src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java

@@ -30,6 +30,8 @@ public class GameController {
 
     private int dealerIndex = 0;
 
+    private volatile Runnable onGameEndedCallback;
+
     private static final int DEALER_OFFSET = 1;
     private static final int SMALL_BLIND_OFFSET = 1;
     private static final int BIG_BLIND_OFFSET = 2;
@@ -45,6 +47,11 @@ public class GameController {
         this.engine = engine;
     }
 
+    /** Set a callback to be invoked when the game ends. */
+    public void setOnGameEndedCallback(Runnable callback) {
+        this.onGameEndedCallback = callback;
+    }
+
     /**
      * Adds a player to the game with the specified name and initial chip count.
      *
@@ -304,5 +311,12 @@ public class GameController {
      */
     public void endGame() {
         engine.getState().setPhase(GamePhase.FINISHED);
+        if (onGameEndedCallback != null) {
+            try {
+                onGameEndedCallback.run();
+            } catch (RuntimeException e) {
+                // Log or handle callback errors
+            }
+        }
     }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/Lobby.java

@@ -9,6 +9,8 @@ import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.rules.RuleEngine;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.state.GameState;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -26,7 +28,9 @@ public class Lobby {
     private final LobbyId id;
     private final String name;
     private final List<String> playerNames = new CopyOnWriteArrayList<>();
+    private final Set<String> absentPlayers = ConcurrentHashMap.newKeySet();
     private volatile GameController gameController;
+    private volatile Runnable onGameEndedCallback;
 
     public Lobby(LobbyId id, String name) {
         this.id = id;
@@ -96,12 +100,91 @@ public class Lobby {
 
     public void initGame(GameController controller) {
         this.gameController = controller;
+        if (controller != null && onGameEndedCallback != null) {
+            controller.setOnGameEndedCallback(onGameEndedCallback);
+        }
+    }
+
+    /**
+     * Set a callback to be invoked when the game in this lobby ends. This will be propagated to the
+     * game controller when a game is started.
+     */
+    public void setOnGameEndedCallback(Runnable callback) {
+        this.onGameEndedCallback = callback;
+        if (gameController != null) {
+            gameController.setOnGameEndedCallback(callback);
+        }
     }
 
     public GameController getGameController() {
         return gameController;
     }
 
+    /**
+     * Check if a player is currently active (not absent) in the lobby.
+     *
+     * @param playerName the player to check
+     * @return true if player is in the active player list
+     */
+    public boolean isPlayerActive(String playerName) {
+        return playerNames.contains(playerName);
+    }
+
+    /**
+     * Mark a player as absent (left the lobby) while the game is still running. The player remains
+     * in the lobby's mappings but is moved to the absent set.
+     *
+     * @param playerName the player to mark as absent
+     * @return true if the player was active and is now absent
+     */
+    public boolean leavePlayer(String playerName) {
+        if (playerName == null) {
+            return false;
+        }
+        boolean removed = playerNames.remove(playerName);
+        if (removed) {
+            absentPlayers.add(playerName);
+        }
+        return removed;
+    }
+
+    /**
+     * Try to restore an absent player back to active. Returns true if player was absent and is now
+     * active again.
+     *
+     * @param playerName the player to restore
+     * @return true if player was restored from absent
+     */
+    public boolean restoreAbsentPlayer(String playerName) {
+        if (playerName == null) {
+            return false;
+        }
+        boolean wasAbsent = absentPlayers.remove(playerName);
+        if (wasAbsent && !playerNames.contains(playerName)) {
+            playerNames.add(playerName);
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Get the set of absent (gone but not removed) players in this lobby.
+     *
+     * @return copy of absent players set
+     */
+    public Set<String> getAbsentPlayers() {
+        return Set.copyOf(absentPlayers);
+    }
+
+    /**
+     * Check if this lobby has any players (active or absent).
+     *
+     * @return true if playerNames or absentPlayers is non-empty
+     */
+    public boolean hasAnyPlayers() {
+        return !playerNames.isEmpty() || !absentPlayers.isEmpty();
+    }
+
     /**
      * Atomically add a player and, if the lobby reached {@code autoStartPlayers} and no game exists
      * yet, create and start a game. The operation is synchronized on the internal player list to
@@ -139,6 +222,11 @@ public class Lobby {
                         game.addPlayer(PlayerId.of(p), defaultStartChips);
                     }
 
