Added Time class + tests

src/main/java/org/hirw/game/Window.java

@@ -5,6 +5,7 @@ import static org.lwjgl.glfw.GLFW.*;
 import static org.lwjgl.opengl.GL11.*;
 import static org.lwjgl.system.MemoryUtil.*;
 
+import org.hirw.game.util.Time;
 import org.lwjgl.Version;
 import org.lwjgl.glfw.*;
 import org.lwjgl.opengl.*;
@@ -82,11 +83,8 @@ public class Window {
 
   private void loop() {
     while (!glfwWindowShouldClose(glfwWindow)) {
-
-      if (Mouse.isPressed(Mouse.Buttons.LEFT)) {
-        System.out.println("LEFT CLICKED");
-      }
-
+      Time.update();
+      System.out.println(Time.deltaTime());
       glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
       glfwSwapBuffers(glfwWindow);
       glfwPollEvents();

src/main/java/org/hirw/game/util/Time.java

@@ -0,0 +1,31 @@
+package org.hirw.game.util;
+
+public final class Time {
+  private static long lastTime = NanoTimer.system().nanoTime();
+  private static long currentTime = NanoTimer.system().nanoTime();
+  private static NanoTimer nanoTimer = NanoTimer.system();
+
+  public interface NanoTimer {
+    long nanoTime();
+
+    static NanoTimer system() {
+      return System::nanoTime;
+    }
+  }
+
+  static void setNanoTimer(NanoTimer timer) {
+    nanoTimer = timer;
+    lastTime = nanoTimer.nanoTime();
+    currentTime = nanoTimer.nanoTime();
+  }
+
+  public static void update() {
+    lastTime = currentTime;
+    currentTime = nanoTimer.nanoTime();
+  }
+
+  public static float deltaTime() {
+    final float ONE_SECOND = 1_000_000_000f;
+    return (currentTime - lastTime) / ONE_SECOND;
+  }
+}

src/test/java/org/hirw/game/util/TimeTest.java

@@ -0,0 +1,64 @@
+package org.hirw.game.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.when;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+class TimeTest {
+  final float ONE_SECOND = 1_000_000_000f;
+  final long ONE_MILLISECOND = 1_000_000L;
+  final float MOE = 0.00000001f; // Margin of error
+
+  private Time.NanoTimer nanoTimer;
+
+  @BeforeEach
+  void setUp() {
+    nanoTimer = Mockito.mock(Time.NanoTimer.class);
+    when(nanoTimer.nanoTime()).thenReturn(0L);
+    Time.setNanoTimer(nanoTimer);
+  }
+
+  @Test
+  void testDeltaTimeInitialState() {
+    float deltaTime = Time.deltaTime();
+    assertEquals(deltaTime, 0f, MOE, "should be 0");
+  }
+
+  @Test
+  void testDeltaTimeAfterUpdate() {
+    when(nanoTimer.nanoTime()).thenReturn(ONE_MILLISECOND);
+
+    Time.update();
+    float deltaTime = Time.deltaTime();
+
+    assertEquals(deltaTime, ONE_MILLISECOND / ONE_SECOND, MOE, "should reflect time difference");
+  }
+
+  @Test
+  void testMultipleUpdates() {
+    when(nanoTimer.nanoTime()).thenReturn(ONE_MILLISECOND).thenReturn(ONE_MILLISECOND * 4);
+
+    Time.update();
+    float delta1 = Time.deltaTime();
+
+    Time.update();
+    float delta2 = Time.deltaTime();
+
+    assertEquals(delta1, ONE_MILLISECOND / ONE_SECOND, MOE, "should be 1/1000th second");
+    assertEquals(delta2, (ONE_MILLISECOND * 3) / ONE_SECOND, MOE, "should be 3/1000th second");
+  }
+
+  @Test
+  void testConsistentStateAcrossCalls() {
+    when(nanoTimer.nanoTime()).thenReturn(ONE_MILLISECOND);
+
+    Time.update();
+    float delta1 = Time.deltaTime();
+    float delta2 = Time.deltaTime();
+
+    assertEquals(delta1, delta2, MOE, "should be equal delta time");
+  }
+}