dino.js

// Copyright (c) 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/**
 * 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 || Runner.config;
  // Logical dimensions of the container.
  this.dimensions = Runner.defaultDimensions;

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

  this.tRex = null;

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

  this.highestScore = 0;
  this.syncHighestScore = false;

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

  this.obstacles = [];

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

  this.playCount = 0;

  // Sound FX.
  this.audioBuffer = null;
  /** @type {Object} */
  this.soundFx = {};

  // 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 {
    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 */
// iPads are returning "MacIntel" for iOS 13 (devices & simulators).
// Chrome on macOS also returns "MacIntel" for navigator.platform,
// but navigator.userAgent includes /Safari/.
// TODO(crbug.com/998999): Fix navigator.userAgent such that it reliably
// returns an agent string containing "CriOS".
const IS_IOS = /CriOS/.test(window.navigator.userAgent) ||
    /iPad|iPhone|iPod|MacIntel/.test(window.navigator.platform) &&
        !(/Safari/.test(window.navigator.userAgent));

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

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

/**
 * Default game configuration.
 */
Runner.config = {
  ACCELERATION: 0.001,
  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,
  GAMEOVER_CLEAR_TIME: 750,
  GAP_COEFFICIENT: 0.6,
  GRAVITY: 0.6,
  INITIAL_JUMP_VELOCITY: 12,
  INVERT_FADE_DURATION: 12000,
  INVERT_DISTANCE: 700,
  MAX_BLINK_COUNT: 3,
  MAX_CLOUDS: 6,
  MAX_OBSTACLE_LENGTH: 3,
  MAX_OBSTACLE_DUPLICATION: 2,
  MAX_SPEED: 13,
  MIN_JUMP_HEIGHT: 35,
  MOBILE_SPEED_COEFFICIENT: 1.2,
  RESOURCE_TEMPLATE_ID: 'audio-resources',
  SPEED: 6,
  SPEED_DROP_COEFFICIENT: 3,
  ARCADE_MODE_INITIAL_TOP_POSITION: 35,
  ARCADE_MODE_TOP_POSITION_PERCENT: 0.1
};


/**
 * 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'
};


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


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


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


/**
 * Runner event names.
 * @enum {string}
 */
Runner.events = {
  ANIM_END: 'webkitAnimationEnd',
  CLICK: 'click',
  KEYDOWN: 'keydown',
  KEYUP: 'keyup',
  POINTERDOWN: 'pointerdown',
  POINTERUP: 'pointerup',
  RESIZE: 'resize',
  TOUCHEND: 'touchend',
  TOUCHSTART: 'touchstart',
  VISIBILITY: 'visibilitychange',
  BLUR: 'blur',
  FOCUS: 'focus',
  LOAD: 'load',
  GAMEPADCONNECTED: 'gamepadconnected',
};

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

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

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

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

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

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

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

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

      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: function(opt_speed) {
    const speed = opt_speed || this.currentSpeed;

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

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

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

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

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

    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.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: function() {
    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: function() {
    if (!this.resizeTimerId_) {
      this.resizeTimerId_ =
          setInterval(this.adjustDimensions.bind(this), 250);
    }
  },

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

    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();
      }
    }
  },

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

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

      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: function() {
    if (this.isArcadeMode()) {
      this.setArcadeMode();
    }
    this.runningTime = 0;
    this.playingIntro = false;
    this.tRex.playingIntro = false;
    this.containerEl.style.webkitAnimation = '';
    this.playCount++;

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

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

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

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

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

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

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

    this.time = now;

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

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

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

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

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

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

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

      // Night mode.
      if (this.invertTimer > this.config.INVERT_FADE_DURATION) {
        this.invertTimer = 0;
        this.invertTrigger = false;
        this.invert(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: function(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);
  },