+                    // Set the callback before starting the game
+                    if (onGameEndedCallback != null) {
+                        game.setOnGameEndedCallback(onGameEndedCallback);
+                    }
+
                     game.startGame();
                     this.gameController = game;
                     LOGGER.info(() -> "Auto-started game in lobby " + id.value());

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/LobbyEventListener.java

@@ -4,4 +4,7 @@ package ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby;
 public interface LobbyEventListener {
     /** Called when a game is automatically started in the given lobby. */
     void onGameStarted(LobbyId lobbyId);
+
+    /** Called when a game in the given lobby ends (phase set to FINISHED). */
+    void onGameEnded(LobbyId lobbyId);
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/LobbyManager.java

@@ -71,6 +71,22 @@ public class LobbyManager {
         if (lobby == null) {
             return false;
         }
+
+        // Check if player was absent and try to restore
+        if (lobby.getAbsentPlayers().contains(username)) {
+            boolean restored = lobby.restoreAbsentPlayer(username);
+            if (restored) {
+                LOGGER.info(
+                        () ->
+                                "User '"
+                                        + username
+                                        + "' rejoined absent slot in lobby "
+                                        + lobbyId.value());
+                playerToLobby.put(username, lobbyId);
+                return true;
+            }
+        }
+
         AddResult result =
                 lobby.addPlayerAndMaybeStart(
                         username, maxPlayersPerLobby, AUTO_START_PLAYERS, DEFAULT_START_CHIPS);
@@ -94,6 +110,8 @@ public class LobbyManager {
                                         + AUTO_START_PLAYERS
                                         + " players; game started.");
                 notifyGameStarted(lobbyId);
+                // Set the game-ended callback
+                notifySetGameEndedCallback(lobbyId);
             }
             return true;
         }
@@ -121,6 +139,29 @@ public class LobbyManager {
         }
     }
 
+    private void notifySetGameEndedCallback(LobbyId lobbyId) {
+        Lobby lobby = activeLobbies.get(lobbyId);
+        if (lobby != null) {
+            lobby.setOnGameEndedCallback(
+                    () -> {
+                        notifyGameEnded(lobbyId);
+                    });
+        }
+    }
+
+    private void notifyGameEnded(LobbyId lobbyId) {
+        for (LobbyEventListener l : listeners) {
+            try {
+                l.onGameEnded(lobbyId);
+            } catch (RuntimeException e) {
+                LOGGER.warning(
+                        () ->
+                                "Listener threw while handling game-end for lobby "
+                                        + lobbyId.value());
+            }
+        }
+    }
+
     public boolean removePlayer(String username) {
         LobbyId id = playerToLobby.remove(username);
         if (id == null) {
@@ -138,6 +179,33 @@ public class LobbyManager {
         return removed;
     }
 
+    /**
+     * Mark a player as absent (left the lobby during a running game). The player is not removed but
+     * moved to the absent set, allowing them to rejoin later without losing their slot.
+     *
+     * @param username the player to mark as absent
+     * @param lobbyId the lobby they're leaving
+     * @return true if the player was marked absent successfully
+     */
+    public boolean leavePlayerFromLobby(String username, LobbyId lobbyId) {
+        Lobby lobby = getLobby(lobbyId);
+        if (lobby == null) {
+            return false;
+        }
+        boolean left = lobby.leavePlayer(username);
+        if (left) {
+            LOGGER.info(
+                    () ->
+                            "User '"
+                                    + username
+                                    + "' left lobby "
+                                    + lobbyId.value()
+                                    + " (marked absent)");
+            // Keep the playerToLobby mapping intact for rejoin
+        }
+        return left;
+    }
+
     public Collection<Lobby> getAllLobbies() {
         return activeLobbies.values();
     }
@@ -169,6 +237,9 @@ public class LobbyManager {
         for (String username : removed.getPlayerNames()) {
             playerToLobby.remove(username);
         }
+        for (String username : removed.getAbsentPlayers()) {
+            playerToLobby.remove(username);
+        }
     }
 
     /**

src/test/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/LobbyManagerTest.java

