calcXPos(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(digitPos, value, opt_highScore) {
    let sourceWidth = DistanceMeter.dimensions.WIDTH;
    let sourceHeight = DistanceMeter.dimensions.HEIGHT;
    let sourceX = DistanceMeter.dimensions.WIDTH * value;
    let sourceY = 0;

    const targetX = digitPos * DistanceMeter.dimensions.DEST_WIDTH;
    const targetY = this.y;
    const targetWidth = DistanceMeter.dimensions.WIDTH;
    const 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 (IS_RTL) {
      if (opt_highScore) {
        this.canvasCtx.translate(
            this.canvasWidth -
                (DistanceMeter.dimensions.WIDTH * (this.maxScoreUnits + 3)),
            this.y);
      } else {
        this.canvasCtx.translate(
            this.canvasWidth - DistanceMeter.dimensions.WIDTH, this.y);
      }
      this.canvasCtx.scale(-1, 1);
    } else {
      const highScoreX =
          this.x - (this.maxScoreUnits * 2) * DistanceMeter.dimensions.WIDTH;
      if (opt_highScore) {
        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(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(deltaTime, distance) {
    let paint = true;
    let playSound = false;

    if (!this.achievement) {
      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', 10);
      } else {
        this.distance = 0;
      }

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

        // Create a string representation of the distance with leading 0.
        const 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.achievement = false;
        this.flashIterations = 0;
        this.flashTimer = 0;
      }
    }

    // Draw the digits if not flashing.
    if (paint) {
      for (let i = this.digits.length - 1; i >= 0; i--) {
        this.draw(i, parseInt(this.digits[i], 10));
      }
    }

    this.drawHighScore();
    return playSound;
  },

  /**
   * Draw the high score.
   */
  drawHighScore() {
    if (parseInt(this.highScore, 10) > 0) {
      this.canvasCtx.save();
      this.canvasCtx.globalAlpha = .8;
      for (let 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(distance) {
    distance = this.getActualDistance(distance);
    const highScoreStr = (this.defaultString +
        distance).substr(-this.maxScoreUnits);

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


  /**
   * Whether a clicked is in the high score area.
   * @param {Event} e Event object.
   * @return {boolean} Whether the click was in the high score bounds.
   */
  hasClickedOnHighScore(e) {
    let x = 0;
    let y = 0;

    if (e.touches) {
      // Bounds for touch differ from pointer.
      const canvasBounds = this.canvas.getBoundingClientRect();
      x = e.touches[0].clientX - canvasBounds.left;
      y = e.touches[0].clientY - canvasBounds.top;
    } else {
      x = e.offsetX;
      y = e.offsetY;
    }

    this.highScoreBounds = this.getHighScoreBounds();
    return x >= this.highScoreBounds.x && x <=
        this.highScoreBounds.x + this.highScoreBounds.width &&
        y >= this.highScoreBounds.y && y <=
        this.highScoreBounds.y + this.highScoreBounds.height;
  },

  /**
   * Get the bounding box for the high score.
   * @return {Object} Object with x, y, width and height properties.
   */
  getHighScoreBounds() {
    return {
      x: (this.x - (this.maxScoreUnits * 2) * DistanceMeter.dimensions.WIDTH) -
          DistanceMeter.config.HIGH_SCORE_HIT_AREA_PADDING,
      y: this.y,
      width: DistanceMeter.dimensions.WIDTH * (this.highScore.length + 1) +
          DistanceMeter.config.HIGH_SCORE_HIT_AREA_PADDING,
      height: DistanceMeter.dimensions.HEIGHT +
          (DistanceMeter.config.HIGH_SCORE_HIT_AREA_PADDING * 2),
    };
  },

  /**
   * Animate flashing the high score to indicate ready for resetting.
   * The flashing stops following this.config.FLASH_ITERATIONS x 2 flashes.
   */
  flashHighScore() {
    const now = getTimeStamp();
    const deltaTime = now - (this.frameTimeStamp || now);
    let paint = true;
    this.frameTimeStamp = now;