  /**
   * Bind relevant key / mouse / touch listeners.
   */
  startListening: function() {
    // 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: function() {
    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: function(e) {
    // Prevent native page scrolling whilst tapping on mobile.
    if (IS_MOBILE && this.playing) {
      e.preventDefault();
    }

    if (this.isCanvasInView()) {
      if (!this.crashed && !this.paused) {
        if (Runner.keycodes.JUMP[e.keyCode] ||
            e.type == Runner.events.TOUCHSTART) {
          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();
            }
            this.loadSounds();
            this.setPlayStatus(true);
            this.update();
            if (window.errorPageController) {
              errorPageController.trackEasterEgg();
            }
          }
          // Start jump.
          if (!this.tRex.jumping && !this.tRex.ducking) {
            this.playSound(this.soundFx.BUTTON_PRESS);
            this.tRex.startJump(this.currentSpeed);
          }
        } else if (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);
          }
        }
        // iOS only triggers touchstart and no pointer events.
      } else if (IS_IOS && this.crashed && e.type == Runner.events.TOUCHSTART &&
          e.currentTarget == this.containerEl) {
        this.handleGameOverClicks(e);
      }
    }
  },

  /**
   * Process key up.
   * @param {Event} e
   */
  onKeyUp: function(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: function(e) {
    if (!this.pollingGamepads) {
      this.pollGamepadState();
    }
  },

  /**
   * rAF loop for gamepad polling.
   */
  pollGamepadState: function() {
    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: function(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: function(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: function(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: function(e) {
    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();
      } 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: function(e) {
    return e.button != null && e.button < 2 &&
        e.type == Runner.events.POINTERUP && e.target == this.canvas;
  },

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

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

  /**
   * Set the initial high score as stored in the user's profile.
   * @param {number} highScore
   */
  initializeHighScore: function(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: function(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: function() {
    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) {
      if (this.canvas) {
        this.gameOverPanel = new GameOverPanel(this.canvas,
            this.spriteDef.TEXT_SPRITE, this.spriteDef.RESTART,
            this.dimensions);
      }
    } else {
      this.gameOverPanel.draw();
    }

    // Update the high score.
    if (this.distanceRan > this.highestScore) {
      this.saveHighScore(this.distanceRan);
    }

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

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

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

  restart: function() {
    if (!this.raqId) {
      this.playCount++;
      this.runningTime = 0;
      this.setPlayStatus(true);
      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.bdayFlashTimer = null;
      this.update();
    }
  },

  setPlayStatus: function(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: function() {
    return document.title == ARCADE_MODE_URL;
  },

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

  /**
   * Sets the scaling for arcade mode.
   */
  setArcadeModeContainerScale: function() {
    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;
    this.containerEl.style.transform = 'scale(' + scale + ') translateY(' +
        translateY + 'px)';
  },

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

  /**
   * Play a sound.
   * @param {AudioBuffer} soundBuffer
   */
  playSound: function(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: function(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;
};


/**
 * 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.
 * @constructor
 */
function GameOverPanel(canvas, textImgPos, restartImgPos, dimensions) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  this.canvasDimensions = dimensions;
  this.textImgPos = textImgPos;
  this.restartImgPos = restartImgPos;
  this.draw();
}


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


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

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

    const centerX = this.canvasDimensions.WIDTH / 2;

    // Game over text.
    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;

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

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

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

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

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


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

/**
 * Check for a collision.
 * @param {!Obstacle} obstacle
 * @param {!Trex} tRex T-rex object.
 * @param {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;
    const 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
 * @constructor
 */
function Obstacle(canvasCtx, type, spriteImgPos, dimensions,
    gapCoefficient, speed, opt_xOffset) {

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

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

  this.init(speed);
}

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

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


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

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

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

    // Check if obstacle can be positioned at various heights.
    if (Array.isArray(this.typeConfig.yPos))  {
      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);
  },

  /**
   * Draw and crop based on size.
   */
  draw: function() {
    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(Runner.imageSprite,
      sourceX, this.spritePos.y,
      sourceWidth * this.size, sourceHeight,
      this.xPos, this.yPos,
      this.typeConfig.width * this.size, this.typeConfig.height);
  },

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

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

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

  /**
   * Calculate a random gap size.
   * - Minimum gap gets wider as speed increses
   * @param {number} gapCoefficient
   * @param {number} speed
   * @return {number} The gap size.
   */
  getGap: function(gapCoefficient, speed) {
    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: function() {
    return this.xPos + this.width > 0;
  },

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

/**
 * Obstacle definitions.
 * minGap: minimum pixel space betweeen obstacles.
 * multipleSpeed: Speed at which multiples are allowed.
 * speedOffset: speed faster / slower than the horizon.
 * minSpeed: Minimum speed which the obstacle can make an appearance.
 *
 * @typedef {{
 *   type: string,
 *   width: number,
 *   height: number,
 *   yPos: number,
 *   multipleSpeed: number,
 *   minGap: number,
 *   minSpeed: number,
 *   collisionBoxes: Array<CollisionBox>,
 * }}
 */
let ObstacleType;