@@ -0,0 +1,172 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/** Unit tests for LobbyManager leave/rejoin functionality. */
+public class LobbyManagerTest {
+    private LobbyManager lobbyManager;
+
+    @BeforeEach
+    void setUp() {
+        lobbyManager = new LobbyManager(4);
+    }
+
+    @Test
+    void testLeavePlayerFromLobbyMarkingAbsent() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+        assertNotNull(lobbyId);
+
+        // Add a player
+        boolean joined = lobbyManager.addPlayerToLobby("Player1", lobbyId);
+        assertTrue(joined);
+
+        Lobby lobby = lobbyManager.getLobby(lobbyId);
+        assertTrue(lobby.getPlayerNames().contains("Player1"));
+        assertTrue(lobby.getAbsentPlayers().isEmpty());
+
+        // Mark player as absent
+        boolean left = lobbyManager.leavePlayerFromLobby("Player1", lobbyId);
+        assertTrue(left);
+
+        // Player should be in absent set
+        assertFalse(lobby.getPlayerNames().contains("Player1"));
+        assertTrue(lobby.getAbsentPlayers().contains("Player1"));
+
+        // Player should still be in the playerToLobby mapping
+        Lobby retrievedLobby = lobbyManager.getLobbyByUsername("Player1");
+        assertEquals(lobbyId, retrievedLobby.getId());
+    }
+
+    @Test
+    void testAbsentPlayerCanRejoin() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+
+        // Add players to fill the lobby almost
+        lobbyManager.addPlayerToLobby("Player1", lobbyId);
+        lobbyManager.addPlayerToLobby("Player2", lobbyId);
+        lobbyManager.addPlayerToLobby("Player3", lobbyId);
+
+        Lobby lobby = lobbyManager.getLobby(lobbyId);
+        assertEquals(3, lobby.getPlayerNames().size());
+
+        // Player1 leaves
+        lobbyManager.leavePlayerFromLobby("Player1", lobbyId);
+        assertEquals(2, lobby.getPlayerNames().size());
+        assertEquals(1, lobby.getAbsentPlayers().size());
+
+        // Player1 tries to rejoin - should succeed without error
+        boolean rejoined = lobbyManager.addPlayerToLobby("Player1", lobbyId);
+        assertTrue(rejoined);
+
+        assertEquals(3, lobby.getPlayerNames().size());
+        assertEquals(0, lobby.getAbsentPlayers().size());
+        assertTrue(lobby.getPlayerNames().contains("Player1"));
+    }
+
+    @Test
+    void testAbsentPlayerDoesNotCountAsFullLobby() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+
+        // Fill lobby completely
+        lobbyManager.addPlayerToLobby("P1", lobbyId);
+        lobbyManager.addPlayerToLobby("P2", lobbyId);
+        lobbyManager.addPlayerToLobby("P3", lobbyId);
+        lobbyManager.addPlayerToLobby("P4", lobbyId);
+
+        Lobby lobby = lobbyManager.getLobby(lobbyId);
+        assertEquals(4, lobby.getPlayerNames().size());
+
+        // P1 leaves
+        lobbyManager.leavePlayerFromLobby("P1", lobbyId);
+        assertEquals(3, lobby.getPlayerNames().size());
+        assertEquals(1, lobby.getAbsentPlayers().size());
+
+        // P1 can rejoin without "LOBBY_FULL" error
+        boolean rejoined = lobbyManager.addPlayerToLobby("P1", lobbyId);