    // Reached the max number of flashes.
    if (this.flashIterations > this.config.FLASH_ITERATIONS * 2) {
      this.cancelHighScoreFlashing();
      return;
    }

    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++;
    }

    if (paint) {
      this.drawHighScore();
    } else {
      this.clearHighScoreBounds();
    }
    // Frame update.
    this.flashingRafId =
        requestAnimationFrame(this.flashHighScore.bind(this));
  },

  /**
   * Draw empty rectangle over high score.
   */
  clearHighScoreBounds() {
    this.canvasCtx.save();
    this.canvasCtx.fillStyle = '#fff';
    this.canvasCtx.rect(this.highScoreBounds.x, this.highScoreBounds.y,
        this.highScoreBounds.width, this.highScoreBounds.height);
    this.canvasCtx.fill();
    this.canvasCtx.restore();
  },

  /**
   * Starts the flashing of the high score.
   */
  startHighScoreFlashing() {
    this.highScoreFlashing = true;
    this.flashHighScore();
  },

  /**
   * Whether high score is flashing.
   * @return {boolean}
   */
  isHighScoreFlashing() {
    return this.highScoreFlashing;
  },

  /**
   * Stop flashing the high score.
   */
  cancelHighScoreFlashing() {
    if (this.flashingRafId) {
      cancelAnimationFrame(this.flashingRafId);
    }
    this.flashIterations = 0;
    this.flashTimer = 0;
    this.highScoreFlashing = false;
    this.clearHighScoreBounds();
    this.drawHighScore();
  },

  /**
   * Clear the high score.
   */
  resetHighScore() {
    this.setHighScore(0);
    this.cancelHighScoreFlashing();
  },

  /**
   * Reset the distance meter back to '00000'.
   */
  reset() {
    this.update(0, 0);
    this.achievement = 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
 * @constructor
 */
function Cloud(canvas, spritePos, containerWidth) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (this.canvas.getContext('2d'));
  this.spritePos = spritePos;
  this.containerWidth = containerWidth;
  this.xPos = containerWidth;
  this.yPos = 0;
  this.remove = false;
  this.gap =
      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() {
    this.yPos = getRandomNum(Cloud.config.MAX_SKY_LEVEL,
        Cloud.config.MIN_SKY_LEVEL);
    this.draw();
  },

  /**
   * Draw the cloud.
   */
  draw() {
    this.canvasCtx.save();
    let sourceWidth = Cloud.config.WIDTH;
    let sourceHeight = Cloud.config.HEIGHT;
    const outputWidth = sourceWidth;
    const outputHeight = sourceHeight;
    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,
        outputWidth, outputHeight);

    this.canvasCtx.restore();
  },

  /**
   * Update the cloud position.
   * @param {number} speed
   */
  update(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() {
    return this.xPos + Cloud.config.WIDTH > 0;
  },
};


/**
 * Background item.
 * Similar to cloud, without random y position.
 * @param {HTMLCanvasElement} canvas Canvas element.
 * @param {Object} spritePos Position of image in sprite.
 * @param {number} containerWidth
 * @param {string} type Element type.
 * @constructor
 */
function BackgroundEl(canvas, spritePos, containerWidth, type) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (this.canvas.getContext('2d'));
  this.spritePos = spritePos;
  this.containerWidth = containerWidth;
  this.xPos = containerWidth;
  this.yPos = 0;
  this.remove = false;
  this.type = type;
  this.gap =
      getRandomNum(BackgroundEl.config.MIN_GAP, BackgroundEl.config.MAX_GAP);
  this.animTimer = 0;
  this.switchFrames = false;

  this.spriteConfig = {};
  this.init();
}

/**
 * Background element object config.
 * Real values assigned when game type changes.
 * @enum {number}
 */
BackgroundEl.config = {
  MAX_BG_ELS: 0,
  MAX_GAP: 0,
  MIN_GAP: 0,
  POS: 0,
  SPEED: 0,
  Y_POS: 0,
  MS_PER_FRAME: 0,  // only needed when BACKGROUND_EL.FIXED is true
};