/** @type {Array<ObstacleType>} */
Obstacle.types = [
  {
    type: 'CACTUS_SMALL',
    width: 17,
    height: 35,
    yPos: 105,
    multipleSpeed: 4,
    minGap: 120,
    minSpeed: 0,
    collisionBoxes: [
      new CollisionBox(0, 7, 5, 27),
      new CollisionBox(4, 0, 6, 34),
      new CollisionBox(10, 4, 7, 14)
    ]
  },
  {
    type: 'CACTUS_LARGE',
    width: 25,
    height: 50,
    yPos: 90,
    multipleSpeed: 7,
    minGap: 120,
    minSpeed: 0,
    collisionBoxes: [
      new CollisionBox(0, 12, 7, 38),
      new CollisionBox(8, 0, 7, 49),
      new CollisionBox(13, 10, 10, 38)
    ]
  },
  {
    type: 'PTERODACTYL',
    width: 46,
    height: 40,
    yPos: [ 100, 75, 50 ], // Variable height.
    yPosMobile: [ 100, 50 ], // Variable height mobile.
    multipleSpeed: 999,
    minSpeed: 8.5,
    minGap: 150,
    collisionBoxes: [
      new CollisionBox(15, 15, 16, 5),
      new CollisionBox(18, 21, 24, 6),
      new CollisionBox(2, 14, 4, 3),
      new CollisionBox(6, 10, 4, 7),
      new CollisionBox(10, 8, 6, 9)
    ],
    numFrames: 2,
    frameRate: 1000/6,
    speedOffset: .8
  }
];


//******************************************************************************
/**
 * T-rex game character.
 * @param {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 = Trex.config;
  // Current status.
  this.status = Trex.status.WAITING;
  this.jumping = false;
  this.ducking = false;
  this.jumpVelocity = 0;
  this.reachedMinHeight = false;
  this.speedDrop = false;
  this.jumpCount = 0;
  this.jumpspotX = 0;

  this.init();
}


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


/**
 * Used in collision detection.
 * @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: function() {
    this.groundYPos = Runner.defaultDimensions.HEIGHT - this.config.HEIGHT -
        Runner.config.BOTTOM_PAD;
    this.yPos = this.groundYPos;
    this.minJumpHeight = this.groundYPos - this.config.MIN_JUMP_HEIGHT;

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

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

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

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

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

    // Game intro animation, T-rex moves in from the left.
    if (this.playingIntro && this.xPos < this.config.START_X_POS) {
      this.xPos += Math.round((this.config.START_X_POS /
          this.config.INTRO_DURATION) * deltaTime);
      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.timer >= this.msPerFrame) {
      this.currentFrame = this.currentFrame ==
          this.currentAnimFrames.length - 1 ? 0 : this.currentFrame + 1;
      this.timer = 0;
    }

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

  /**
   * Draw the t-rex to a particular position.
   * @param {number} x
   * @param {number} y
   */
  draw: function(x, y) {
    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;

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

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

    // Ducking.
    if (this.ducking && this.status != Trex.status.CRASHED) {
      this.canvasCtx.drawImage(Runner.imageSprite, sourceX, sourceY,
          sourceWidth, sourceHeight,
          this.xPos, this.yPos,
          this.config.WIDTH_DUCK, 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: function() {
    this.blinkDelay = Math.ceil(Math.random() * Trex.BLINK_TIMING);
  },

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

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

  /**
   * Update frame for a jump.
   * @param {number} deltaTime
   */
  updateJump: function(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 {
      this.yPos += Math.round(this.jumpVelocity * framesElapsed);
    }

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

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

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

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

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

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

  /**
   * Reset the t-rex to running at start of game.
   */
  reset: function() {
    this.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.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: function(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: function(canvasWidth) {
    this.x = canvasWidth - (DistanceMeter.dimensions.DEST_WIDTH *
        (this.maxScoreUnits + 1));
  },

