fix intersections starting independently with no coordination

main/src/main/java/sd/IntersectionProcess.java

@@ -171,6 +171,29 @@ public class IntersectionProcess {
         System.out.println("  Routing configured.");
     }
 
+    /**
+     * Requests permission for a traffic light to turn green.
+     * Blocks until permission is granted (no other light is green).
+     * 
+     * @param direction The direction requesting green light
+     */
+    public void requestGreenLight(String direction) {
+        trafficCoordinationLock.lock();
+        currentGreenDirection = direction;
+    }
+    
+    /**
+     * Releases the green light permission, allowing another light to turn green.
+     * 
+     * @param direction The direction releasing green light
+     */
+    public void releaseGreenLight(String direction) {
+        if (direction.equals(currentGreenDirection)) {
+            currentGreenDirection = null;
+            trafficCoordinationLock.unlock();
+        }
+    }
+    
     /**
      * Starts all traffic light threads.
      */

main/src/main/java/sd/engine/TrafficLightThread.java

@@ -29,7 +29,6 @@ public class TrafficLightThread implements Runnable {
 
     @Override
     public void run() {
-        // Capture the current thread reference
         this.currentThread = Thread.currentThread();
         this.running = true;
         System.out.println("[" + light.getId() + "] Traffic light thread started.");
@@ -37,29 +36,37 @@ public class TrafficLightThread implements Runnable {
         try {
             while (running && !Thread.currentThread().isInterrupted()) {
                 
-                // --- GREEN Phase ---
+                // Request permission to turn green (blocks until granted)
-                light.changeState(TrafficLightState.GREEN);
+                process.requestGreenLight(light.getDirection());
-                System.out.println("[" + light.getId() + "] State: GREEN");
                 
-                processGreenLightQueue();
+                try {
+                    // --- GREEN Phase ---
+                    light.changeState(TrafficLightState.GREEN);
+                    System.out.println("[" + light.getId() + "] State: GREEN");
                     
-                if (!running || Thread.currentThread().isInterrupted()) break;
+                    processGreenLightQueue();
                     
-                // Wait for green duration
+                    if (!running || Thread.currentThread().isInterrupted()) break;
-                Thread.sleep((long) (light.getGreenTime() * 1000));
                     
-                if (!running || Thread.currentThread().isInterrupted()) break;
+                    // Wait for green duration
+                    Thread.sleep((long) (light.getGreenTime() * 1000));
                     
-                // --- RED Phase ---
+                    if (!running || Thread.currentThread().isInterrupted()) break;
-                light.changeState(TrafficLightState.RED);
+
-                System.out.println("[" + light.getId() + "] State: RED");
+                    // --- RED Phase ---
+                    light.changeState(TrafficLightState.RED);
+                    System.out.println("[" + light.getId() + "] State: RED");
+                    
+                } finally {
+                    // Always release the green light permission
+                    process.releaseGreenLight(light.getDirection());
+                }
                 
                 // Wait for red duration
                 Thread.sleep((long) (light.getRedTime() * 1000));
             }
         } catch (InterruptedException e) {
             System.out.println("[" + light.getId() + "] Traffic light thread interrupted.");
-            // Restore interrupt status
             Thread.currentThread().interrupt();
         } finally {
             this.running = false;

main/src/test/java/sd/TrafficLightCoordinationTest.java

@@ -1,18 +1,18 @@
 package sd;
 
-import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.AfterEach;
-import org.junit.jupiter.api.BeforeEach;
-
-import sd.model.TrafficLight;
-import sd.model.TrafficLightState;
-
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 
-import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.AfterEach;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import sd.model.TrafficLight;
+import sd.model.TrafficLightState;
 
 /**
  * Test class to verify traffic light coordination within an intersection.
@@ -108,7 +108,7 @@ public class TrafficLightCoordinationTest {
         assertTrue(maxGreenSimultaneously.get() <= 1, 
             "At most ONE light should be GREEN at any time. Found: " + maxGreenSimultaneously.get());
         
-        System.out.println("\n✅ Traffic light coordination working correctly!");
+        System.out.println("\nTraffic light coordination working correctly!");
     }
 
     /**