BackgroundEl.prototype = {
  /**
   * Initialise the element setting the y position.
   */
  init() {
    this.spriteConfig = Runner.spriteDefinition.BACKGROUND_EL[this.type];
    if (this.spriteConfig.FIXED) {
      this.xPos = this.spriteConfig.FIXED_X_POS;
    }
    this.yPos = BackgroundEl.config.Y_POS - this.spriteConfig.HEIGHT +
        this.spriteConfig.OFFSET;
    this.draw();
  },

  /**
   * Draw the element.
   */
  draw() {
    this.canvasCtx.save();
    let sourceWidth = this.spriteConfig.WIDTH;
    let sourceHeight = this.spriteConfig.HEIGHT;
    let sourceX = this.spriteConfig.X_POS;
    const outputWidth = sourceWidth;
    const outputHeight = sourceHeight;

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

    this.canvasCtx.drawImage(
        Runner.imageSprite, sourceX, this.spritePos.y, sourceWidth,
        sourceHeight, this.xPos, this.yPos, outputWidth, outputHeight);

    this.canvasCtx.restore();
  },

  /**
   * Update the background element position.
   * @param {number} speed
   */
  update(speed) {
    if (!this.remove) {
      if (this.spriteConfig.FIXED) {
        this.animTimer += speed;
        if (this.animTimer > BackgroundEl.config.MS_PER_FRAME) {
          this.animTimer = 0;
          this.switchFrames = !this.switchFrames;
        }

        if (this.spriteConfig.FIXED_Y_POS_1 &&
            this.spriteConfig.FIXED_Y_POS_2) {
          this.yPos = this.switchFrames ? this.spriteConfig.FIXED_Y_POS_1 :
                                          this.spriteConfig.FIXED_Y_POS_2;
        }
      } else {
        // Fixed speed, regardless of actual game speed.
        this.xPos -= BackgroundEl.config.SPEED;
      }
      this.draw();

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

  /**
   * Check if the element is visible on the stage.
   * @return {boolean}
   */
  isVisible() {
    return this.xPos + this.spriteConfig.WIDTH > 0;
  },
};



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

/**
 * Nightmode shows a moon and stars on the horizon.
 * @param {HTMLCanvasElement} canvas
 * @param {number} spritePos
 * @param {number} containerWidth
 * @constructor
 */
function NightMode(canvas, spritePos, containerWidth) {
  this.spritePos = spritePos;
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (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.
   */
  update(activated) {
    // 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 (let 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(currentPos, speed) {
    if (currentPos < -NightMode.config.WIDTH) {
      currentPos = this.containerWidth;
    } else {
      currentPos -= speed;
    }
    return currentPos;
  },

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

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

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

    // Stars.
    if (this.drawStars) {
      for (let i = 0; i < NightMode.config.NUM_STARS; i++) {
        this.canvasCtx.drawImage(
            Runner.origImageSprite, 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.origImageSprite, 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() {
    const segmentSize = Math.round(this.containerWidth /
        NightMode.config.NUM_STARS);

    for (let 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.spriteDefinitionByType.original.HDPI.STAR.y +
            NightMode.config.STAR_SIZE * 2 * i;
      } else {
        this.stars[i].sourceY =
            Runner.spriteDefinitionByType.original.LDPI.STAR.y +
            NightMode.config.STAR_SIZE * i;
      }
    }
  },

  reset() {
    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} lineConfig Configuration object.
 * @constructor
 */
function HorizonLine(canvas, lineConfig) {
  let sourceX = lineConfig.SOURCE_X;
  let sourceY = lineConfig.SOURCE_Y;

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

  this.spritePos = {x: sourceX, y: sourceY};
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  this.sourceDimensions = {};
  this.dimensions = lineConfig;

  this.sourceXPos = [this.spritePos.x, this.spritePos.x +
      this.dimensions.WIDTH];
  this.xPos = [];
  this.yPos = 0;
  this.bumpThreshold = 0.5;

  this.setSourceDimensions(lineConfig);
  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(newDimensions) {
    for (const dimension in newDimensions) {
      if (dimension !== 'SOURCE_X' && dimension !== 'SOURCE_Y') {
        if (IS_HIDPI) {
          if (dimension !== 'YPOS') {
            this.sourceDimensions[dimension] = newDimensions[dimension] * 2;
          }
        } else {
          this.sourceDimensions[dimension] = newDimensions[dimension];
        }
        this.dimensions[dimension] = newDimensions[dimension];
      }
    }

