Fix ZeldaZach review for PR 7496: deck path in ctor, server-time baseline, single-query sync check

cockatrice/CMakeLists.txt

@@ -29,6 +29,7 @@ set(cockatrice_SOURCES
     src/interface/deck_loader/loaded_deck.cpp
     src/interface/deck_sync/deck_sync_profile.cpp
     src/interface/deck_sync/deck_sync_sidecar.cpp
+    src/interface/deck_sync/deck_sync_manager.cpp
     src/interface/widgets/dialogs/dlg_connect.cpp
     src/interface/widgets/dialogs/dlg_convert_deck_to_cod_format.cpp
     src/interface/widgets/dialogs/dlg_create_game.cpp
@@ -57,6 +58,8 @@ set(cockatrice_SOURCES
     src/interface/widgets/dialogs/dlg_shared_decks_preview.cpp
     src/interface/widgets/dialogs/dlg_settings.cpp
     src/interface/widgets/dialogs/dlg_startup_card_check.cpp
+    src/interface/widgets/dialogs/dlg_sync_conflict.cpp
+    src/interface/widgets/dialogs/dlg_sync_onboarding.cpp
     src/interface/widgets/dialogs/dlg_tip_of_the_day.cpp
     src/interface/widgets/dialogs/dlg_update.cpp
     src/interface/widgets/dialogs/dlg_view_log.cpp

cockatrice/src/client/settings/cache_settings.cpp

@@ -25,6 +25,7 @@
 #include <libcockatrice/settings/commander_bracket_settings.h>
 #include <libcockatrice/settings/debug_settings.h>
 #include <libcockatrice/settings/deck_editor_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/game_filters_settings.h>
 #include <libcockatrice/settings/game_settings.h>
@@ -155,6 +156,7 @@ SettingsCache::SettingsCache()
     cardsDisplaySettings = new CardsDisplaySettings(settingsPath, this);
     interfaceSettings = new InterfaceSettings(settingsPath, this);
     deckEditorSettings = new DeckEditorSettings(settingsPath, this);
+    deckSyncSettings = new DeckSyncSettings(settingsPath, this);
     pathsSettings = new PathsSettings(settingsPath, this);
     visualDeckStorageSettings = new VisualDeckStorageSettings(settingsPath, this);
     appearanceSettings = new AppearanceSettings(settingsPath, this);
@@ -456,6 +458,11 @@ DeckEditorSettings &SettingsCache::deckEditor() const
     return *deckEditorSettings;
 }
 
+DeckSyncSettings &SettingsCache::deckSync() const
+{
+    return *deckSyncSettings;
+}
+
 VisualDeckStorageSettings &SettingsCache::visualDeckStorage() const
 {
     return *visualDeckStorageSettings;

cockatrice/src/client/settings/cache_settings.h

@@ -33,6 +33,7 @@ class ChatSettings;
 class CommanderBracketSettings;
 class DebugSettings;
 class DeckEditorSettings;
+class DeckSyncSettings;
 class DownloadSettings;
 class GameFiltersSettings;
 class GameSettings;
@@ -82,6 +83,7 @@ private:
     CardsDisplaySettings *cardsDisplaySettings = nullptr;
     InterfaceSettings *interfaceSettings = nullptr;
     DeckEditorSettings *deckEditorSettings = nullptr;
+    DeckSyncSettings *deckSyncSettings = nullptr;
     PathsSettings *pathsSettings = nullptr;
     VisualDeckStorageSettings *visualDeckStorageSettings = nullptr;
     AppearanceSettings *appearanceSettings = nullptr;
@@ -150,6 +152,7 @@ public:
     [[nodiscard]] CardsDisplaySettings &cardsDisplay() const;
     [[nodiscard]] InterfaceSettings &userInterface() const;
     [[nodiscard]] DeckEditorSettings &deckEditor() const;
+    [[nodiscard]] DeckSyncSettings &deckSync() const;
     [[nodiscard]] PathsSettings &paths() const;
     [[nodiscard]] VisualDeckStorageSettings &visualDeckStorage() const;
     [[nodiscard]] AppearanceSettings &appearance() const;

cockatrice/src/interface/deck_sync/deck_sync_conflict.h

@@ -0,0 +1,50 @@
+/**
+ * @file deck_sync_conflict.h
+ * @brief Conflict payload between the sync engine and the conflict dialog.
+ */
+
+#ifndef DECK_SYNC_CONFLICT_H
+#define DECK_SYNC_CONFLICT_H
+
+#include <QString>
+
+/** @brief How a both-sides-changed deck is resolved. */
+enum DeckSyncResolutionAction
+{
+    DeckSyncKeepMine,     ///< Upload the local deck, discarding the server version.
+    DeckSyncKeepServer,   ///< Download the server deck, discarding local changes.
+    DeckSyncKeepBoth,     ///< Keep the local deck and store the server version aside.
+    DeckSyncUseMerge,     ///< Upload the auto-merged deck (only when mergeable).
+    DeckSyncSkipConflict, ///< Leave both sides untouched for this run.
+};
+
+/** @brief Sync direction for one run. */
+enum class DeckSyncMode
+{
+    TwoWay,       ///< Mirror both sides (default).
+    UploadOnly,   ///< Push local content; never download or propagate deletes.
+    DownloadOnly, ///< Take server content; never upload or push deletes.
+    Separate,     ///< First-sync verdict only: keep this profile out of sync.
+};
+
+/** @brief One deck that changed both locally and on the server. */
+struct DeckSyncConflict
+{
+    QString relativePath;      ///< Local deck path relative to the deck folder.
+    QString localAbsolutePath; ///< Absolute local file path.
+    QString serverDeckString;  ///< Server deck content in native format.
+    QString baseDeckString;    ///< Last-synced content, empty when no snapshot exists.
+    bool hasBase = false;      ///< False when the base snapshot is missing.
+    qint64 localSecs = 0;      ///< Local modification time, seconds since epoch.
+    qint64 serverSecs = 0;     ///< Server modification time, seconds since epoch.
+};
+
+/** @brief The dialog verdict for one DeckSyncConflict. */
+struct DeckSyncResolution
+{
+    DeckSyncResolutionAction action = DeckSyncKeepMine;
+    bool rememberSession = false; ///< Reuse this verdict for later conflicts this run.
+    bool rememberAlways = false;  ///< Store this verdict as the conflict default setting.
+};
+
+#endif // DECK_SYNC_CONFLICT_H

