/**
 * @author       Digitsensitive <digit.sensitivee@gmail.com>
 * @copyright    2018 Digitsensitive
 * @description  Juno Core: Game Loop
 *
 * This is the core game loop of the juno html5 game framework.
 * Juno uses a fixed update time step with a variable rendering. This
 * means, that it updates with a fixed time step, but can drop rendering
 * frames to catch up.
 *
 * The Game Loop makes use of EventEmitter to communicate.
 *
 * Regarding Request Animation Frame Juno uses the most basic implementation.
 *
 * Regarding Time Juno uses the most basic implementation of performane.now
 * (see polyfills/performance.now.ts).
 *
 * References:
 * http://gameprogrammingpatterns.com/game-loop.html (Date: 2018-09-09)
 * https://gafferongames.com/post/fix_your_timestep (Date: 2018-09-09)
 * http://www.koonsolo.com/news/dewitters-gameloop (Date: 2018-09-09)
 * https://github.com/sethvincent/gameloop (Date: 2018-09-09)
 *
 * @license      {@link https://github.com/digitsensitive/juno-console/blob/master/license.txt|MIT License}
 */

import { EventEmitter } from "eventemitter3";
import { ElapsedTime } from "../polyfills/performance.now";

export class GameLoop extends EventEmitter {
  private accumulator: number;
  private currentTime: number;
  private fps: number;
  private paused: boolean;
  private step: number;

  constructor() {
    super();
    this.fps = 60;
    this.paused = false;
    this.step = 1 / this.fps;
  }

  /**
   * Start the game loop
   * @param state [name of the state to start]
   */
  public start(state: string): void {
    this.init();
    this.paused = false;
    this.currentTime = ElapsedTime();
    this.accumulator = 0;
    this.frame();
  }

  /**
   * Execution of one frame (= tick).
   */
  private frame(): void {
    if (!this.paused) {
      let newTime = ElapsedTime();

      // it is important that frameTime is in seconds
      // because this.step is also in seconds
      let frameTime = (newTime - this.currentTime) / 1000;

      this.currentTime = newTime;
      this.accumulator += frameTime;

      while (this.accumulator >= this.step) {
        this.update(this.step);
        this.accumulator -= this.step;
      }

      this.render(this.accumulator / this.step);
      requestAnimationFrame(this.frame.bind(this));
    }
  }

  /**
   * Init the game
   */
  private init(): void {
    this.emit("init");
  }

  /**
   * Update the game
   * @param interval [interval in seconds]
   */
  private update(interval: number): void {
    this.emit("update", interval);
  }

  /**
   * Render the game
   * @param delta [delta in seconds]
   */
  private render(delta: number): void {
    this.emit("render", delta);
  }
}