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

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

  /**
   * Draw the horizon line.
   */
  draw() {
    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(pos, increment) {
    const line1 = pos;
    const 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(deltaTime, speed) {
    const 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() {
    this.xPos[0] = 0;
    this.xPos[1] = this.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 =
      /** @type {CanvasRenderingContext2D} */ (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;
  this.altGameModeActive = false;

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

  // Background elements
  this.backgroundEls = [];
  this.lastEl = null;
  this.backgroundSpeed = this.config.BG_CLOUD_SPEED;

  // Horizon
  this.horizonLine = null;
  this.horizonLines = [];
  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() {
    Obstacle.types = Runner.spriteDefinitionByType.original.OBSTACLES;
    this.addCloud();
    // Multiple Horizon lines
    for (let i = 0; i < Runner.spriteDefinition.LINES.length; i++) {
      this.horizonLines.push(
          new HorizonLine(this.canvas, Runner.spriteDefinition.LINES[i]));
    }

    this.nightMode = new NightMode(this.canvas, this.spritePos.MOON,
        this.dimensions.WIDTH);
  },

  /**
   * Update obstacle definitions based on the speed of the game.
   */
  adjustObstacleSpeed: function() {
    for (let i = 0; i < Obstacle.types.length; i++) {
      if (Runner.slowDown) {
        Obstacle.types[i].multipleSpeed = Obstacle.types[i].multipleSpeed / 2;
        Obstacle.types[i].minGap *= 1.5;
        Obstacle.types[i].minSpeed = Obstacle.types[i].minSpeed / 2;

        // Convert variable y position obstacles to fixed.
        if (typeof (Obstacle.types[i].yPos) == 'object') {
          Obstacle.types[i].yPos = Obstacle.types[i].yPos[0];
          Obstacle.types[i].yPosMobile = Obstacle.types[i].yPos[0];
        }
      }
    }
  },

  /**
   * Update sprites to correspond to change in sprite sheet.
   * @param {number} spritePos
   */
  enableAltGameMode: function(spritePos) {
    // Clear existing horizon objects.
    this.clouds = [];
    this.backgroundEls = [];

    this.altGameModeActive = true;
    this.spritePos = spritePos;

    Obstacle.types = Runner.spriteDefinition.OBSTACLES;
    this.adjustObstacleSpeed();

    Obstacle.MAX_GAP_COEFFICIENT = Runner.spriteDefinition.MAX_GAP_COEFFICIENT;
    Obstacle.MAX_OBSTACLE_LENGTH = Runner.spriteDefinition.MAX_OBSTACLE_LENGTH;

    BackgroundEl.config = Runner.spriteDefinition.BACKGROUND_EL_CONFIG;

    this.horizonLines = [];
    for (let i = 0; i < Runner.spriteDefinition.LINES.length; i++) {
      this.horizonLines.push(
          new HorizonLine(this.canvas, Runner.spriteDefinition.LINES[i]));
    }
    this.reset();
  },

  /**
   * @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(deltaTime, currentSpeed, updateObstacles, showNightMode) {
    this.runningTime += deltaTime;

    if (this.altGameModeActive) {
      this.updateBackgroundEls(deltaTime, currentSpeed);
    }

    for (let i = 0; i < this.horizonLines.length; i++) {
      this.horizonLines[i].update(deltaTime, currentSpeed);
    }

    if (!this.altGameModeActive || Runner.spriteDefinition.HAS_CLOUDS) {
      this.nightMode.update(showNightMode);
      this.updateClouds(deltaTime, currentSpeed);
    }