cockatrice/src/interface/deck_sync/deck_sync_manager.h

@@ -0,0 +1,230 @@
+/**
+ * @file deck_sync_manager.h
+ * @brief One-shot two-way deck collection sync between local files and the server.
+ */
+
+#ifndef DECK_SYNC_MANAGER_H
+#define DECK_SYNC_MANAGER_H
+
+#include "deck_sync_conflict.h"
+#include "deck_sync_profile.h"
+
+#include <QDateTime>
+#include <QList>
+#include <QMap>
+#include <QObject>
+#include <QSet>
+#include <QString>
+#include <QVariant>
+#include <functional>
+#include <libcockatrice/settings/deck_sync_settings.h>
+
+class AbstractClient;
+class CommandContainer;
+class DeckList;
+class QFileSystemWatcher;
+class QTimer;
+class Response;
+class ServerInfo_DeckStorage_Folder;
+
+/**
+ * @class DeckSyncManager
+ * @brief Runs a manual whole-collection sync: local *.cod files vs server deck storage.
+ *
+ * Each syncNow() run loads the profile sidecar, scans the deck folder, fetches
+ * the server tree and mirrors both sides: unknown local decks upload, unknown
+ * server decks download, and decks changed on both sides resolve newer-wins
+ * (ties go to the server). Matching is path-based; a rename looks like a
+ * delete plus an add. Before a download overwrites a locally modified file, the
+ * local content is copied to a .sync-backup file next to it.
+ *
+ * Ownership: fire-and-forget per run, driven by PendingCommand round trips.
+ * Both-sides-changed decks pause the queue for a verdict via conflictRequest /
+ * resolveConflict (or the conflict default setting); the first sync of an
+ * unknown profile pauses for an onboarding verdict via firstSyncDetected /
+ * continueFirstSync. No watcher and no auto triggers — those arrive later.
+ *
+ * Deletes are tombstones, never silent: a locally deleted deck is pushed with
+ * Command_DeckDel, a server-side delete is applied locally only after copying
+ * the file to a .sync-backup, and resurrection reuses the stored deck id.
+ */
+class DeckSyncManager : public QObject
+{
+    Q_OBJECT
+public:
+    /** @brief Computes the color identity string uploaded alongside a deck. May be empty. */
+    using ColorIdentityProvider = std::function<QString(const DeckList &)>;
+
+    explicit DeckSyncManager(QObject *parent = nullptr);
+
+    void setClient(AbstractClient *_client);
+    void setServerIdentity(const QString &host, int port, const QString &username);
+    void setDeckPath(const QString &path);
+    void setColorIdentityProvider(ColorIdentityProvider provider);
+    void setConflictDefault(int conflictDefault);
+    void setAutoSyncEnabled(bool enabled);
+    void setSyncIntervalMinutes(int minutes);
+    [[nodiscard]] bool isSyncing() const;
+
+public slots:
+    void syncNow();
+    /** @brief Continues a paused first sync after the onboarding verdict. */
+    void continueFirstSync(DeckSyncMode mode);
+    /** @brief Aborts a paused first sync without writing any verdict. */
+    void cancelFirstSync();
+    /** @brief Applies the dialog verdict for the active conflict and resumes. */
+    void resolveConflict(const DeckSyncResolution &resolution);
+
+signals:
+    void syncStarted();
+    void statusMessage(const QString &message);
+    void syncFinished(int uploaded, int downloaded, int unchanged, int skipped, const QString &error);
+    /** @brief First sync with content on both sides: the tab must ask how to proceed. */
+    void firstSyncDetected(int localCount, int serverCount);
+    /** @brief A both-sides-changed deck needs a verdict; the queue pauses until resolveConflict. */
+    void conflictRequest(const DeckSyncConflict &conflict);
+
+private slots:
+    void onListFinished(const Response &response, const CommandContainer &commandContainer);
+    void onCheckFinished(const Response &response, const CommandContainer &commandContainer);
+    void
+    onEnsureDirFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onUploadFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onDownloadFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+    void
+    onDeleteFinished(const Response &response, const CommandContainer &commandContainer, const QVariant &extraData);
+
+private:
+    struct ServerFile
+    {
+        int id = -1;
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+    struct LocalFile
+    {
+        QString absolutePath;
+        QString hash;
+        QDateTime modified;
+    };
+    struct UploadOp
+    {
+        QString relativePath;
+        QString serverPath;
+        int deckId = -1;
+        QString guid;
+    };
+    struct DownloadOp
+    {
+        QString relativePath;
+        int deckId = -1;
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+    struct ActiveConflict
+    {
+        bool active = false;
+        QString relativePath;
+        QString serverPath;
+        int deckId = -1;
+        QString serverGuid;
+        QString serverHash;
+        qint64 serverLastModifiedSecs = 0;
+        QString serverDeckString;
+    };
+    struct ConflictDownload
+    {
+        QString serverPath;
+        int deckId = -1;
+        QString guid;
+        QString hash;
+        qint64 lastModifiedSecs = 0;
+    };
+
+    void failSync(const QString &error);
+    void finishSync();
+    void fetchFullList();
+    void sendSyncCheck(qint64 lastSyncSecs);
+    void autoSyncSoon();
+    [[nodiscard]] bool canAutoRun() const;
+    void onDebounceTimeout();
+    void onWatchedPathChanged(const QString &path);
+    void refreshWatcher();
+    void refreshInterval();
+    [[nodiscard]] DeckSyncProfile buildProfile() const;
+    void collectServerFiles(const ServerInfo_DeckStorage_Folder &folder,
+                            const QString &prefix,
+                            QMap<QString, ServerFile> &files,
+                            QSet<QString> &dirs);
+    void processNext();
+    void processNextUpload();
+    void sendMkDirFor(const QString &dir);
+    void sendUpload(const UploadOp &op);
+    void sendDownload(const DownloadOp &op);
+    void sendDelete(const QString &relativePath, int deckId);
+    void snapshotLocalFile(const QString &relativePath, const QString &guid);
+    QString readBaseSnapshot(const QString &guid) const;
+    void classify(const QMap<QString, ServerFile> &serverFiles);
+    void queueConflict(const QString &relativePath,
+                       const QString &serverPath,
+                       int deckId,
+                       const QString &serverGuid,
+                       const QString &serverHash,
+                       qint64 serverLastModifiedSecs);
+    void handleConflictDownload(const QString &relativePath, const QString &serverDeckString);
+    void applyDownloadedDeck(const QString &relativePath,
+                             int deckId,
+                             const QString &deckString,
+                             const QString &guid,
+                             const QString &hash,
+                             qint64 lastModifiedSecs,
+                             bool hasLastModified);
+    void applyUpload(const QString &relativePath, const QString &serverPath, int deckId, const QString &guid);
+    void applyDownload(const QString &relativePath,
+                       int deckId,
+                       const QString &guid,
+                       const QString &hash,
+                       qint64 lastModifiedSecs);
+    static QString parentDir(const QString &path);
+    QString uniqueLocalPath(const QString &relativePath) const;
+
+    AbstractClient *client = nullptr;
+    QString serverHost;
+    int serverPort = 0;
+    QString serverUsername;
+    QString deckPath;
+    ColorIdentityProvider colorIdentityProvider;
+    bool syncing = false;
+
+    QString sidecarPath;
+    QMap<QString, LocalFile> localFiles;
+    QSet<QString> ensuredDirs;
+    QList<UploadOp> pendingUploads;
+    QList<DownloadOp> pendingDownloads;
+    QList<QString> pendingDeletes;
+    QSet<QString> conflictPaths;
+    QMap<QString, ConflictDownload> pendingConflicts;
+    QMap<QString, ServerFile> stashedServerFiles;
+    ActiveConflict activeConflict;
+    DeckSyncMode syncMode = DeckSyncMode::TwoWay;
+    QFileSystemWatcher *watcher = nullptr;
+    QTimer *debounceTimer = nullptr;
+    QTimer *intervalTimer = nullptr;
+    bool autoSyncEnabled = true;
+    int syncIntervalMinutes = 5;
+    bool offline = false;
+    qint64 pendingServerTime = -1;
+    int conflictDefault = DeckSyncConflictAsk;
+    int sessionDefault = -1;
+    bool firstSyncRun = false;
+    int uploadedCount = 0;
+    int downloadedCount = 0;
+    int unchangedCount = 0;
+    int skippedCount = 0;
+};
+
+#endif // DECK_SYNC_MANAGER_H

cockatrice/src/interface/deck_sync/deck_sync_sidecar.cpp

@@ -5,6 +5,7 @@
 #include <QFileInfo>
 #include <QJsonDocument>
 #include <QJsonObject>
+#include <QSaveFile>
 
 namespace
 {
@@ -15,9 +16,16 @@ DeckSyncSidecar::DeckSyncSidecar(const QString &_filePath) : filePath(_filePath)
 {
 }
 
+QString DeckSyncSidecar::pathForDeckPath(const QString &deckPath, const QString &profileKey)
+{
+    return QDir(deckPath).filePath(".sync/" + profileKey + ".json");
+}
+
 bool DeckSyncSidecar::load()
 {
     recordMap.clear();
+    mode = MODE_LINKED;
+    lastSyncSecs = 0;
 
     QFile file(filePath);
     if (!file.exists()) {
@@ -30,6 +38,9 @@ bool DeckSyncSidecar::load()
     if (!document.isObject() || !document.object().value("records").isObject()) {
         return false;
     }
+    const QJsonValue modeValue = document.object().value("mode");
+    mode = modeValue.isString() ? modeValue.toString() : MODE_LINKED;
+    lastSyncSecs = static_cast<qint64>(document.object().value("lastSync").toDouble());
     const QJsonObject records = document.object().value("records").toObject();
     for (auto it = records.begin(); it != records.end(); ++it) {
         if (!it.value().isObject()) {
@@ -61,15 +72,17 @@ bool DeckSyncSidecar::save() const
     }
     QJsonObject root;
     root.insert("version", SIDECAR_FORMAT_VERSION);
+    root.insert("mode", mode);
+    root.insert("lastSync", static_cast<double>(lastSyncSecs));
     root.insert("records", records);
 
     QDir().mkpath(QFileInfo(filePath).absolutePath());
-    QFile file(filePath);
-    if (!file.open(QIODevice::WriteOnly | QIODevice::Truncate)) {
+    QSaveFile file(filePath);
+    if (!file.open(QIODevice::WriteOnly)) {
         return false;
     }
     file.write(QJsonDocument(root).toJson(QJsonDocument::Compact));
-    return true;
+    return file.commit();
 }
 
 bool DeckSyncSidecar::contains(const QString &relativePath) const

cockatrice/src/interface/deck_sync/deck_sync_sidecar.h

@@ -30,10 +30,18 @@ struct DeckSyncRecord
 class DeckSyncSidecar
 {
 public:
-    QString filePath; ///< Full path of the sidecar JSON file.
+    static constexpr const char *MODE_LINKED = "linked";
+    static constexpr const char *MODE_SEPARATE = "separate";
+
+    QString filePath;           ///< Full path of the sidecar JSON file.
+    QString mode = MODE_LINKED; ///< Linked normally; separate keeps this profile out of sync.
+    qint64 lastSyncSecs = 0;    ///< Last completed sync, seconds since epoch; 0 means never.
 
     explicit DeckSyncSidecar(const QString &_filePath);
 
+    /** @brief Sidecar file path for a deck folder and profile key. */
+    static QString pathForDeckPath(const QString &deckPath, const QString &profileKey);
+
     /** @brief Loads records; a missing file loads as empty and returns true. */
     bool load();
     /** @brief Stores records, creating parent folders as needed. */

