Game Loop Pattern (#1083)

* Add game loop module

* Add game loop module

* Fix merge issue

* Implement game loop module

* Implement game loop module

* Implement time based game loop

* implement VariableStepGameLoop

* Implement FixedStepGameLoop

* Add UT

* Add Unit tests

* Fix checkstyle issues

* Add README.md

* Fix code review issues

* Fix code review issues

* update README.md

game-loop/src/main/java/com/iluwatar/gameloop/FixedStepGameLoop.java

@@ -0,0 +1,62 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+/**
+ * For fixed-step game loop, a certain amount of real time has elapsed since the
+ * last turn of the game loop. This is how much game time need to be simulated for
+ * the game’s “now” to catch up with the player’s.
+ */
+public class FixedStepGameLoop extends GameLoop {
+
+  /**
+   * 20 ms per frame = 50 FPS.
+   */
+  private static final long MS_PER_FRAME = 20;
+
+  @Override
+  protected void processGameLoop() {
+    var previousTime = System.currentTimeMillis();
+    var lag = 0L;
+    while (isGameRunning()) {
+      var currentTime = System.currentTimeMillis();
+      var elapsedTime = currentTime - previousTime;
+      previousTime = currentTime;
+      lag += elapsedTime;
+
+      processInput();
+
+      while (lag >= MS_PER_FRAME) {
+        update();
+        lag -= MS_PER_FRAME;
+      }
+
+      render();
+    }
+  }
+
+  protected void update() {
+    controller.moveBullet(0.5f * MS_PER_FRAME / 1000);
+  }
+}