+        assertTrue(rejoined);
+    }
+
+    @Test
+    void testPlayerToLobbyMappingPreservedAfterLeave() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+        lobbyManager.addPlayerToLobby("Player1", lobbyId);
+
+        // Verify mapping exists
+        Lobby beforeLeave = lobbyManager.getLobbyByUsername("Player1");
+        assertEquals(lobbyId, beforeLeave.getId());
+
+        // Player leaves
+        lobbyManager.leavePlayerFromLobby("Player1", lobbyId);
+
+        // Mapping should still exist for rejoin
+        Lobby afterLeave = lobbyManager.getLobbyByUsername("Player1");
+        assertEquals(lobbyId, afterLeave.getId());
+    }
+
+    @Test
+    void testRemoveLobbyCleanupsBothActiveAndAbsentPlayers() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+
+        lobbyManager.addPlayerToLobby("P1", lobbyId);
+        lobbyManager.addPlayerToLobby("P2", lobbyId);
+        lobbyManager.leavePlayerFromLobby("P1", lobbyId);
+
+        // Verify both are in lobby
+        Lobby lobby = lobbyManager.getLobby(lobbyId);
+        assertTrue(lobby.getPlayerNames().contains("P2"));
+        assertTrue(lobby.getAbsentPlayers().contains("P1"));
+
+        // Remove lobby
+        lobbyManager.removeLobby(lobbyId);
+
+        // Both players should be removed from playerToLobby
+        assertNull(lobbyManager.getLobbyByUsername("P1"));
+        assertNull(lobbyManager.getLobbyByUsername("P2"));
+    }
+
+    @Test
+    void testLeaveNonExistentPlayerFromLobby() {
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+
+        // Try to remove a player that was never added
+        boolean left = lobbyManager.leavePlayerFromLobby("NonExistent", lobbyId);
+        assertFalse(left);
+    }
+
+    @Test
+    void testLeavePlayerFromNonExistentLobby() {
+        LobbyId nonExistentId = LobbyId.of(999);
+
+        // Try to remove from non-existent lobby
+        boolean left = lobbyManager.leavePlayerFromLobby("Player1", nonExistentId);
+        assertFalse(left);
+    }
+
+    @Test
+    void testGameStartedWithAbsentPlayerTracking() {
+        // This test verifies that when a game starts with 4 players,
+        // the absent players tracking is initialized and works
+        LobbyId lobbyId = lobbyManager.createNewLobby("Test Lobby");
+
+        lobbyManager.addPlayerToLobby("P1", lobbyId);
+        lobbyManager.addPlayerToLobby("P2", lobbyId);
+        lobbyManager.addPlayerToLobby("P3", lobbyId);
+        // Game should start when 4th player joins
+        lobbyManager.addPlayerToLobby("P4", lobbyId);
+
+        Lobby lobby = lobbyManager.getLobby(lobbyId);
+        assertNotNull(lobby.getGameController(), "Game should be started");
+        assertEquals(4, lobby.getPlayerNames().size());
+        assertEquals(0, lobby.getAbsentPlayers().size());
+
+        // P1 leaves during game
+        boolean left = lobbyManager.leavePlayerFromLobby("P1", lobbyId);
+        assertTrue(left);
+
+        assertEquals(3, lobby.getPlayerNames().size());
+        assertEquals(1, lobby.getAbsentPlayers().size());
+    }
+}