cockatrice/src/interface/widgets/dialogs/dlg_sync_conflict.cpp

@@ -0,0 +1,178 @@
+#include "dlg_sync_conflict.h"
+
+#include <QCheckBox>
+#include <QDateTime>
+#include <QDialogButtonBox>
+#include <QFile>
+#include <QLabel>
+#include <QListWidget>
+#include <QLocale>
+#include <QRadioButton>
+#include <QVBoxLayout>
+#include <libcockatrice/deck_list/deck_diff.h>
+#include <libcockatrice/deck_list/deck_list.h>
+#include <libcockatrice/deck_list/tree/deck_list_card_node.h>
+#include <libcockatrice/deck_list/tree/inner_deck_list_node.h>
+
+namespace
+{
+QString formatTime(qint64 secsSinceEpoch)
+{
+    return QLocale().toString(QDateTime::fromSecsSinceEpoch(secsSinceEpoch), QLocale::ShortFormat);
+}
+
+int zoneCardCount(const DeckList &deck, const QString &zone)
+{
+    int count = 0;
+    deck.forEachCard([&count, &zone](InnerDecklistNode *zoneNode, DecklistCardNode *card) {
+        if (zoneNode->getName() == zone) {
+            count += card->getNumber();
+        }
+    });
+    return count;
+}
+} // namespace
+
+DlgSyncConflict::DlgSyncConflict(const DeckSyncConflict &_conflict, QWidget *parent)
+    : QDialog(parent), conflict(_conflict)
+{
+    auto *layout = new QVBoxLayout(this);
+
+    titleLabel = new QLabel(this);
+    detailLabel = new QLabel(this);
+    detailLabel->setWordWrap(true);
+    keepMineRadio = new QRadioButton(this);
+    keepServerRadio = new QRadioButton(this);
+    keepBothRadio = new QRadioButton(this);
+    mergeRadio = new QRadioButton(this);
+    changesList = new QListWidget(this);
+    mergeSummaryLabel = new QLabel(this);
+    mergeSummaryLabel->setWordWrap(true);
+    rememberSessionBox = new QCheckBox(this);
+    rememberAlwaysBox = new QCheckBox(this);
+
+    layout->addWidget(titleLabel);
+    layout->addWidget(detailLabel);
+    layout->addWidget(keepMineRadio);
+    layout->addWidget(keepServerRadio);
+    layout->addWidget(keepBothRadio);
+    layout->addWidget(mergeRadio);
+    layout->addWidget(changesList);
+    layout->addWidget(mergeSummaryLabel);
+    layout->addWidget(rememberSessionBox);
+    layout->addWidget(rememberAlwaysBox);
+
+    auto *buttons = new QDialogButtonBox(QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+    connect(buttons, &QDialogButtonBox::accepted, this, &QDialog::accept);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    layout->addWidget(buttons);
+
+    DeckList localDeck;
+    DeckList serverDeck;
+    QFile localFile(conflict.localAbsolutePath);
+    decksParsed = localFile.open(QIODevice::ReadOnly) && localDeck.loadFromFile_Native(&localFile) &&
+                  serverDeck.loadFromString_Native(conflict.serverDeckString);
+    localIsNewer = conflict.localSecs >= conflict.serverSecs;
+
+    if (decksParsed) {
+        shownChanges = DeckDiff::diff(localDeck, serverDeck);
+        if (conflict.hasBase) {
+            DeckList baseDeck;
+            if (baseDeck.loadFromString_Native(conflict.baseDeckString)) {
+                const DeckDiffResult localChanges = DeckDiff::diff(baseDeck, localDeck);
+                const DeckDiffResult remoteChanges = DeckDiff::diff(baseDeck, serverDeck);
+                mergeable = DeckDiff::canAutoMerge(localChanges, remoteChanges);
+                if (mergeable) {
+                    const DeckList merged = DeckDiff::merge(baseDeck, localChanges, remoteChanges);
+                    mergedMainCount = zoneCardCount(merged, "main");
+                    mergedSideCount = zoneCardCount(merged, "side");
+                }
+            }
+        }
+    }
+
+    if (localIsNewer) {
+        keepMineRadio->setChecked(true);
+    } else {
+        keepServerRadio->setChecked(true);
+    }
+    mergeRadio->setEnabled(mergeable);
+
+    connect(keepMineRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(keepServerRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(keepBothRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    connect(mergeRadio, &QRadioButton::toggled, this, &DlgSyncConflict::updateRememberBoxes);
+    updateRememberBoxes();
+    retranslateUi();
+}
+
+void DlgSyncConflict::retranslateUi()
+{
+    setWindowTitle(tr("Sync conflict"));
+    titleLabel->setText(conflict.relativePath);
+    if (!decksParsed) {
+        detailLabel->setText(tr("This deck changed on both sides, but one version could not be read."));
+    } else {
+        detailLabel->setText(tr("This deck changed both on this device and on the server."));
+    }
+
+    keepMineRadio->setText(
+        tr("Keep my deck (%1%2)").arg(formatTime(conflict.localSecs), localIsNewer ? tr(", newest") : QString()));
+    keepServerRadio->setText(tr("Keep the server deck (%1%2)")
+                                 .arg(formatTime(conflict.serverSecs), localIsNewer ? QString() : tr(", newest")));
+    keepBothRadio->setText(tr("Keep both (store the server version aside)"));
+    mergeRadio->setText(mergeable ? tr("Use the auto-merge") : tr("Use the auto-merge (not available)"));
+
+    changesList->clear();
+    for (const DeckCardChange &change : shownChanges.changes) {
+        changesList->addItem(
+            tr("%1: %2 %3 → %4").arg(change.zone).arg(change.cardName).arg(change.oldCount).arg(change.newCount));
+    }
+
+    if (mergeable) {
+        mergeSummaryLabel->setText(tr("Auto-merge proposal: union of both sides (%1 main, %2 side).")
+                                       .arg(mergedMainCount)
+                                       .arg(mergedSideCount));
+    } else if (!conflict.hasBase) {
+        mergeSummaryLabel->setText(tr("No base snapshot exists, so the versions cannot be merged automatically."));
+    } else {
+        mergeSummaryLabel->setText(tr("The changes overlap, so they cannot be merged automatically."));
+    }
+
+    rememberSessionBox->setText(tr("Remember for the rest of this sync"));
+    rememberAlwaysBox->setText(tr("Always use this rule"));
+}
+
+void DlgSyncConflict::updateRememberBoxes()
+{
+    const bool single = keepMineRadio->isChecked() || keepServerRadio->isChecked();
+    rememberSessionBox->setEnabled(single);
+    rememberAlwaysBox->setEnabled(single);
+    if (!single) {
+        rememberSessionBox->setChecked(false);
+        rememberAlwaysBox->setChecked(false);
+    }
+}
+
+DeckSyncResolutionAction DlgSyncConflict::selectedAction() const
+{
+    if (keepServerRadio->isChecked()) {
+        return DeckSyncKeepServer;
+    }
+    if (keepBothRadio->isChecked()) {
+        return DeckSyncKeepBoth;
+    }
+    if (mergeRadio->isChecked()) {
+        return DeckSyncUseMerge;
+    }
+    return DeckSyncKeepMine;
+}
+
+DeckSyncResolution DlgSyncConflict::resolution() const
+{
+    DeckSyncResolution result;
+    result.action = selectedAction();
+    result.rememberSession = rememberSessionBox->isEnabled() && rememberSessionBox->isChecked();
+    result.rememberAlways = rememberAlwaysBox->isEnabled() && rememberAlwaysBox->isChecked();
+    return result;
+}

cockatrice/src/interface/widgets/dialogs/dlg_sync_conflict.h

@@ -0,0 +1,63 @@
+/**
+ * @file dlg_sync_conflict.h
+ * @brief Dialog for resolving a deck that changed both locally and on the server.
+ */
+
+#ifndef DLG_SYNC_CONFLICT_H
+#define DLG_SYNC_CONFLICT_H
+
+#include "../../deck_sync/deck_sync_conflict.h"
+
+#include <QDialog>
+#include <QString>
+#include <libcockatrice/deck_list/deck_diff.h>
+
+class QCheckBox;
+class QLabel;
+class QListWidget;
+class QRadioButton;
+
+/**
+ * @class DlgSyncConflict
+ * @brief Shows both versions of a conflicted deck and returns the verdict.
+ *
+ * Radio choice between Keep mine / Keep server (newest preselected), Keep both
+ * and Use merge (enabled only when the changes touch disjoint cards). The
+ * change list shows card-count deltas between the two versions; the merge
+ * summary describes the auto-merge proposal. Rejected dialogs resolve as Skip.
+ */
+class DlgSyncConflict : public QDialog
+{
+    Q_OBJECT
+public:
+    explicit DlgSyncConflict(const DeckSyncConflict &conflict, QWidget *parent = nullptr);
+    [[nodiscard]] DeckSyncResolution resolution() const;
+    void retranslateUi();
+
+private slots:
+    void updateRememberBoxes();
+
+private:
+    DeckSyncResolutionAction selectedAction() const;
+
+    DeckSyncConflict conflict;
+    bool decksParsed = false;
+    bool localIsNewer = true;
+    DeckDiffResult shownChanges;
+    bool mergeable = false;
+    int mergedMainCount = 0;
+    int mergedSideCount = 0;
+
+    QLabel *titleLabel;
+    QLabel *detailLabel;
+    QRadioButton *keepMineRadio;
+    QRadioButton *keepServerRadio;
+    QRadioButton *keepBothRadio;
+    QRadioButton *mergeRadio;
+    QListWidget *changesList;
+    QLabel *mergeSummaryLabel;
+    QCheckBox *rememberSessionBox;
+    QCheckBox *rememberAlwaysBox;
+};
+
+#endif // DLG_SYNC_CONFLICT_H

cockatrice/src/interface/widgets/dialogs/dlg_sync_onboarding.cpp

@@ -0,0 +1,59 @@
+#include "dlg_sync_onboarding.h"
+
+#include <QDialogButtonBox>
+#include <QLabel>
+#include <QRadioButton>
+#include <QVBoxLayout>
+
+DlgSyncOnboarding::DlgSyncOnboarding(int _localCount, int _serverCount, QWidget *parent)
+    : QDialog(parent), localCount(_localCount), serverCount(_serverCount)
+{
+    auto *layout = new QVBoxLayout(this);
+
+    detailLabel = new QLabel(this);
+    detailLabel->setWordWrap(true);
+    mergeRadio = new QRadioButton(this);
+    uploadRadio = new QRadioButton(this);
+    downloadRadio = new QRadioButton(this);
+    separateRadio = new QRadioButton(this);
+    mergeRadio->setChecked(true);
+
+    layout->addWidget(detailLabel);
+    layout->addWidget(mergeRadio);
+    layout->addWidget(uploadRadio);
+    layout->addWidget(downloadRadio);
+    layout->addWidget(separateRadio);
+
+    auto *buttons = new QDialogButtonBox(QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+    connect(buttons, &QDialogButtonBox::accepted, this, &QDialog::accept);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    layout->addWidget(buttons);
+
+    retranslateUi();
+}
+
+void DlgSyncOnboarding::retranslateUi()
+{
+    setWindowTitle(tr("Sync decks to this server"));
+    detailLabel->setText(tr("You have %1 local decks and %2 decks stored on this server. How should they be combined?")
+                             .arg(localCount)
+                             .arg(serverCount));
+    mergeRadio->setText(tr("Merge: keep everything, sync both ways (recommended)"));
+    uploadRadio->setText(tr("Upload mine: push local decks to the server"));
+    downloadRadio->setText(tr("Download server: take the server decks"));
+    separateRadio->setText(tr("Keep separate: do not sync with this server"));
+}
+
+DeckSyncMode DlgSyncOnboarding::selectedMode() const
+{
+    if (uploadRadio->isChecked()) {
+        return DeckSyncMode::UploadOnly;
+    }
+    if (downloadRadio->isChecked()) {
+        return DeckSyncMode::DownloadOnly;
+    }
+    if (separateRadio->isChecked()) {
+        return DeckSyncMode::Separate;
+    }
+    return DeckSyncMode::TwoWay;
+}

cockatrice/src/interface/widgets/dialogs/dlg_sync_onboarding.h

@@ -0,0 +1,43 @@
+/**
+ * @file dlg_sync_onboarding.h
+ * @brief First-sync choice when local decks and server decks meet.
+ */
+
+#ifndef DLG_SYNC_ONBOARDING_H
+#define DLG_SYNC_ONBOARDING_H
+
+#include "../../deck_sync/deck_sync_conflict.h"
+
+#include <QDialog>
+
+class QLabel;
+class QRadioButton;
+
+/**
+ * @class DlgSyncOnboarding
+ * @brief Asks how to combine a non-empty local collection with a non-empty server collection.
+ *
+ * Merge (two-way) is the default. Upload and download restrict the run to one
+ * direction; keeping separate records the profile as sync-disabled.
+ * Rejected dialogs cancel the sync without recording anything.
+ */
+class DlgSyncOnboarding : public QDialog
+{
+    Q_OBJECT
+public:
+    DlgSyncOnboarding(int _localCount, int _serverCount, QWidget *parent = nullptr);
+    [[nodiscard]] DeckSyncMode selectedMode() const;
+    void retranslateUi();
+
+private:
+    int localCount = 0;
+    int serverCount = 0;
+
+    QLabel *detailLabel;
+    QRadioButton *mergeRadio;
+    QRadioButton *uploadRadio;
+    QRadioButton *downloadRadio;
+    QRadioButton *separateRadio;
+};
+
+#endif // DLG_SYNC_ONBOARDING_H

cockatrice/src/interface/widgets/settings_page/general_settings_page.cpp

@@ -28,6 +28,7 @@
 #include <QtGlobal>
 #include <libcockatrice/card/card_localization.h>
 #include <libcockatrice/settings/cards_display_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/paths_settings.h>
 #include <libcockatrice/settings/personal_settings.h>
@@ -332,12 +333,45 @@ GeneralSettingsPage::GeneralSettingsPage()
     pathsGroupBox = new QGroupBox;
     pathsGroupBox->setLayout(pathsGrid);
 
