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CMakeLists.txt

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+cmake_minimum_required(VERSION 3.16)
+project(ChatServer LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_AUTOMOC ON)
+
+find_package(Qt6 REQUIRED COMPONENTS Core Network)
+
+add_executable(ChatServer
+    main.cpp
+    ChatServer.h
+    ChatServer.cpp
+)
+
+target_link_libraries(ChatServer PRIVATE Qt6::Core Qt6::Network)

ChatServer.cpp

@@ -0,0 +1,201 @@
+#include "ChatServer.h"
+
+#include <QJsonObject>
+#include <QJsonArray>
+#include <QJsonDocument>
+#include <QDateTime>
+#include <QDebug>
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Constructor
+// ─────────────────────────────────────────────────────────────────────────────
+ChatServer::ChatServer(QObject *parent) : QTcpServer(parent)
+{
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Qt calls this automatically when a new client connects
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::incomingConnection(qintptr socketDescriptor)
+{
+    auto *socket = new QTcpSocket(this);
+
+    // Qt docs recommend checking setSocketDescriptor actually succeeded
+    if (!socket->setSocketDescriptor(socketDescriptor)) {
+        qWarning() << "[Server] Failed to set descriptor:" << socket->errorString();
+        delete socket;
+        return;
+    }
+
+    // wire up this socket's signals to our slots
+    connect(socket, &QTcpSocket::readyRead,    this, &ChatServer::onReadyRead);
+    connect(socket, &QTcpSocket::disconnected, this, &ChatServer::onDisconnected);
+
+    m_clients[socket] = "";  // socket known, username not yet
+    qInfo() << "[Server] New connection from" << socket->peerAddress().toString();
+
+    sendSystemMessage(socket, QString("Connected to server. %1 user(s) online.")
+                      .arg(m_clients.size()));
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Fires when data arrives from any client
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::onReadyRead()
+{
+    auto *socket = qobject_cast<QTcpSocket *>(sender());
+    if (!socket) return;
+
+    // append whatever arrived to this client's buffer
+    m_buffers[socket] += socket->readAll();
+
+    // guard against a client sending huge amounts of data with no newlines
+    if (m_buffers[socket].size() > MAX_BUFFER_SIZE) {
+        qWarning() << "[Server] Buffer overflow from"
+                   << m_clients.value(socket, "unknown") << "— disconnecting.";
+        socket->disconnectFromHost();
+        return;
+    }
+
+    // extract and process complete messages (delimited by \n)
+    while (m_buffers[socket].contains('\n')) {
+        int idx         = m_buffers[socket].indexOf('\n');
+        QByteArray line = m_buffers[socket].left(idx).trimmed();
+        m_buffers[socket] = m_buffers[socket].mid(idx + 1);
+
+        if (line.isEmpty()) continue;
+        processFrame(socket, line);
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Fires when a client disconnects
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::onDisconnected()
+{
+    // Qt docs: use deleteLater() when deleting sender() of disconnected signal
+    auto *socket = qobject_cast<QTcpSocket *>(sender());
+    if (!socket) return;
+
+    QString username = m_clients.value(socket, "Unknown");
+    m_clients.remove(socket);
+    m_buffers.remove(socket);
+    socket->deleteLater();
+
+    qInfo() << "[Server]" << username << "disconnected.";
+    broadcast(nullptr, buildFrame("system", "Server",
+              username + " has left the chat.", ""));
+    broadcastUserList();
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Handle one complete JSON message from a client
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::processFrame(QTcpSocket *socket, const QByteArray &data)
+{
+    // parse raw bytes into a JSON document
+    QJsonParseError err;
+    QJsonDocument doc = QJsonDocument::fromJson(data, &err);
+    if (err.error != QJsonParseError::NoError || !doc.isObject()) {
+        qWarning() << "[Server] Bad JSON from client:" << err.errorString();
+        return;
