src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/ServerApp.java

@@ -1,5 +1,8 @@
 package ch.unibas.dmi.dbis.cs108.casono.server;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingRequest;
@@ -64,7 +67,7 @@ public class ServerApp {
 
         ResponseDispatcher responseDispatcher = new ResponseDispatcher(sessionManager);
 
-        registerCommands(dispatcher, router, responseDispatcher);
+        registerCommands(dispatcher, router, responseDispatcher, userRegistry);
 
         networkManager.start();
     }
@@ -79,8 +82,14 @@ public class ServerApp {
     private static void registerCommands(
             CommandParserDispatcher parserDispatcher,
             CommandRouter commandRouter,
-            ResponseDispatcher responseDispatcher) {
+            ResponseDispatcher responseDispatcher,
+            UserRegistry userRegistry) {
         parserDispatcher.register("PING", new PingParser());
         commandRouter.register(PingRequest.class, new PingHandler(responseDispatcher));
+
+        parserDispatcher.register("CHECK_USERNAME", new CheckUsernameParser());
+        commandRouter.register(
+                CheckUsernameRequest.class,
+                new CheckUsernameHandler(responseDispatcher, userRegistry));
     }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameHandler.java

@@ -0,0 +1,44 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Optional;
+
+/** Handles {@link CheckUsernameRequest}s to check whether a username is available. */
+public class CheckUsernameHandler implements CommandHandler<CheckUsernameRequest> {
+    private final ResponseDispatcher responseDispatcher;
+    private final UserRegistry userRegistry;
+
+    /**
+     * Creates a new handler for checking username availability.
+     *
+     * @param responseDispatcher the dispatcher used to send the response
+     * @param userRegistry the registry used to look up existing users
+     */
+    public CheckUsernameHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
+        this.responseDispatcher = responseDispatcher;
+        this.userRegistry = userRegistry;
+    }
+
+    /**
+     * Executes the username availability check for the given request.
+     *
+     * <p>If no user exists for the requested username, the username is reported as {@link
+     * UsernameAvailability#FREE}; otherwise, it is reported as {@link UsernameAvailability#TAKEN}.
+     *
+     * @param request the request to execute
+     */
+    @Override
+    public void execute(CheckUsernameRequest request) {
+        Optional<User> user = userRegistry.getByUsername(request.getUsername());
+        UsernameAvailability availability;
+        if (user.isEmpty()) {
+            availability = UsernameAvailability.FREE;
+        } else {
+            availability = UsernameAvailability.TAKEN;
+        }
+        responseDispatcher.dispatch(new CheckUsernameResponse(request.getContext(), availability));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameParser.java

@@ -0,0 +1,21 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.PrimitiveRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.accessor.RequestParameterAccessor;
+
+/** Parses a primitive request into a {@link CheckUsernameRequest}. */
+public class CheckUsernameParser implements CommandParser<CheckUsernameRequest> {
+    /**
+     * Extracts the required {@code USERNAME} parameter from the incoming request.
+     *
+     * @param primitiveRequest the request to parse
+     * @return {@link CheckUsernameRequest} containing the username
+     */
+    @Override
+    public CheckUsernameRequest parse(PrimitiveRequest primitiveRequest) {
+        RequestParameterAccessor accessor =
+                new RequestParameterAccessor(primitiveRequest.parameters());
+        return new CheckUsernameRequest(primitiveRequest.context(), accessor.require("USERNAME"));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameRequest.java

@@ -0,0 +1,30 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+
+/** Request implementation used to check whether a username is available or already taken */
+public class CheckUsernameRequest extends Request {
+    private final String username;
+
+    /**
+     * Constructs a new CheckUsernameRequest with the given context and username to check
+     *
+     * @param context the {@link RequestContext} containing information for responding to the
+     *     request
+     * @param username the username to check for availability
+     */
+    public CheckUsernameRequest(RequestContext context, String username) {
+        super(context);
+        this.username = username;
+    }
+
+    /**
+     * Returns the provided username in the request
+     *
+     * @return username to check
+     */
+    public String getUsername() {
+        return username;
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameResponse.java

@@ -0,0 +1,18 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.SuccessResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.builder.ResponseBodyBuilder;
+
+/** Response indicating the availability status of a username check. */
+public class CheckUsernameResponse extends SuccessResponse {
+    /**
+     * Creates a new response to respond to the username availability check to
+     *
+     * @param context the {@link RequestContext} associated with the request
+     * @param availability the availability status of the requested username
+     */
+    public CheckUsernameResponse(RequestContext context, UsernameAvailability availability) {
+        super(context, new ResponseBodyBuilder().param("STATUS", availability).build());
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/UsernameAvailability.java

@@ -0,0 +1,10 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick;
+
+/** Represents the availability status of a username */
+enum UsernameAvailability {
+    /** Username is available */
+    FREE,
+
+    /** Username is already in use */
+    TAKEN
+}