+    // deck sync settings
+    conflictDefaultSelector.addItem("", DeckSyncConflictAsk);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepMine);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepServer);
+    conflictDefaultSelector.addItem("", DeckSyncConflictKeepNewer);
+    int conflictIndex = conflictDefaultSelector.findData(settings.deckSync().getConflictDefault());
+    if (conflictIndex >= 0) {
+        conflictDefaultSelector.setCurrentIndex(conflictIndex);
+    }
+    connect(&conflictDefaultSelector, qOverload<int>(&QComboBox::currentIndexChanged), this, [this](int index) {
+        SettingsCache::instance().deckSync().setConflictDefault(conflictDefaultSelector.itemData(index).toInt());
+    });
+
+    autoSyncCheckBox.setChecked(settings.deckSync().getAutoSync());
+    connect(&autoSyncCheckBox, &QCheckBox::toggled, &SettingsCache::instance().deckSync(),
+            &DeckSyncSettings::setAutoSync);
+
+    syncIntervalEdit.setMinimum(1);
+    syncIntervalEdit.setMaximum(60);
+    syncIntervalEdit.setValue(settings.deckSync().getSyncIntervalMinutes());
+    connect(&syncIntervalEdit, qOverload<int>(&QSpinBox::valueChanged), &SettingsCache::instance().deckSync(),
+            &DeckSyncSettings::setSyncIntervalMinutes);
+
+    auto *deckSyncGrid = new QGridLayout;
+    deckSyncGrid->addWidget(&conflictDefaultLabel, 0, 0);
+    deckSyncGrid->addWidget(&conflictDefaultSelector, 0, 1);
+    deckSyncGrid->addWidget(&autoSyncCheckBox, 1, 0, 1, 2);
+    deckSyncGrid->addWidget(&syncIntervalLabel, 2, 0);
+    deckSyncGrid->addWidget(&syncIntervalEdit, 2, 1);
+    deckSyncGroupBox = new QGroupBox;
+    deckSyncGroupBox->setLayout(deckSyncGrid);
+
     auto *mainLayout = new QVBoxLayout;
     mainLayout->addWidget(languageGroupBox);
     mainLayout->addWidget(versionGroupBox);
     mainLayout->addWidget(cardDatabaseGroupBox);
     mainLayout->addWidget(startupGroupBox);
     mainLayout->addWidget(pathsGroupBox);
+    mainLayout->addWidget(deckSyncGroupBox);
     mainLayout->addStretch();
 
     GeneralSettingsPage::retranslateUi();
@@ -607,5 +641,16 @@ void GeneralSettingsPage::retranslateUi()
     cardDatabasePathLabel.setText(tr("Card database:"));
     customCardDatabasePathLabel.setText(tr("Custom database directory:"));
     tokenDatabasePathLabel.setText(tr("Token database:"));
+    deckSyncGroupBox->setTitle(tr("Deck sync"));
+    conflictDefaultLabel.setText(tr("When a deck changed on both sides:"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictAsk), tr("Ask me every time"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepMine), tr("Keep my deck"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepServer),
+                                        tr("Keep the server deck"));
+    conflictDefaultSelector.setItemText(conflictDefaultSelector.findData(DeckSyncConflictKeepNewer),
+                                        tr("Keep the newer deck"));
+    autoSyncCheckBox.setText(tr("Sync decks automatically"));
+    syncIntervalLabel.setText(tr("Check every:"));
+    syncIntervalEdit.setSuffix(" " + tr("min"));
     resetAllPathsButton->setText(tr("Reset all paths"));
 }

cockatrice/src/interface/widgets/settings_page/general_settings_page.h

@@ -52,6 +52,7 @@ private:
     QGroupBox *cardDatabaseGroupBox;
     QGroupBox *startupGroupBox;
     QGroupBox *pathsGroupBox;
+    QGroupBox *deckSyncGroupBox;
 
     QLabel languageLabel;
     QComboBox languageBox;
@@ -101,6 +102,12 @@ private:
     QLineEdit *tokenDatabasePathEdit;
     QPushButton *resetAllPathsButton;
     QLabel *allPathsResetLabel;
+
+    QLabel conflictDefaultLabel;
+    QComboBox conflictDefaultSelector;
+    QCheckBox autoSyncCheckBox;
+    QLabel syncIntervalLabel;
+    QSpinBox syncIntervalEdit;
 };
 
 #endif // COCKATRICE_GENERAL_SETTINGS_PAGE_H

cockatrice/src/interface/widgets/tabs/tab_deck_storage.cpp

@@ -4,10 +4,14 @@
 #include "../../deck_loader/deck_file_format.h"
 #include "../../deck_loader/deck_loader.h"
 #include "../../deck_loader/loaded_deck.h"
+#include "../../deck_sync/deck_sync_conflict.h"
+#include "../../deck_sync/deck_sync_manager.h"
 #include "../../pixel_map_generator.h"
 #include "../cards/additional_info/deck_color_identity.h"
 #include "../deck_share/deck_share_utils.h"
 #include "../deck_share/share_bar_widget.h"
+#include "../dialogs/dlg_sync_conflict.h"
+#include "../dialogs/dlg_sync_onboarding.h"
 #include "../interface/widgets/server/remote/remote_decklist_tree_widget.h"
 #include "../interface/widgets/utility/get_text_with_max.h"
 #include "abstract_client.h"
@@ -32,6 +36,7 @@
 #include <QInputDialog>
 #include <QItemSelection>
 #include <QItemSelectionModel>
+#include <QLabel>
 #include <QLineEdit>
 #include <QMessageBox>
 #include <QModelIndex>
@@ -63,6 +68,7 @@
 #include <libcockatrice/protocol/pb/response_deck_download.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_upload.pb.h>
 #include <libcockatrice/protocol/pending_command.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/network_settings.h>
 #include <libcockatrice/settings/paths_settings.h>
 #include <libcockatrice/utility/string_limits.h>
@@ -178,6 +184,9 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
     rightVbox->addWidget(shareBar);
     rightVbox->addWidget(serverDirView);
     rightVbox->addLayout(rightToolBarLayout);
+    syncStatusLabel = new QLabel(this);
+    syncStatusLabel->setWordWrap(true);
+    rightVbox->addWidget(syncStatusLabel);
     rightGroupBox = new QGroupBox;
     rightGroupBox->setLayout(rightVbox);
 
@@ -229,6 +238,10 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
     aPublishDeck->setIcon(QPixmap("theme:icons/lock"));
     connect(aPublishDeck, &QAction::triggered, this, &TabDeckStorage::actPublishDeck);
 
+    aSyncNow = new QAction(this);
+    aSyncNow->setIcon(qApp->style()->standardIcon(QStyle::SP_BrowserReload));
+    connect(aSyncNow, &QAction::triggered, this, &TabDeckStorage::actSyncNow);
+
     // Add actions to toolbars
     leftToolBar->addAction(aOpenLocalDeck);
     leftToolBar->addAction(aRenameLocal);
@@ -240,11 +253,41 @@ TabDeckStorage::TabDeckStorage(TabSupervisor *_tabSupervisor,
 
     rightToolBar->addAction(aOpenRemoteDeck);
     rightToolBar->addAction(aDownload);
+    rightToolBar->addAction(aSyncNow);
     rightToolBar->addAction(aShareDecks);
     rightToolBar->addAction(aPublishDeck);
     rightToolBar->addAction(aNewFolder);
     rightToolBar->addAction(aDeleteRemoteDeck);
 
+    syncManager = new DeckSyncManager(this);
+    syncManager->setClient(client);
+    syncManager->setDeckPath(SettingsCache::instance().paths().getDeckPath());
+    connect(&SettingsCache::instance().paths(), &PathsSettings::deckPathChanged, this, [this] {
+        const QString deckPath = SettingsCache::instance().paths().getDeckPath();
+        localDirModel->setRootPath(deckPath);
+        localDirView->setRootIndex(localDirModel->index(localDirModel->rootPath(), 0));
+        syncManager->setDeckPath(deckPath);
+    });
+    syncManager->setColorIdentityProvider(
+        [](const DeckList &deck) { return getDeckColorIdentity(deck, CardDatabaseManager::query()); });
+    syncManager->setConflictDefault(SettingsCache::instance().deckSync().getConflictDefault());
+    syncManager->setAutoSyncEnabled(SettingsCache::instance().deckSync().getAutoSync());
+    syncManager->setSyncIntervalMinutes(SettingsCache::instance().deckSync().getSyncIntervalMinutes());
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::conflictDefaultChanged, syncManager,
+            &DeckSyncManager::setConflictDefault);
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::autoSyncChanged, syncManager,
+            &DeckSyncManager::setAutoSyncEnabled);
+    connect(&SettingsCache::instance().deckSync(), &DeckSyncSettings::syncIntervalMinutesChanged, syncManager,
+            &DeckSyncManager::setSyncIntervalMinutes);
+    connect(syncManager, &DeckSyncManager::statusMessage, this, &TabDeckStorage::onSyncStatusMessage);
+    connect(syncManager, &DeckSyncManager::syncFinished, this, &TabDeckStorage::onSyncFinished);
+    connect(syncManager, &DeckSyncManager::firstSyncDetected, this, &TabDeckStorage::onFirstSyncDetected);
+    connect(syncManager, &DeckSyncManager::conflictRequest, this, &TabDeckStorage::onConflictRequest);
+    if (currentUserInfo != nullptr) {
+        syncUsername = QString::fromStdString(currentUserInfo->name());
+    }
+    syncManager->setServerIdentity(client->serverName(), client->serverPort(), syncUsername);
+
     retranslateUi();
 