+    }
+
+    QJsonObject obj  = doc.object();
+    QString type     = obj["type"].toString();
+    QString username = obj["username"].toString().trimmed();
+
+    // ── JOIN ──────────────────────────────────────────────────────────────────
+    if (type == "join") {
+        if (username.isEmpty()) {
+            sendSystemMessage(socket, "Username cannot be empty.");
+            return;
+        }
+
+        // check for duplicate username (case insensitive)
+        for (auto it = m_clients.begin(); it != m_clients.end(); ++it) {
+            if (it.value().toLower() == username.toLower() && it.key() != socket) {
+                sendSystemMessage(socket, "Username '" + username + "' is already taken.");
+                return;
+            }
+        }
+
+        m_clients[socket] = username;
+        qInfo() << "[Server]" << username << "joined.";
+
+        sendSystemMessage(socket, "Welcome, " + username + "!");
+        broadcast(socket, buildFrame("system", "Server",
+                  username + " joined the chat.", ""));
+        broadcastUserList();
+        return;
+    }
+
+    // ── GUARD: must have a username before doing anything else ────────────────
+    if (m_clients.value(socket).isEmpty()) {
+        sendSystemMessage(socket, "Please join with a username first.");
+        return;
+    }
+
+    // ── MESSAGE ───────────────────────────────────────────────────────────────
+    if (type == "message") {
+        QString text = obj["text"].toString().trimmed();
+        if (text.isEmpty()) return;
+
+        QString ts = QDateTime::currentDateTime().toString("hh:mm");
+        qInfo() << "[Server]" << username << ":" << text;
+        broadcast(nullptr, buildFrame("message", username, text, ts));
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Package data into a JSON frame ready to send
+// ─────────────────────────────────────────────────────────────────────────────
+QByteArray ChatServer::buildFrame(const QString &type, const QString &from,
+                                   const QString &text, const QString &ts)
+{
+    QJsonObject obj;
+    obj["type"] = type;
+    obj["from"] = from;
+    obj["text"] = text;
+    obj["ts"]   = ts;
+    // Compact = no spaces, everything on one line. \n is the message delimiter.
+    return QJsonDocument(obj).toJson(QJsonDocument::Compact) + "\n";
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Send a frame to all clients, optionally excluding one
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::broadcast(QTcpSocket *exclude, const QByteArray &frame)
+{
+    for (auto *s : m_clients.keys()) {
+        if (s != exclude && s->state() == QAbstractSocket::ConnectedState)
+            s->write(frame);
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Send a system notification to one specific client
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::sendSystemMessage(QTcpSocket *socket, const QString &text)
+{
+    socket->write(buildFrame("system", "Server", text, ""));
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+//  Send the current online user list to all clients
+// ─────────────────────────────────────────────────────────────────────────────
+void ChatServer::broadcastUserList()
+{
+    // QJsonArray is explicitly included — don't rely on indirect includes
+    QJsonArray names;
+    for (const QString &name : m_clients.values())
+        if (!name.isEmpty()) names.append(name);
+
+    QJsonObject obj;
+    obj["type"]  = "userlist";
+    obj["users"] = names;
+
+    QByteArray frame = QJsonDocument(obj).toJson(QJsonDocument::Compact) + "\n";
+    for (auto *s : m_clients.keys())
+        s->write(frame);
+}

ChatServer.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <QTcpServer>
+#include <QTcpSocket>
+#include <QMap>
+#include <QByteArray>
+#include <QString>
+
+class ChatServer : public QTcpServer
+{
+    Q_OBJECT
+
+public:
+    explicit ChatServer(QObject *parent = nullptr);
+
+protected:
+    void incomingConnection(qintptr socketDescriptor) override;
+
+private slots:
+    void onReadyRead();
+    void onDisconnected();
+
+private:
+    void processFrame(QTcpSocket *socket, const QByteArray &data);
+    void broadcast(QTcpSocket *exclude, const QByteArray &frame);
+    void sendSystemMessage(QTcpSocket *socket, const QString &text);
+    void broadcastUserList();
+    QByteArray buildFrame(const QString &type, const QString &from,
+                          const QString &text, const QString &ts);
+
+    // max bytes we buffer per client before dropping them (prevents memory abuse)
+    static constexpr int MAX_BUFFER_SIZE = 1024 * 64;  // 64 KB
+
+    QMap<QTcpSocket *, QString>    m_clients;  // socket → username
+    QMap<QTcpSocket *, QByteArray> m_buffers;  // socket → incoming data buffer
+};

main.cpp

@@ -0,0 +1,213 @@
+#include <QCoreApplication>
+#include <QDebug>
+#include "ChatServer.h"
+
+class ChatServer : public QTcpServer
+{
+    Q_OBJECT
+
+public:
+    explicit ChatServer(QObject *parent = nullptr) : QTcpServer(parent) {}
+
+protected:
+    // Qt calls this automatically when a new client connects
+    void incomingConnection(qintptr socketDescriptor) override
+    {
+        auto *socket = new QTcpSocket(this);
+
+        // Qt docs recommend checking this actually succeeded
+        if (!socket->setSocketDescriptor(socketDescriptor)) {
+            qWarning() << "[Server] Failed to set descriptor:" << socket->errorString();
+            delete socket;
+            return;
+        }
+
+        // wire up this socket's signals to our slots
+        connect(socket, &QTcpSocket::readyRead,    this, &ChatServer::onReadyRead);
+        connect(socket, &QTcpSocket::disconnected, this, &ChatServer::onDisconnected);
+
+        m_clients[socket] = "";   // socket known, username not yet
+        qInfo() << "[Server] New connection from" << socket->peerAddress().toString();
+
+        sendSystemMessage(socket, QString("Connected to server. %1 user(s) online.")
+                          .arg(m_clients.size()));
+    }
+
+private slots:
+    // fires when data arrives from any client
+    void onReadyRead()
+    {
+        auto *socket = qobject_cast<QTcpSocket *>(sender());
+        if (!socket) return;
+
+        // append whatever arrived to this client's buffer
+        m_buffers[socket] += socket->readAll();
+
+        // extract complete messages (delimited by \n)
+        while (m_buffers[socket].contains('\n')) {
+            int idx        = m_buffers[socket].indexOf('\n');
+            QByteArray line = m_buffers[socket].left(idx).trimmed();
+            m_buffers[socket] = m_buffers[socket].mid(idx + 1);
+
+            if (line.isEmpty()) continue;
+            processFrame(socket, line);
+        }
+    }
+
+    // fires when a client disconnects
+    void onDisconnected()
+    {
+        auto *socket = qobject_cast<QTcpSocket *>(sender());
+        if (!socket) return;
+
+        // look up their username before removing them
+        QString username = m_clients.value(socket, "Unknown");
+        m_clients.remove(socket);
+        m_buffers.remove(socket);
+        socket->deleteLater();
+
+        qInfo() << "[Server]" << username << "disconnected.";
+        broadcast(nullptr, buildFrame("system", "Server",
+                  username + " has left the chat.", ""));
+    }
+
+private:
+    // handle one complete JSON message from a client
+    void processFrame(QTcpSocket *socket, const QByteArray &data)
+    {
+        // parse the raw bytes into a JSON document
+        QJsonParseError err;
+        QJsonDocument doc = QJsonDocument::fromJson(data, &err);
+        if (err.error != QJsonParseError::NoError || !doc.isObject()) {
+            qWarning() << "[Server] Bad JSON from client:" << err.errorString();
+            return;
+        }
+
+        QJsonObject obj  = doc.object();
+        QString type     = obj["type"].toString();
+        QString username = obj["username"].toString().trimmed();
+
+        // ── JOIN ──────────────────────────────────────────────────────────
+        if (type == "join") {
+            if (username.isEmpty()) {
+                sendSystemMessage(socket, "Username cannot be empty.");
+                return;