  /**
   * Draw a digit to canvas.
   * @param {number} digitPos Position of the digit.
   * @param {number} value Digit value 0-9.
   * @param {boolean=} opt_highScore Whether drawing the high score.
   */
  draw: function(digitPos, value, opt_highScore) {
    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 (opt_highScore) {
      // Left of the current score.
      const highScoreX = this.x - (this.maxScoreUnits * 2) *
          DistanceMeter.dimensions.WIDTH;
      this.canvasCtx.translate(highScoreX, this.y);
    } else {
      this.canvasCtx.translate(this.x, this.y);
    }

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

    this.canvasCtx.restore();
  },

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

  /**
   * Update the distance meter.
   * @param {number} distance
   * @param {number} deltaTime
   * @return {boolean} Whether the acheivement sound fx should be played.
   */
  update: function(deltaTime, distance) {
    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) {
        // Acheivement 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: function() {
    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: function(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: function(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: function() {
    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: function() {
    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: function() {
    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: function() {
    if (this.flashingRafId) {
      cancelAnimationFrame(this.flashingRafId);
    }
    this.flashIterations = 0;
    this.flashTimer = 0;
    this.highScoreFlashing = false;
    this.clearHighScoreBounds();
    this.drawHighScore();
  },

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

  /**
   * Reset the distance meter back to '00000'.
   */
  reset: function() {
    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.cloudGap = getRandomNum(Cloud.config.MIN_CLOUD_GAP,
      Cloud.config.MAX_CLOUD_GAP);

  this.init();
}


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


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

  /**
   * Draw the cloud.
   */
  draw: function() {
    this.canvasCtx.save();
    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: function(speed) {
    if (!this.remove) {
      this.xPos -= Math.ceil(speed);
      this.draw();

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

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


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

/**
 * Nightmode shows a moon and stars on the horizon.
 * @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: function(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: function(currentPos, speed) {
    if (currentPos < -NightMode.config.WIDTH) {
      currentPos = this.containerWidth;
    } else {
      currentPos -= speed;
    }
    return currentPos;
  },

  draw: function() {
    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.spriteDefinition.LDPI.STAR.x;

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

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

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

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

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

  // Do star placement.
  placeStars: function() {
    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.spriteDefinition.HDPI.STAR.y +
            NightMode.config.STAR_SIZE * 2 * i;
      } else {
        this.stars[i].sourceY = Runner.spriteDefinition.LDPI.STAR.y +
            NightMode.config.STAR_SIZE * i;
      }
    }
  },

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

};


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

/**
 * Horizon Line.
 * Consists of two connecting lines. Randomly assigns a flat / bumpy horizon.
 * @param {HTMLCanvasElement} canvas
 * @param {Object} spritePos Horizon position in sprite.
 * @constructor
 */
function HorizonLine(canvas, spritePos) {
  this.spritePos = spritePos;
  this.canvas = canvas;
  this.canvasCtx =
      /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  this.sourceDimensions = {};
  this.dimensions = HorizonLine.dimensions;
  this.sourceXPos = [this.spritePos.x, this.spritePos.x +
      this.dimensions.WIDTH];
  this.xPos = [];
  this.yPos = 0;
  this.bumpThreshold = 0.5;

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


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


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

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

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

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

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

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

  /**
   * Update the x position of an indivdual piece of the line.
   * @param {number} pos Line position.
   * @param {number} increment
   */
  updateXPos: function(pos, increment) {
    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: function(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: function() {
    this.xPos[0] = 0;
    this.xPos[1] = HorizonLine.dimensions.WIDTH;
  }
};


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

/**
 * Horizon background class.
 * @param {HTMLCanvasElement} canvas
 * @param {Object} spritePos Sprite positioning.
 * @param {Object} dimensions Canvas dimensions.
 * @param {number} gapCoefficient
 * @constructor
 */
function Horizon(canvas, spritePos, dimensions, gapCoefficient) {
  this.canvas = canvas;
  this.canvasCtx =
      /** @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;

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

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


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


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

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

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

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

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

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

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

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

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

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

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

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

      this.obstacleHistory.unshift(obstacleType.type);

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

  /**
   * Returns whether the previous two obstacles are the same as the next one.
   * Maximum duplication is set in config value MAX_OBSTACLE_DUPLICATION.
   * @return {boolean}
   */
  duplicateObstacleCheck: function(nextObstacleType) {
    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: function() {
    this.obstacles = [];
    this.horizonLine.reset();
    this.nightMode.reset();
  },

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

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