     QWidget *mainWidget = new QWidget(this);
@@ -273,6 +316,8 @@ void TabDeckStorage::retranslateUi()
     aShareDecks->setText(tr("Share decks"));
     aPublishDeck->setText(tr("Publish/unpublish deck"));
     aOpenDecksFolder->setText(tr("Open decks folder"));
+    aSyncNow->setText(tr("Sync now"));
+    renderSyncStatus();
     shareBar->retranslateUi();
     if (shareBar->isVisible()) {
         onServerSelectionChanged();
@@ -300,6 +345,8 @@ QString TabDeckStorage::getTargetPath() const
 
 void TabDeckStorage::handleConnected(const ServerInfo_User &userInfo)
 {
+    syncUsername = QString::fromStdString(userInfo.name());
+    syncManager->setServerIdentity(client->serverName(), client->serverPort(), syncUsername);
     setRemoteEnabled(userInfo.user_level() & ServerInfo_User::IsRegistered);
 }
 
@@ -317,6 +364,7 @@ void TabDeckStorage::handleConnectionChanged(ClientStatus status)
 
 void TabDeckStorage::setRemoteEnabled(bool enabled)
 {
+    remoteEnabled = enabled;
     aUpload->setEnabled(enabled);
     aOpenRemoteDeck->setEnabled(enabled);
     aDownload->setEnabled(enabled);
@@ -324,6 +372,7 @@ void TabDeckStorage::setRemoteEnabled(bool enabled)
     aPublishDeck->setEnabled(enabled);
     aNewFolder->setEnabled(enabled);
     aDeleteRemoteDeck->setEnabled(enabled);
+    aSyncNow->setEnabled(enabled && !syncManager->isSyncing());
 
     if (enabled) {
         serverDirView->refreshTree();
@@ -527,6 +576,87 @@ void TabDeckStorage::actOpenDecksFolder()
     QDesktopServices::openUrl(QUrl::fromLocalFile(dir));
 }
 
+void TabDeckStorage::actSyncNow()
+{
+    syncManager->setDeckPath(SettingsCache::instance().paths().getDeckPath());
+    aSyncNow->setEnabled(false);
+    syncManager->syncNow();
+}
+
+void TabDeckStorage::onFirstSyncDetected(int localCount, int serverCount)
+{
+    DlgSyncOnboarding dialog(localCount, serverCount, this);
+    if (dialog.exec() == QDialog::Accepted) {
+        syncManager->continueFirstSync(dialog.selectedMode());
+    } else {
+        syncManager->cancelFirstSync();
+    }
+}
+
+void TabDeckStorage::onConflictRequest(const DeckSyncConflict &conflict)
+{
+    DlgSyncConflict dialog(conflict, this);
+    if (dialog.exec() != QDialog::Accepted) {
+        DeckSyncResolution skip;
+        skip.action = DeckSyncSkipConflict;
+        syncManager->resolveConflict(skip);
+        return;
+    }
+    const DeckSyncResolution resolution = dialog.resolution();
+    if (resolution.rememberAlways &&
+        (resolution.action == DeckSyncKeepMine || resolution.action == DeckSyncKeepServer)) {
+        SettingsCache::instance().deckSync().setConflictDefault(
+            resolution.action == DeckSyncKeepMine ? DeckSyncConflictKeepMine : DeckSyncConflictKeepServer);
+    }
+    syncManager->resolveConflict(resolution);
+}
+
+void TabDeckStorage::onSyncStatusMessage(const QString &message)
+{
+    syncStatusType = SyncStatusType::Working;
+    syncStatusDetail = message;
+    renderSyncStatus();
+}
+
+void TabDeckStorage::onSyncFinished(int uploaded, int downloaded, int unchanged, int skipped, const QString &error)
+{
+    aSyncNow->setEnabled(remoteEnabled);
+    if (!error.isEmpty()) {
+        syncStatusType = SyncStatusType::Failed;
+        syncStatusDetail = error;
+    } else {
+        syncStatusType = SyncStatusType::Finished;
+        syncStatusDetail.clear();
+        lastSyncSummary = {uploaded, downloaded, unchanged, skipped};
+    }
+    renderSyncStatus();
+    if (error.isEmpty()) {
+        serverDirView->refreshTree();
+    }
+}
+
+void TabDeckStorage::renderSyncStatus()
+{
+    switch (syncStatusType) {
+        case SyncStatusType::Working:
+            syncStatusLabel->setText(syncStatusDetail);
+            break;
+        case SyncStatusType::Failed:
+            syncStatusLabel->setText(tr("Sync failed: %1").arg(syncStatusDetail));
+            break;
+        case SyncStatusType::Finished:
+            syncStatusLabel->setText(tr("Sync complete: %1 uploaded, %2 downloaded, %3 unchanged, %4 skipped.")
+                                         .arg(lastSyncSummary.uploaded)
+                                         .arg(lastSyncSummary.downloaded)
+                                         .arg(lastSyncSummary.unchanged)
+                                         .arg(lastSyncSummary.skipped));
+            break;
+        case SyncStatusType::Idle:
+            syncStatusLabel->setText(tr("Not synced yet"));
+            break;
+    }
+}
+
 void TabDeckStorage::actRemoteDoubleClick(const QModelIndex &curRight)
 {
     if (dynamic_cast<RemoteDeckList_TreeModel::FileNode *>(serverDirView->getNode(curRight))) {

cockatrice/src/interface/widgets/tabs/tab_deck_storage.h

@@ -19,15 +19,19 @@
 #include <qtmetamacros.h>
 
 struct LoadedDeck;
+struct DeckSyncConflict;
+struct DeckSyncResolution;
 class ServerInfo_User;
 class QTreeView;
 class QFileSystemModel;
 class QToolBar;
 class QGroupBox;
 class QTimer;
+class QLabel;
 class CommandContainer;
 class Response;
 class ShareBarWidget;
+class DeckSyncManager;
 class QAction;
 class QModelIndex;
 class TabSupervisor;
@@ -50,6 +54,29 @@ private:
     QAction *aOpenLocalDeck, *aRenameLocal, *aUpload, *aNewLocalFolder, *aDeleteLocalDeck;
     QAction *aOpenDecksFolder;
     QAction *aOpenRemoteDeck, *aDownload, *aShareDecks, *aPublishDeck, *aNewFolder, *aDeleteRemoteDeck;
+    QAction *aSyncNow;
+    QLabel *syncStatusLabel;
+    DeckSyncManager *syncManager;
+    QString syncUsername;
+    bool remoteEnabled = false;
+    enum class SyncStatusType
+    {
+        Idle,
+        Working,
+        Finished,
+        Failed
+    };
+    struct SyncSummary
+    {
+        int uploaded = 0;
+        int downloaded = 0;
+        int unchanged = 0;
+        int skipped = 0;
+    };
+    SyncStatusType syncStatusType = SyncStatusType::Idle;
+    QString syncStatusDetail;
+    SyncSummary lastSyncSummary;
+    void renderSyncStatus();
     bool visibilityRefreshStarted = false;
     QTimer *visibilityRefreshTimer;
     QStringList visibilityFailures;
@@ -108,6 +135,12 @@ private slots:
     void deleteFolderFinished(const Response &response, const CommandContainer &commandContainer);
     void deleteDeckFinished(const Response &response, const CommandContainer &commandContainer);
 
+    void actSyncNow();
+    void onFirstSyncDetected(int localCount, int serverCount);
+    void onConflictRequest(const DeckSyncConflict &conflict);
+    void onSyncStatusMessage(const QString &message);
+    void onSyncFinished(int uploaded, int downloaded, int unchanged, int skipped, const QString &error);
+
 public:
     TabDeckStorage(TabSupervisor *_tabSupervisor, AbstractClient *_client, const ServerInfo_User *currentUserInfo);
     void retranslateUi() override;

libcockatrice_deck_list/libcockatrice/deck_list/deck_diff.cpp

@@ -13,12 +13,36 @@ namespace
 {
 using ZoneCardKey = QPair<QString, QString>;
 
+struct ZoneCardInfo
+{
+    int count = 0;
+    QString cardSetShortName;
+    QString cardSetNumber;
+    QString cardProviderId;
+};
+
+QMap<ZoneCardKey, ZoneCardInfo> zoneCardInfos(const DeckList &deck)
+{
+    QMap<ZoneCardKey, ZoneCardInfo> infos;
+    deck.forEachCard([&infos](InnerDecklistNode *zone, DecklistCardNode *card) {
+        ZoneCardInfo &info = infos[{zone->getName(), card->getName()}];
+        if (info.count == 0) {
+            info.cardSetShortName = card->getCardSetShortName();
+            info.cardSetNumber = card->getCardCollectorNumber();
+            info.cardProviderId = card->getCardProviderId();
+        }
+        info.count += card->getNumber();
+    });
+    return infos;
+}
+
 QMap<ZoneCardKey, int> zoneCardCounts(const DeckList &deck)
 {
     QMap<ZoneCardKey, int> counts;
-    deck.forEachCard([&counts](InnerDecklistNode *zone, DecklistCardNode *card) {
-        counts[{zone->getName(), card->getName()}] += card->getNumber();
-    });
+    const QMap<ZoneCardKey, ZoneCardInfo> infos = zoneCardInfos(deck);
+    for (auto it = infos.begin(); it != infos.end(); ++it) {
+        counts.insert(it.key(), it.value().count);
+    }
     return counts;
 }
 
@@ -34,17 +58,25 @@ bool changeLessThan(const DeckCardChange &a, const DeckCardChange &b)
 DeckDiffResult DeckDiff::diff(const DeckList &oldDeck, const DeckList &newDeck)
 {
     const QMap<ZoneCardKey, int> oldCounts = zoneCardCounts(oldDeck);
-    const QMap<ZoneCardKey, int> newCounts = zoneCardCounts(newDeck);
+    const QMap<ZoneCardKey, ZoneCardInfo> newInfos = zoneCardInfos(newDeck);
 
     QSet<ZoneCardKey> keys(oldCounts.keyBegin(), oldCounts.keyEnd());
-    keys.unite(QSet<ZoneCardKey>(newCounts.keyBegin(), newCounts.keyEnd()));
+    keys.unite(QSet<ZoneCardKey>(newInfos.keyBegin(), newInfos.keyEnd()));
 
