feat: implement circuit breaker with closed/open/half-open states (#51)

- Add CircuitBreaker class in new network package with three states:
  CLOSED (requests pass), OPEN (requests blocked), HALF_OPEN (one probe)
- Trips after 3 consecutive heartbeat failures, 10s cooldown before probe
- Integrate into GameNetworkHandler: gates saveMove and sendHeartbeat
- Connection UI reflects breaker state (Disconnected/Reconnecting/latency)

core/src/main/java/com/group14/regicidechess/network/CircuitBreaker.java

@@ -0,0 +1,77 @@
+package com.group14.regicidechess.network;
+
+/**
+ * Simple circuit breaker with three states: CLOSED, OPEN, HALF_OPEN.
+ *
+ * CLOSED:    All requests pass. Consecutive failures are counted.
+ * OPEN:      Requests blocked. After cooldown, transitions to HALF_OPEN.
+ * HALF_OPEN: One probe request allowed. Success -> CLOSED, failure -> OPEN.
+ */
+public class CircuitBreaker {
+
+    public enum State { CLOSED, OPEN, HALF_OPEN }
+
+    private State state = State.CLOSED;
+    private int failureCount = 0;
+    private long openedAt = 0L;
+    private boolean halfOpenProbeUsed = false;
+
+    private final int failureThreshold;
+    private final long cooldownMs;
+
+    public CircuitBreaker(int failureThreshold, long cooldownMs) {
+        this.failureThreshold = failureThreshold;
+        this.cooldownMs = cooldownMs;
+    }
+
+    public synchronized boolean allowRequest() {
+        advanceIfCooldownElapsed();
+        switch (state) {
+            case CLOSED:
+                return true;
+            case HALF_OPEN:
+                if (!halfOpenProbeUsed) {
+                    halfOpenProbeUsed = true;
+                    return true;
+                }
+                return false;
+            case OPEN:
+            default:
+                return false;
+        }
+    }
+
+    public synchronized void recordSuccess() {
+        failureCount = 0;
+        state = State.CLOSED;
+        halfOpenProbeUsed = false;
+    }
+
+    public synchronized void recordFailure() {
+        failureCount++;
+        if (state == State.HALF_OPEN || failureCount >= failureThreshold) {
+            state = State.OPEN;
+            openedAt = System.currentTimeMillis();
+            halfOpenProbeUsed = false;
+        }
+    }
+
+    public synchronized State getState() {
+        advanceIfCooldownElapsed();
+        return state;
+    }
+
+    public synchronized void reset() {
+        state = State.CLOSED;
+        failureCount = 0;
+        openedAt = 0L;
+        halfOpenProbeUsed = false;
+    }
+
+    private void advanceIfCooldownElapsed() {
+        if (state == State.OPEN && System.currentTimeMillis() - openedAt >= cooldownMs) {
+            state = State.HALF_OPEN;
+            halfOpenProbeUsed = false;
+        }
+    }
+}

core/src/main/java/com/group14/regicidechess/screens/game/GameNetworkHandler.java

@@ -6,6 +6,7 @@
 import com.badlogic.gdx.scenes.scene2d.ui.Image;
 import com.badlogic.gdx.scenes.scene2d.ui.Label;
 import com.group14.regicidechess.database.DatabaseManager;
+import com.group14.regicidechess.network.CircuitBreaker;
 import com.group14.regicidechess.model.Move;
 import com.group14.regicidechess.model.Player;
 
@@ -48,6 +49,7 @@ public interface Listener {
     // Connection UI refs — updated directly here to keep GameScreen clean
     private final Image connectionIcon;
     private final Label connectionLabel;
+    private final CircuitBreaker circuitBreaker = new CircuitBreaker(3, 10_000L);
 
     public GameNetworkHandler(String gameId, Player localPlayer, Listener listener,
                                Image connectionIcon, Label connectionLabel) {
@@ -72,13 +74,16 @@ public void start() {
     // Outgoing
     // ─────────────────────────────────────────────────────────────────────────
 
-    /** Saves a completed move (with optional promotion) to Firebase. */
     public void saveMove(Move move) {
+        if (!circuitBreaker.allowRequest()) {
+            Gdx.app.log("GameNetworkHandler", "Circuit breaker OPEN — move send blocked");
+            return;
+        }
         DatabaseManager.getInstance().getApi().saveMove(gameId, move, () -> {});
     }
 
-    /** Sends a heartbeat pulse. Call every HEARTBEAT_INTERVAL seconds from render(). */
     public void sendHeartbeat() {
+        if (!circuitBreaker.allowRequest()) return;
         DatabaseManager.getInstance().getApi().sendHeartbeat(gameId, localPlayer.isWhite());
     }
 
@@ -118,19 +123,20 @@ private void startGameOverListener() {
     }
 
     private void startHeartbeat() {
-        // Send our first beat immediately
-        DatabaseManager.getInstance().getApi().sendHeartbeat(gameId, localPlayer.isWhite());
+        sendHeartbeat();
 
         DatabaseManager.getInstance().getApi()
             .listenForHeartbeat(
                 gameId,
-                !localPlayer.isWhite(), // watch opponent's heartbeat
+                !localPlayer.isWhite(),
                 HEARTBEAT_TIMEOUT_MS,
                 latency -> Gdx.app.postRunnable(() -> {
+                    circuitBreaker.recordSuccess();
                     updateConnectionUI(latency);
                     listener.onHeartbeatLatency(latency);
                 }),
                 () -> Gdx.app.postRunnable(() -> {
+                    circuitBreaker.recordFailure();
                     connectionIcon.setColor(Color.RED);
                     connectionLabel.setText("Lost");
                     listener.onOpponentDisconnected();
@@ -143,6 +149,17 @@ private void startHeartbeat() {
     // ─────────────────────────────────────────────────────────────────────────
 
     private void updateConnectionUI(long latencyMs) {
+        CircuitBreaker.State cbState = circuitBreaker.getState();
+        if (cbState == CircuitBreaker.State.OPEN) {
+            connectionIcon.setColor(Color.RED);
+            connectionLabel.setText("Disconnected");
+            return;
+        }
+        if (cbState == CircuitBreaker.State.HALF_OPEN) {
+            connectionIcon.setColor(Color.ORANGE);
+            connectionLabel.setText("Reconnecting...");
+            return;
+        }
         if (latencyMs < 150)      connectionIcon.setColor(Color.GREEN);
         else if (latencyMs < 500) connectionIcon.setColor(Color.ORANGE);
         else                      connectionIcon.setColor(Color.RED);