+            }
+
+            // check for duplicate username (case insensitive)
+            for (auto it = m_clients.begin(); it != m_clients.end(); ++it) {
+                if (it.value().toLower() == username.toLower() && it.key() != socket) {
+                    sendSystemMessage(socket, "Username '" + username + "' is already taken.");
+                    return;
+                }
+            }
+
+            // store the username against this socket
+            m_clients[socket] = username;
+            qInfo() << "[Server]" << username << "joined.";
+
+            // welcome the new user
+            sendSystemMessage(socket, "Welcome, " + username + "!");
+
+            // tell everyone else they joined
+            broadcast(socket, buildFrame("system", "Server",
+                      username + " joined the chat.", ""));
+
+            // send updated user list to all clients
+            broadcastUserList();
+            return;
+        }
+
+        // ── GUARD: must have joined before doing anything else ────────────
+        if (m_clients.value(socket).isEmpty()) {
+            sendSystemMessage(socket, "Please join with a username first.");
+            return;
+        }
+
+        // ── MESSAGE ───────────────────────────────────────────────────────
+        if (type == "message") {
+            QString text = obj["text"].toString().trimmed();
+            if (text.isEmpty()) return;
+
+            QString ts = QDateTime::currentDateTime().toString("hh:mm");
+            qInfo() << "[Server]" << username << ":" << text;
+
+            // broadcast to everyone including the sender
+            broadcast(nullptr, buildFrame("message", username, text, ts));
+        }
+    }
+
+    // package data into a JSON frame ready to send
+    QByteArray buildFrame(const QString &type, const QString &from,
+                           const QString &text, const QString &ts)
+    {
+        QJsonObject obj;
+        obj["type"] = type;
+        obj["from"] = from;
+        obj["text"] = text;
+        obj["ts"]   = ts;
+        // Compact = no spaces, everything on one line, \n is the message delimiter
+        return QJsonDocument(obj).toJson(QJsonDocument::Compact) + "\n";
+    }
+
+    // send a frame to all clients, optionally excluding one
+    void broadcast(QTcpSocket *exclude, const QByteArray &frame)
+    {
+        for (auto *s : m_clients.keys()) {
+            if (s != exclude && s->state() == QAbstractSocket::ConnectedState)
+                s->write(frame);
+        }
+    }
+
+    // send a system notification to one specific client
+    void sendSystemMessage(QTcpSocket *socket, const QString &text)
+    {
+        socket->write(buildFrame("system", "Server", text, ""));
+    }
+
+    // send the current online user list to all clients
+    void broadcastUserList()
+    {
+        QJsonArray names;
+        for (const QString &name : m_clients.values())
+            if (!name.isEmpty()) names.append(name);
+
+        QJsonObject obj;
+        obj["type"]  = "userlist";
+        obj["users"] = names;
+
+        QByteArray frame = QJsonDocument(obj).toJson(QJsonDocument::Compact) + "\n";
+        for (auto *s : m_clients.keys())
+            s->write(frame);
+    }
+
+    // socket → username  ("" if not yet joined)
+    QMap<QTcpSocket *, QString>    m_clients;
+
+    // socket → incoming data buffer
+    QMap<QTcpSocket *, QByteArray> m_buffers;
+};
+
+// ─────────────────────────────────────────────────────────────────────────────
+int main(int argc, char *argv[])
+{
+    QCoreApplication app(argc, argv);
+
+    quint16 port = 54321;
+    ChatServer server;
+
+    if (!server.listen(QHostAddress::Any, port)) {
+        qCritical() << "[Server] Failed to listen on port" << port
+                    << ":" << server.errorString();
+        return 1;
+    }
+
+    qInfo() << "╔══════════════════════════════════╗";
+    qInfo() << "║   Qt Chat Server  — port" << port << " ║";
+    qInfo() << "╚══════════════════════════════════╝";
+    qInfo() << "Waiting for connections...";
+
+    return app.exec();
+}
+
+#include "main.moc"