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

  /**
   * Update background element positions. Also handles creating new elements.
   * @param {number} elSpeed
   * @param {Array<Object>} bgElArray
   * @param {number} maxBgEl
   * @param {Function} bgElAddFunction
   * @param {number} frequency
   */
  updateBackgroundEl(elSpeed, bgElArray, maxBgEl, bgElAddFunction, frequency) {
    const numElements = bgElArray.length;

    if (numElements) {
      for (let i = numElements - 1; i >= 0; i--) {
        bgElArray[i].update(elSpeed);
      }

      const lastEl = bgElArray[numElements - 1];

      // Check for adding a new element.
      if (numElements < maxBgEl &&
          (this.dimensions.WIDTH - lastEl.xPos) > lastEl.gap &&
          frequency > Math.random()) {
        bgElAddFunction();
      }
    } else {
      bgElAddFunction();
    }
  },

  /**
   * Update the cloud positions.
   * @param {number} deltaTime
   * @param {number} speed
   */
  updateClouds(deltaTime, speed) {
    const elSpeed = this.cloudSpeed / 1000 * deltaTime * speed;
    this.updateBackgroundEl(
        elSpeed, this.clouds, this.config.MAX_CLOUDS, this.addCloud.bind(this),
        this.cloudFrequency);

    // Remove expired elements.
    this.clouds = this.clouds.filter((obj) => !obj.remove);
  },

  /**
   * Update the background element positions.
   * @param {number} deltaTime
   * @param {number} speed
   */
  updateBackgroundEls(deltaTime, speed) {
    this.updateBackgroundEl(
        deltaTime, this.backgroundEls, BackgroundEl.config.MAX_BG_ELS,
        this.addBackgroundEl.bind(this), this.cloudFrequency);

    // Remove expired elements.
    this.backgroundEls = this.backgroundEls.filter((obj) => !obj.remove);
  },

  /**
   * Update the obstacle positions.
   * @param {number} deltaTime
   * @param {number} currentSpeed
   */
  updateObstacles(deltaTime, currentSpeed) {
    const updatedObstacles = this.obstacles.slice(0);

    for (let i = 0; i < this.obstacles.length; i++) {
      const 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) {
      const 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() {
    this.obstacles.shift();
  },

  /**
   * Add a new obstacle.
   * @param {number} currentSpeed
   */
  addNewObstacle(currentSpeed) {
    const obstacleCount =
        Obstacle.types[Obstacle.types.length - 1].type != 'COLLECTABLE' ||
            (Runner.isAltGameModeEnabled() && !this.altGameModeActive ||
             this.altGameModeActive) ?
        Obstacle.types.length - 1 :
        Obstacle.types.length - 2;
    const obstacleTypeIndex =
        obstacleCount > 0 ? getRandomNum(0, obstacleCount) : 0;
    const obstacleType = Obstacle.types[obstacleTypeIndex];

    // Check for multiples of the same type of obstacle.
    // Also check obstacle is available at current speed.
    if ((obstacleCount > 0 && this.duplicateObstacleCheck(obstacleType.type)) ||
        currentSpeed < obstacleType.minSpeed) {
      this.addNewObstacle(currentSpeed);
    } else {
      const obstacleSpritePos = this.spritePos[obstacleType.type];

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

      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(nextObstacleType) {
    let duplicateCount = 0;

    for (let 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() {
    this.obstacles = [];
    for (let l = 0; l < this.horizonLines.length; l++) {
      this.horizonLines[l].reset();
    }

    this.nightMode.reset();
  },

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

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

  /**
   * Add a random background element to the horizon.
   */
  addBackgroundEl() {
    const backgroundElTypes =
        Object.keys(Runner.spriteDefinition.BACKGROUND_EL);

    if (backgroundElTypes.length > 0) {
      let index = getRandomNum(0, backgroundElTypes.length - 1);
      let type = backgroundElTypes[index];

      // Add variation if available.
      while (type == this.lastEl && backgroundElTypes.length > 1) {
        index = getRandomNum(0, backgroundElTypes.length - 1);
        type = backgroundElTypes[index];
      }

      this.lastEl = type;
      this.backgroundEls.push(new BackgroundEl(
          this.canvas, this.spritePos.BACKGROUND_EL, this.dimensions.WIDTH,
          type));
    }
  },
};