Add: UserRegistry to store all active users and grand synchronized access

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserRegistry.java

@@ -0,0 +1,42 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+import java.util.Iterator;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class UserRegistry {
+    private final ConcurrentHashMap<UserId, User> byId = new ConcurrentHashMap<>();
+
+    public synchronized Optional<User> registerIfAvailable(String name, SessionId sessionId) {
+        Iterator<User> users = byId.elements().asIterator();
+        while(users.hasNext()) {
+            User user = users.next();
+
+            if (user.getName() == name) {
+                return Optional.empty();
+            }
+        }
+
+        User user = new User(new UserId(), name, sessionId);
+        byId.put(user.getId(), user);
+        return Optional.of(user);
+    }
+
+    public Optional<User> findBySessionId(SessionId sessionId) {
+        Iterator<User> users = byId.elements().asIterator();
+        while(users.hasNext()) {
+            User user = users.next();
+            Optional<SessionId> currentSessionId = user.getSessionId();
+            
+            if (currentSessionId.isEmpty()) {
+                continue;
+            }
+
+            if (currentSessionId.get() == sessionId) {
+                return Optional.of(user);
+            }
+        }
+        return Optional.empty();
+    }
+}