     DeckDiffResult result;
     for (const ZoneCardKey &key : keys) {
         const int oldCount = oldCounts.value(key, 0);
-        const int newCount = newCounts.value(key, 0);
-        if (oldCount != newCount) {
-            result.changes.append({key.first, key.second, oldCount, newCount});
+        const ZoneCardInfo newInfo = newInfos.value(key, ZoneCardInfo());
+        if (oldCount != newInfo.count) {
+            DeckCardChange change;
+            change.zone = key.first;
+            change.cardName = key.second;
+            change.oldCount = oldCount;
+            change.newCount = newInfo.count;
+            change.cardSetShortName = newInfo.cardSetShortName;
+            change.cardSetNumber = newInfo.cardSetNumber;
+            change.cardProviderId = newInfo.cardProviderId;
+            result.changes.append(change);
         }
     }
     std::sort(result.changes.begin(), result.changes.end(), changeLessThan);
@@ -69,21 +101,41 @@ DeckList
 DeckDiff::merge(const DeckList &baseDeck, const DeckDiffResult &localChanges, const DeckDiffResult &remoteChanges)
 {
     QMap<ZoneCardKey, int> counts = zoneCardCounts(baseDeck);
+    QMap<ZoneCardKey, DeckCardChange> printings;
     for (const DeckCardChange &change : localChanges.changes) {
         counts[{change.zone, change.cardName}] = change.newCount;
+        printings[{change.zone, change.cardName}] = change;
     }
     for (const DeckCardChange &change : remoteChanges.changes) {
         counts[{change.zone, change.cardName}] = change.newCount;
+        printings[{change.zone, change.cardName}] = change;
     }
 
-    DeckList merged(baseDeck.getMetadata(), DecklistNodeTree(), baseDeck.getSideboardPlans());
-    QList<ZoneCardKey> keys = counts.keys();
-    std::sort(keys.begin(), keys.end());
-    for (const ZoneCardKey &key : keys) {
-        const int count = qMax(0, counts.value(key));
-        for (int i = 0; i < count; ++i) {
-            merged.addCard(key.second, key.first);
+    DeckList merged = baseDeck;
+    QMap<ZoneCardKey, QList<DecklistCardNode *>> nodesByKey;
+    merged.forEachCard([&nodesByKey](InnerDecklistNode *zone, DecklistCardNode *card) {
+        nodesByKey[{zone->getName(), card->getName()}].append(card);
+    });
+
+    for (auto it = counts.begin(); it != counts.end(); ++it) {
+        const ZoneCardKey &key = it.key();
+        const int target = qMax(0, it.value());
+        QList<DecklistCardNode *> nodes = nodesByKey.value(key);
+        if (target == 0) {
+            for (DecklistCardNode *node : nodes) {
+                merged.getTree()->deleteNode(node);
+            }
+        } else if (nodes.isEmpty()) {
+            const DeckCardChange printing = printings.value(key);
+            merged.getTree()->addCard(printing.cardName, target, printing.zone, -1, printing.cardSetShortName,
+                                      printing.cardSetNumber, printing.cardProviderId, true);
+        } else {
+            nodes.first()->setNumber(target);
+            for (int i = 1; i < nodes.size(); ++i) {
+                merged.getTree()->deleteNode(nodes.at(i));
+            }
         }
     }
+    merged.refreshDeckHash();
     return merged;
 }

libcockatrice_deck_list/libcockatrice/deck_list/deck_diff.h

@@ -14,10 +14,13 @@ class DeckList;
 /** @brief A single card-count change within one zone. */
 struct DeckCardChange
 {
-    QString zone;     ///< Zone name, e.g. "main".
-    QString cardName; ///< Card name.
-    int oldCount = 0; ///< Copies in the old deck.
-    int newCount = 0; ///< Copies in the new deck.
+    QString zone;             ///< Zone name, e.g. "main".
+    QString cardName;         ///< Card name.
+    int oldCount = 0;         ///< Copies in the old deck.
+    int newCount = 0;         ///< Copies in the new deck.
+    QString cardSetShortName; ///< Printing set carried from the new side, empty when unchanged.
+    QString cardSetNumber;    ///< Printing collector number carried from the new side.
+    QString cardProviderId;   ///< Printing provider id carried from the new side.
     /** @brief Copies added (positive) or removed (negative). */
     [[nodiscard]] int delta() const
     {
@@ -40,9 +43,9 @@ struct DeckDiffResult
  * @class DeckDiff
  * @brief Pure card-count diff and three-way merge for deck sync.
  *
- * Compares card names and copy counts per zone only; printing information
- * (set, collector number, provider) is not tracked, so a merged deck keeps
- * base printings with merged counts.
+ * Compares card names and copy counts per zone only; a merged deck keeps
+ * base printings for existing cards and carries the changed side's printing
+ * for newly added cards.
  */
 class DeckDiff
 {

libcockatrice_protocol/libcockatrice/protocol/pb/CMakeLists.txt

@@ -26,6 +26,7 @@ set(PROTO_FILES
     command_deck_share_list.proto
     command_deck_share_list_mine.proto
     command_deck_share_remove.proto
+    command_deck_sync_check.proto
     command_deck_upload.proto
     command_del_counter.proto
     command_delete_arrow.proto
@@ -149,6 +150,7 @@ set(PROTO_FILES
     response_deck_share_download.proto
     response_deck_share_list.proto
     response_deck_share_list_mine.proto
+    response_deck_sync_status.proto
     response_deck_upload.proto
     response_dump_zone.proto
     response_forgotpasswordrequest.proto

libcockatrice_protocol/libcockatrice/protocol/pb/command_deck_sync_check.proto

@@ -0,0 +1,11 @@
+syntax = "proto2";
+import "session_commands.proto";
+
+message Command_DeckSyncCheck {
+    extend SessionCommand {
+        optional Command_DeckSyncCheck ext = 1034;
+    }
+    // Seconds since Unix epoch of the client's last completed sync.
+    // 0 means "never synced": the server reports every stored row.
+    optional uint64 last_sync_time = 1 [default = 0];
+}

libcockatrice_protocol/libcockatrice/protocol/pb/response.proto

@@ -85,6 +85,7 @@ message Response {
         DECK_SHARE_LIST = 1104;         // Response listing shared decks
         DECK_SHARE_DOWNLOAD = 1105;     // Response for shared deck download
         DECK_SHARE_LIST_MINE = 1106;    // Response listing the caller's own shares
+        DECK_SYNC_STATUS = 1107;        // Response with changed decks since a timestamp
         CARD_ART_RULE_LIST = 1200;      // Response containing a list of card art rules
     }
 

libcockatrice_protocol/libcockatrice/protocol/pb/response_deck_sync_status.proto

@@ -0,0 +1,21 @@
+syntax = "proto2";
+import "response.proto";
+
+message DeckSyncChangedFile {
+    // Server-relative folder path without file extension, e.g. "Modern/Affinity".
+    optional string path = 1;
+    optional uint32 deck_id = 2;
+    optional string guid = 3;
+    optional string hash = 4;
+    optional uint64 last_modified = 5;
+    optional bool deleted = 6 [default = false];
+}
+
+message Response_DeckSyncStatus {
+    extend Response {
+        optional Response_DeckSyncStatus ext = 1107;
+    }
+    repeated DeckSyncChangedFile changed_files = 1;
+    // Server clock at query time; the client adopts it as its sync baseline.
+    optional uint64 server_time = 2;
+}

libcockatrice_protocol/libcockatrice/protocol/pb/session_commands.proto

@@ -36,6 +36,7 @@ message SessionCommand {
         DECK_DOWNLOAD_PUBLIC = 1031;
         DECK_SHARE_LIST_MINE = 1032;
         DECK_SHARE_REMOVE = 1033;
+        DECK_SYNC_CHECK = 1034;
         REPLAY_LIST = 1100;
         REPLAY_DOWNLOAD = 1101;
         REPLAY_MODIFY_MATCH = 1102;

libcockatrice_settings/CMakeLists.txt

@@ -12,6 +12,7 @@ set(HEADERS
     libcockatrice/settings/commander_bracket_settings.h
     libcockatrice/settings/debug_settings.h
     libcockatrice/settings/deck_editor_settings.h
+    libcockatrice/settings/deck_sync_settings.h
     libcockatrice/settings/download_settings.h
     libcockatrice/settings/game_filters_settings.h
     libcockatrice/settings/game_settings.h
@@ -45,6 +46,7 @@ add_library(
   libcockatrice/settings/chat_settings.cpp
   libcockatrice/settings/debug_settings.cpp
   libcockatrice/settings/deck_editor_settings.cpp
+  libcockatrice/settings/deck_sync_settings.cpp
   libcockatrice/settings/download_settings.cpp
   libcockatrice/settings/game_filters_settings.cpp
   libcockatrice/settings/game_settings.cpp

libcockatrice_settings/libcockatrice/settings/deck_sync_settings.cpp

@@ -0,0 +1,44 @@
+#include "deck_sync_settings.h"
+
+#include "libcockatrice/settings/settings_manager.h"
+
+#include <QVariant>
+#include <QtGlobal>
+
+DeckSyncSettings::DeckSyncSettings(const QString &settingPath, QObject *parent)
+    : SettingsManager(settingPath + "deck_sync.ini", "decksync", QString(), parent)
+{
+}
+
+int DeckSyncSettings::getConflictDefault() const
+{
+    return getValue("conflictDefault", QString(), QString(), DeckSyncConflictAsk).toInt();
+}
+
+void DeckSyncSettings::setConflictDefault(int _conflictDefault)
+{
+    setValue(_conflictDefault, "conflictDefault");
+    emit conflictDefaultChanged(_conflictDefault);
+}
+
+bool DeckSyncSettings::getAutoSync() const
+{
+    return getValue("autoSync", QString(), QString(), true).toBool();
+}
+
+void DeckSyncSettings::setAutoSync(bool _autoSync)
+{
+    setValue(_autoSync, "autoSync");
+    emit autoSyncChanged(_autoSync);
+}
+
+int DeckSyncSettings::getSyncIntervalMinutes() const
+{
+    return qBound(1, getValue("syncIntervalMinutes", QString(), QString(), 5).toInt(), 60);
+}
+
+void DeckSyncSettings::setSyncIntervalMinutes(int _syncIntervalMinutes)
+{
+    setValue(_syncIntervalMinutes, "syncIntervalMinutes");
+    emit syncIntervalMinutesChanged(_syncIntervalMinutes);
+}