src/test/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/LobbyTest.java

@@ -0,0 +1,195 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.GameController;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.engine.GameEngine;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.engine.RoundManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.engine.TurnManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.rules.RuleEngine;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.state.GameState;
+import java.util.ArrayList;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/** Unit tests for Lobby absent players tracking functionality. */
+public class LobbyTest {
+    private Lobby lobby;
+
+    @BeforeEach
+    void setUp() {
+        lobby = new Lobby(LobbyId.of(1), "Test Lobby");
+    }
+
+    @Test
+    void testAddPlayerSuccessfully() {
+        boolean added = lobby.addPlayer("Player1", 4);
+        assertTrue(added);
+        assertEquals(1, lobby.getPlayerNames().size());
+        assertTrue(lobby.getPlayerNames().contains("Player1"));
+    }
+
+    @Test
+    void testAddPlayerToFullLobby() {
+        lobby.addPlayer("Player1", 4);
+        lobby.addPlayer("Player2", 4);
+        lobby.addPlayer("Player3", 4);
+        lobby.addPlayer("Player4", 4);
+
+        // Try to add 5th player to a 4-player lobby
+        boolean added = lobby.addPlayer("Player5", 4);
+        assertFalse(added);
+    }
+
+    @Test
+    void testLeavePlayerMovesToAbsent() {
+        lobby.addPlayer("Player1", 4);
+        assertTrue(lobby.getPlayerNames().contains("Player1"));
+        assertTrue(lobby.getAbsentPlayers().isEmpty());
+
+        // Player leaves
+        boolean left = lobby.leavePlayer("Player1");
+        assertTrue(left);
+
+        // Player should be in absent set
+        assertFalse(lobby.getPlayerNames().contains("Player1"));
+        assertTrue(lobby.getAbsentPlayers().contains("Player1"));
+    }
+
+    @Test
+    void testRestoreAbsentPlayerSuccessfully() {
+        lobby.addPlayer("Player1", 4);
+        lobby.leavePlayer("Player1");
+
+        // Restore the absent player
+        boolean restored = lobby.restoreAbsentPlayer("Player1");
+        assertTrue(restored);
+
+        // Player should be back in active list
+        assertTrue(lobby.getPlayerNames().contains("Player1"));
+        assertFalse(lobby.getAbsentPlayers().contains("Player1"));
+    }
+
+    @Test
+    void testRestoreAbsentPlayerCannotRestoreTwice() {
+        lobby.addPlayer("Player1", 4);
+        lobby.leavePlayer("Player1");
+
+        boolean restored1 = lobby.restoreAbsentPlayer("Player1");
+        assertTrue(restored1);
+
+        // Try to restore again - should fail
+        boolean restored2 = lobby.restoreAbsentPlayer("Player1");
+        assertFalse(restored2);
+    }
+
+    @Test
+    void testHasAnyPlayersWithActiveOnly() {
+        lobby.addPlayer("Player1", 4);
+        assertTrue(lobby.hasAnyPlayers());
+    }
+
+    @Test
+    void testHasAnyPlayersWithAbsentOnly() {
+        lobby.addPlayer("Player1", 4);
+        lobby.leavePlayer("Player1");
+        assertTrue(lobby.hasAnyPlayers());
+    }
+
+    @Test
+    void testHasAnyPlayersEmpty() {
+        assertFalse(lobby.hasAnyPlayers());
+    }
+
+    @Test
+    void testIsPlayerActive() {
+        lobby.addPlayer("Player1", 4);
+        assertTrue(lobby.isPlayerActive("Player1"));
+
+        lobby.leavePlayer("Player1");
+        assertFalse(lobby.isPlayerActive("Player1"));
+
+        lobby.restoreAbsentPlayer("Player1");
+        assertTrue(lobby.isPlayerActive("Player1"));
+    }
+
+    @Test
+    void testMultiplePlayersLeaveAndRejoin() {
+        // Add 4 players
+        lobby.addPlayer("P1", 4);
+        lobby.addPlayer("P2", 4);
+        lobby.addPlayer("P3", 4);
+        lobby.addPlayer("P4", 4);
+
+        // All active
+        assertEquals(4, lobby.getPlayerNames().size());
+        assertEquals(0, lobby.getAbsentPlayers().size());
+
+        // P1 and P2 leave
+        lobby.leavePlayer("P1");
+        lobby.leavePlayer("P2");
+
+        assertEquals(2, lobby.getPlayerNames().size());
+        assertEquals(2, lobby.getAbsentPlayers().size());
+
+        // P1 comes back
+        lobby.restoreAbsentPlayer("P1");
+
+        assertEquals(3, lobby.getPlayerNames().size());
+        assertEquals(1, lobby.getAbsentPlayers().size());
+        assertTrue(lobby.getPlayerNames().contains("P1"));
+        assertTrue(lobby.getAbsentPlayers().contains("P2"));
+    }
+
+    @Test
+    void testGameEndedCallbackInvoked() {
+        // Create a simple game setup
+        GameState state = new GameState();
+        GameEngine engine =
+                new GameEngine(
+                        state,
+                        new RuleEngine(new ArrayList<>()),
+                        new RoundManager(),
+                        new TurnManager());
+        GameController gameController = new GameController(engine);
+
+        // Track if callback was invoked
+        boolean[] callbackInvoked = {false};
+
+        // Set callback
+        lobby.setOnGameEndedCallback(() -> callbackInvoked[0] = true);
+
+        // Simulate game ending
+        gameController.endGame();
+        // Callback should NOT be invoked yet because it's not registered in the
+        // controller
+
+        // Now register the callback in the controller
+        gameController.setOnGameEndedCallback(() -> callbackInvoked[0] = true);
+        gameController.endGame();
+
+        assertTrue(callbackInvoked[0], "Game ended callback should have been invoked");
+    }
+
+    @Test
+    void testRenamePlayerInActiveList() {
+        lobby.addPlayer("OldName", 4);
+        assertTrue(lobby.getPlayerNames().contains("OldName"));
+
+        boolean renamed = lobby.renamePlayer("OldName", "NewName");
+        assertTrue(renamed);
+
+        assertFalse(lobby.getPlayerNames().contains("OldName"));
+        assertTrue(lobby.getPlayerNames().contains("NewName"));
+    }
+
+    @Test
+    void testRenamePlayerCannotRenameIfAbsent() {
+        lobby.addPlayer("Player1", 4);
+        lobby.leavePlayer("Player1");
+
+        // Should NOT be able to rename an absent player directly
+        boolean renamed = lobby.renamePlayer("Player1", "Player2");
+        assertFalse(renamed);
+    }
+}