libcockatrice_settings/libcockatrice/settings/deck_sync_settings.h

@@ -0,0 +1,43 @@
+#ifndef DECK_SYNC_SETTINGS_H
+#define DECK_SYNC_SETTINGS_H
+
+#include "settings_manager.h"
+
+#include <QString>
+#include <qtmetamacros.h>
+
+class QObject;
+
+enum DeckSyncConflictDefault
+{
+    DeckSyncConflictAsk = 0,    ///< Ask every time a deck changed on both sides.
+    DeckSyncConflictKeepMine,   ///< Silently keep the local deck.
+    DeckSyncConflictKeepServer, ///< Silently keep the server deck.
+    DeckSyncConflictKeepNewer,  ///< Silently keep whichever side changed last.
+};
+
+class DeckSyncSettings : public SettingsManager
+{
+    Q_OBJECT
+    friend class SettingsCache;
+
+public:
+    explicit DeckSyncSettings(const QString &settingPath, QObject *parent = nullptr);
+
+    [[nodiscard]] int getConflictDefault() const;
+    void setConflictDefault(int _conflictDefault);
+    [[nodiscard]] bool getAutoSync() const;
+    void setAutoSync(bool _autoSync);
+    [[nodiscard]] int getSyncIntervalMinutes() const;
+    void setSyncIntervalMinutes(int _syncIntervalMinutes);
+
+signals:
+    void conflictDefaultChanged(int conflictDefault);
+    void autoSyncChanged(bool autoSync);
+    void syncIntervalMinutesChanged(int syncIntervalMinutes);
+
+private:
+    DeckSyncSettings(const DeckSyncSettings & /*other*/);
+};
+
+#endif // DECK_SYNC_SETTINGS_H

libcockatrice_settings/libcockatrice/settings/paths_settings.cpp

@@ -69,6 +69,7 @@ QString PathsSettings::getRedirectCachePath() const
 void PathsSettings::setDeckPath(const QString &_deckPath)
 {
     setValue(_deckPath, "decks");
+    emit deckPathChanged();
 }
 
 void PathsSettings::setFiltersPath(const QString &_filtersPath)

libcockatrice_settings/libcockatrice/settings/paths_settings.h

@@ -39,6 +39,7 @@ public:
     void setSpoilerDatabasePath(const QString &_spoilerDatabasePath);
 
 signals:
+    void deckPathChanged();
     void cardDatabasePathChanged();
     void picsPathChanged();
     void themeChanged();

servatrice/servatrice.ini.example

@@ -488,3 +488,10 @@ max_shares_per_day=50
 ; Minimum enforced: 1.
 ; Default: 20
 max_versions=20
+
+; How many days a deleted deck stays restorable as a tombstone before the
+; cleanup clock purges it (with its versions) for good. Re-uploading the
+; same deck within this window resurrects it instead of creating a copy.
+; A value of 0 purges tombstones on the next cleanup run.
+; Default: 30
+tombstone_days=30

servatrice/src/servatrice.cpp

@@ -460,6 +460,7 @@ bool Servatrice::initServer()
 
     deckShareCleanupClock = new QTimer(this);
     connect(deckShareCleanupClock, SIGNAL(timeout()), this, SLOT(cleanupExpiredDeckShares()));
+    connect(deckShareCleanupClock, SIGNAL(timeout()), this, SLOT(purgeDeckTombstones()));
     const int deckShareCleanupInterval = getDeckShareCleanupInterval();
     if (deckShareCleanupInterval > 0) {
         qDebug() << "Starting deck share cleanup clock, interval" << deckShareCleanupInterval << "ms";
@@ -689,6 +690,11 @@ void Servatrice::cleanupExpiredDeckShares()
     servatriceDatabaseInterface->cleanupExpiredDeckShares();
 }
 
+void Servatrice::purgeDeckTombstones()
+{
+    servatriceDatabaseInterface->purgeDeckTombstones(getDeckSyncTombstoneDays());
+}
+
 void Servatrice::statusUpdate()
 {
     if (!servatriceDatabaseInterface->checkSql()) {
@@ -1127,6 +1133,11 @@ int Servatrice::getDeckSyncMaxVersions() const
     return qMax(1, settingsCache->value("deck_sync/max_versions", 20).toInt());
 }
 
+int Servatrice::getDeckSyncTombstoneDays() const
+{
+    return qMax(0, settingsCache->value("deck_sync/tombstone_days", 30).toInt());
+}
+
 int Servatrice::getNumberOfTCPPools() const
 {
     return settingsCache->value("server/number_pools", 1).toInt();

servatrice/src/servatrice.h

@@ -157,6 +157,7 @@ private slots:
     void statusUpdate();
     void shutdownTimeout();
     void cleanupExpiredDeckShares();
+    void purgeDeckTombstones();
 
 protected:
     void doSendIslMessage(const IslMessage &msg, int _serverId) override;
@@ -293,6 +294,7 @@ public:
     int getDeckShareMaxDecksPerShare() const;
     int getDeckShareMaxSharesPerDay() const;
     int getDeckSyncMaxVersions() const;
+    int getDeckSyncTombstoneDays() const;
     int getMaxUsersPerAddress() const;
     int getMessageCountingInterval() const override;
     int getMaxMessageCountPerInterval() const override;

servatrice/src/servatrice_database_interface.cpp

@@ -1088,8 +1088,8 @@ DeckList *Servatrice_DatabaseInterface::getDeckFromDatabase(int deckId, int user
 {
     checkSql();
 
-    QSqlQuery *query =
-        prepareQuery("select content from {prefix}_decklist_files where id = :id and id_user = :id_user");
+    QSqlQuery *query = prepareQuery(
+        "select content from {prefix}_decklist_files where id = :id and id_user = :id_user and deleted = 0");
     query->bindValue(":id", deckId);
     query->bindValue(":id_user", userId);
     execSqlQuery(query);
@@ -1240,6 +1240,19 @@ void Servatrice_DatabaseInterface::cleanupExpiredDeckShares()
     execSqlQuery(query);
 }
 
+void Servatrice_DatabaseInterface::purgeDeckTombstones(int olderThanDays)
+{
+    checkSql();
+
+    // DB-native clock: tombstones are stamped with NOW() on this same
+    // database, so the grace window is immune to app/db clock skew.
+    // The null arm only catches manually flagged rows.
+    QSqlQuery *query = prepareQuery("delete from {prefix}_decklist_files where deleted = 1 and (last_modified < "
+                                    "DATE_SUB(NOW(), INTERVAL :days DAY) or last_modified is null)");
+    query->bindValue(":days", qMax(0, olderThanDays));
+    execSqlQuery(query);
+}
+
 bool Servatrice_DatabaseInterface::getDeckSharesForUser(int userId, QList<DeckShareSummaryRecord> &shares)
 {
     checkSql();

servatrice/src/servatrice_database_interface.h

@@ -140,6 +140,11 @@ public:
     bool getDeckShareItem(const QString &token, int itemId, QString &content);
     /** @brief Deletes all expired share bundles (cascades to their items). */
     void cleanupExpiredDeckShares();
+    /**
+     * @brief Hard-deletes tombstoned decks older than the grace period (cascades to versions).
+     * @param olderThanDays Tombstones older than this many days are purged; 0 purges all tombstones.
+     */
+    void purgeDeckTombstones(int olderThanDays);
 
     int getNextGameId() override;
     int getNextReplayId() override;

servatrice/src/serversocketinterface.cpp

@@ -96,6 +96,7 @@
 #include <libcockatrice/protocol/pb/command_deck_share_list.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_share_list_mine.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_share_remove.pb.h>
+#include <libcockatrice/protocol/pb/command_deck_sync_check.pb.h>
 #include <libcockatrice/protocol/pb/command_deck_upload.pb.h>
 #include <libcockatrice/protocol/pb/command_get_server_stats.pb.h>
 #include <libcockatrice/protocol/pb/command_replay_delete_match.pb.h>
@@ -130,6 +131,7 @@
 #include <libcockatrice/protocol/pb/response_deck_share_download.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_list.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_share_list_mine.pb.h>
+#include <libcockatrice/protocol/pb/response_deck_sync_status.pb.h>
 #include <libcockatrice/protocol/pb/response_deck_upload.pb.h>
 #include <libcockatrice/protocol/pb/response_forgotpasswordrequest.pb.h>
 #include <libcockatrice/protocol/pb/response_get_admin_notes.pb.h>
@@ -353,6 +355,8 @@ Response::ResponseCode AbstractServerSocketInterface::processExtendedSessionComm
             return cmdDeckUpload(cmd.GetExtension(Command_DeckUpload::ext), rc);
         case SessionCommand::DECK_DOWNLOAD:
             return cmdDeckDownload(cmd.GetExtension(Command_DeckDownload::ext), rc);
+        case SessionCommand::DECK_SYNC_CHECK:
+            return cmdDeckSyncCheck(cmd.GetExtension(Command_DeckSyncCheck::ext), rc);
         case SessionCommand::REPLAY_LIST:
             return cmdReplayList(cmd.GetExtension(Command_ReplayList::ext), rc);
         case SessionCommand::REPLAY_DOWNLOAD:
@@ -683,7 +687,8 @@ bool AbstractServerSocketInterface::deckListHelper(int folderId,
 
     query = sqlInterface->prepareQuery("select id, name, upload_time, is_public, banner_card_name, "
                                        "banner_card_provider, color_identity, tags, guid, hash, last_modified from "
-                                       "{prefix}_decklist_files where id_folder = :id_folder and id_user = :id_user");
+                                       "{prefix}_decklist_files where id_folder = :id_folder and id_user = :id_user "
+                                       "and deleted = 0");
     query->bindValue(":id_folder", folderId);
     query->bindValue(":id_user", userId);
     if (!sqlInterface->execSqlQuery(query)) {
@@ -828,8 +833,8 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckSetVisibility(const
     sqlInterface->checkSql();
 
     if (cmd.has_deck_id()) {
-        QSqlQuery *query =
-            sqlInterface->prepareQuery("select 1 from {prefix}_decklist_files where id = :id and id_user = :id_user");
+        QSqlQuery *query = sqlInterface->prepareQuery(
+            "select 1 from {prefix}_decklist_files where id = :id and id_user = :id_user and deleted = 0");
         query->bindValue(":id", cmd.deck_id());
         query->bindValue(":id_user", userInfo->id());
         sqlInterface->execSqlQuery(query);
@@ -867,6 +872,60 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckSetVisibility(const
     return Response::RespOk;
 }
 
+bool AbstractServerSocketInterface::syncCheckHelper(int userId, qint64 sinceSecs, Response_DeckSyncStatus *status)
+{
+    // Single query over the user's files: folder rows are hard-deleted while
+    // their files linger as tombstones, so walking the live folder tree would
+    // miss deletes pointing at nonexistent folders. The client only checks
+    // changed_files_size(), so folder paths are not reconstructed here.
+    QSqlQuery *query = sqlInterface->prepareQuery("select id, guid, hash, last_modified, deleted from "
+                                                  "{prefix}_decklist_files where id_user = :id_user "
+                                                  "and (last_modified is null or last_modified >= "
+                                                  "FROM_UNIXTIME(:since))");
+    query->bindValue(":id_user", userId);
+    query->bindValue(":since", sinceSecs);
+    if (!sqlInterface->execSqlQuery(query)) {
+        return false;
+    }
+    while (query->next()) {
+        DeckSyncChangedFile *changed = status->add_changed_files();
+        changed->set_deck_id(query->value(0).toUInt());
+        changed->set_guid(query->value(1).toString().toStdString());
+        changed->set_hash(query->value(2).toString().toStdString());
+        const QDateTime lastModified = query->value(3).toDateTime();
+        if (lastModified.isValid()) {
+            changed->set_last_modified(lastModified.toSecsSinceEpoch());
+        }
+        changed->set_deleted(query->value(4).toBool());
+    }
+    return true;
+}
+
+Response::ResponseCode AbstractServerSocketInterface::cmdDeckSyncCheck(const Command_DeckSyncCheck &cmd,
+                                                                       ResponseContainer &rc)
+{
+    if (authState != PasswordRight) {
+        return Response::RespFunctionNotAllowed;
+    }
+
+    sqlInterface->checkSql();
+
+    Response_DeckSyncStatus *re = new Response_DeckSyncStatus;
+    if (!syncCheckHelper(userInfo->id(), static_cast<qint64>(cmd.last_sync_time()), re)) {
+        delete re;
+        return Response::RespContextError;
+    }
+
+    QSqlQuery *clockQuery = sqlInterface->prepareQuery("select unix_timestamp(now())");
+    if (!sqlInterface->execSqlQuery(clockQuery) || !clockQuery->next()) {
+        delete re;
+        return Response::RespContextError;
+    }
+    re->set_server_time(clockQuery->value(0).toULongLong());
+    rc.setResponseExtension(re);
+    return Response::RespOk;
+}
+
 Response::ResponseCode AbstractServerSocketInterface::cmdDeckDownloadPublic(const Command_DeckDownloadPublic &cmd,
                                                                             ResponseContainer &rc)
 {
@@ -939,7 +998,8 @@ void AbstractServerSocketInterface::deckDelDirHelper(int basePathId)
         deckDelDirHelper(query->value(0).toInt());
     }
 
-    query = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id_folder = :id_folder");
+    query = sqlInterface->prepareQuery("update {prefix}_decklist_files set deleted = 1, last_modified = NOW() "
+                                       "where id_folder = :id_folder and deleted = 0");
     query->bindValue(":id_folder", basePathId);
     sqlInterface->execSqlQuery(query);
 
@@ -990,18 +1050,17 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckDel(const Command_D
 
     sqlInterface->checkSql();
     QSqlQuery *query =
-        sqlInterface->prepareQuery("select id from {prefix}_decklist_files where id = :id and id_user = :id_user");
+        sqlInterface->prepareQuery("update {prefix}_decklist_files set deleted = 1, last_modified = NOW() where id = "
+                                   ":id and id_user = :id_user and deleted = 0");
     query->bindValue(":id", cmd.deck_id());
     query->bindValue(":id_user", userInfo->id());
-    sqlInterface->execSqlQuery(query);
-    if (!query->next()) {
+    if (!sqlInterface->execSqlQuery(query)) {
+        return Response::RespContextError;
+    }
+    if (query->numRowsAffected() == 0) {
         return Response::RespNameNotFound;
     }
 
-    query = sqlInterface->prepareQuery("delete from {prefix}_decklist_files where id = :id");
-    query->bindValue(":id", cmd.deck_id());
-    sqlInterface->execSqlQuery(query);
-
     return Response::RespOk;
 }
 
@@ -1169,9 +1228,11 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckUpload(const Comman
         }
         const QString oldHash = currentQuery->value(1).toString();
 
-        QString updateQuery = "update {prefix}_decklist_files set name=:name, upload_time=NOW(), content=:content, "
-                              "banner_card_name=:banner_card_name, banner_card_provider=:banner_card_provider, "
-                              "color_identity=:color_identity, tags=:tags, guid=:guid, hash=:hash, last_modified=NOW()";
+        QString updateQuery =
+            "update {prefix}_decklist_files set name=:name, upload_time=NOW(), content=:content, "
+            "banner_card_name=:banner_card_name, banner_card_provider=:banner_card_provider, "
+            "color_identity=:color_identity, tags=:tags, guid=:guid, hash=:hash, last_modified=NOW(), "
+            "deleted=0";
         if (cmd.has_is_public()) {
             updateQuery += ", is_public=:is_public";
         }
@@ -1373,7 +1434,7 @@ Response::ResponseCode AbstractServerSocketInterface::cmdDeckShareCreate(const C
         // issues its own query on the same cached statement set.
         QSqlQuery *query =
             sqlInterface->prepareQuery("select id, color_identity from {prefix}_decklist_files where id_folder = "
-                                       ":id_folder and id_user = :id_user");
+                                       ":id_folder and id_user = :id_user and deleted = 0");
         query->bindValue(":id_folder", folderId);
         query->bindValue(":id_user", userInfo->id());
         if (!sqlInterface->execSqlQuery(query)) {

servatrice/src/serversocketinterface.h

@@ -56,6 +56,7 @@ class Command_DeckShareList;
 class Command_DeckShareListMine;
 class Command_DeckShareRemove;
 class Command_DeckShareDownload;
+class Command_DeckSyncCheck;
 class Command_ReplayList;
 class Command_ReplayDownload;
 class Command_ReplayModifyMatch;
@@ -107,6 +108,7 @@ class Command_ViewLogHistory;
 class Command_WarnUser;
 class QObject;
 class ResponseContainer;
+class Response_DeckSyncStatus;
 
 class AbstractServerSocketInterface : public Server_ProtocolHandler
 {
@@ -157,6 +159,8 @@ private:
     bool storeDeckVersion(int deckId, const QString &content, const QString &hash);
     DeckList *getDeckFromDatabase(int deckId);
     Response::ResponseCode cmdDeckDownload(const Command_DeckDownload &cmd, ResponseContainer &rc);
+    bool syncCheckHelper(int userId, qint64 sinceSecs, Response_DeckSyncStatus *status);
+    Response::ResponseCode cmdDeckSyncCheck(const Command_DeckSyncCheck &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckDownloadPublic(const Command_DeckDownloadPublic &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckShareCreate(const Command_DeckShareCreate &cmd, ResponseContainer &rc);
     Response::ResponseCode cmdDeckShareList(const Command_DeckShareList &cmd, ResponseContainer &rc);

tests/deck_sync_sidecar_test.cpp

@@ -119,6 +119,29 @@ TEST(DeckSyncSidecarTest, RemoveDropsRecord)
     EXPECT_FALSE(reloaded.contains("a.cod"));
 }
 
+TEST(DeckSyncSidecarTest, ModeRoundTripsAndDefaultsToLinked)
+{
+    QTemporaryDir dir;
+    ASSERT_TRUE(dir.isValid());
+    DeckSyncSidecar sidecar(sidecarPath(dir));
+    EXPECT_EQ(sidecar.mode, DeckSyncSidecar::MODE_LINKED);
+    EXPECT_EQ(sidecar.lastSyncSecs, 0);
+    sidecar.mode = DeckSyncSidecar::MODE_SEPARATE;
+    sidecar.lastSyncSecs = 1791739200;
+    EXPECT_TRUE(sidecar.save());
+
+    DeckSyncSidecar reloaded(sidecarPath(dir));
+    EXPECT_TRUE(reloaded.load());
+    EXPECT_EQ(reloaded.mode, DeckSyncSidecar::MODE_SEPARATE);
+    EXPECT_EQ(reloaded.lastSyncSecs, 1791739200);
+}
+
+TEST(DeckSyncSidecarTest, PathForDeckPathBuildsSidecarPath)
+{
+    QString path = DeckSyncSidecar::pathForDeckPath("/decks", "abc123");
+    EXPECT_EQ(path, QString("/decks/.sync/abc123.json"));
+}
+
 int main(int argc, char **argv)
 {
     ::testing::InitGoogleTest(&argc, argv);

tests/settings/settings_defaults_test.cpp

@@ -11,6 +11,7 @@
 #include <libcockatrice/settings/cards_display_settings.h>
 #include <libcockatrice/settings/chat_settings.h>
 #include <libcockatrice/settings/deck_editor_settings.h>
+#include <libcockatrice/settings/deck_sync_settings.h>
 #include <libcockatrice/settings/download_settings.h>
 #include <libcockatrice/settings/game_settings.h>
 #include <libcockatrice/settings/interface_settings.h>
@@ -531,6 +532,52 @@ TEST_F(SettingsDefaultsTest, DeckEditor_VdeStartupTab_SetAndGet)
     ASSERT_EQ(s.getVdeStartupTab(), VdeStartupTabContext);
 }
 
+// --- DeckSyncSettings ---
+
+TEST_F(SettingsDefaultsTest, DeckSync_ConflictDefault_Default)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictAsk);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_ConflictDefault_SetAndGet)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    s.setConflictDefault(DeckSyncConflictKeepNewer);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictKeepNewer);
+    s.setConflictDefault(DeckSyncConflictKeepMine);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictKeepMine);
+    s.setConflictDefault(DeckSyncConflictAsk);
+    ASSERT_EQ(s.getConflictDefault(), DeckSyncConflictAsk);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_AutoSync_Default)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getAutoSync(), true);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_AutoSync_SetAndGet)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    s.setAutoSync(false);
+    ASSERT_EQ(s.getAutoSync(), false);
+    s.setAutoSync(true);
+    ASSERT_EQ(s.getAutoSync(), true);
+}
+
+TEST_F(SettingsDefaultsTest, DeckSync_SyncIntervalMinutes_DefaultAndBounds)
+{
+    DeckSyncSettings s(settingsPath, nullptr);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 5);
+    s.setSyncIntervalMinutes(15);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 15);
+    s.setSyncIntervalMinutes(0);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 1);
+    s.setSyncIntervalMinutes(600);
+    ASSERT_EQ(s.getSyncIntervalMinutes(), 60);
+}
+
 // --- NetworkSettings ---
 
 TEST_F(SettingsDefaultsTest, Network_ClientID_Default)