// Copyright (C) 2024 Jarek Kobus
// Copyright (C) 2024 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0

#ifndef TASKING_TASKTREE_H
#define TASKING_TASKTREE_H

#include "tasking_global.h"

#include <QtCore/QList>
#include <QtCore/QObject>

#include <memory>

QT_BEGIN_NAMESPACE
template <class T>
class QFuture;

namespace Tasking {

class Do;
class For;
class Group;
class GroupItem;
using GroupItems = QList<GroupItem>;

Q_NAMESPACE_EXPORT(TASKING_EXPORT)

// WorkflowPolicy:
// 1. When all children finished with success -> report success, otherwise:
//    a) Report error on first error and stop executing other children (including their subtree).
//    b) On first error - continue executing all children and report error afterwards.
// 2. When all children finished with error -> report error, otherwise:
//    a) Report success on first success and stop executing other children (including their subtree).
//    b) On first success - continue executing all children and report success afterwards.
// 3. Stops on first finished child. In sequential mode it will never run other children then the first one.
//    Useful only in parallel mode.
// 4. Always run all children, let them finish, ignore their results and report success afterwards.
// 5. Always run all children, let them finish, ignore their results and report error afterwards.

enum class WorkflowPolicy
{
    StopOnError,          // 1a - Reports error on first child error, otherwise success (if all children were success).
    ContinueOnError,      // 1b - The same, but children execution continues. Reports success when no children.
    StopOnSuccess,        // 2a - Reports success on first child success, otherwise error (if all children were error).
    ContinueOnSuccess,    // 2b - The same, but children execution continues. Reports error when no children.
    StopOnSuccessOrError, // 3  - Stops on first finished child and report its result.
    FinishAllAndSuccess,  // 4  - Reports success after all children finished.
    FinishAllAndError     // 5  - Reports error after all children finished.
};
Q_ENUM_NS(WorkflowPolicy)

enum class SetupResult
{
    Continue,
    StopWithSuccess,
    StopWithError
};
Q_ENUM_NS(SetupResult)

enum class DoneResult
{
    Success,
    Error
};
Q_ENUM_NS(DoneResult)

enum class DoneWith
{
    Success,
    Error,
    Cancel
};
Q_ENUM_NS(DoneWith)

enum class CallDone
{
    Never = 0,
    OnSuccess = 1 << 0,
    OnError   = 1 << 1,
    OnCancel  = 1 << 2,
    Always = OnSuccess | OnError | OnCancel
};
Q_ENUM_NS(CallDone)
Q_DECLARE_FLAGS(CallDoneFlags, CallDone)
Q_DECLARE_OPERATORS_FOR_FLAGS(CallDoneFlags)

TASKING_EXPORT DoneResult toDoneResult(bool success);
TASKING_EXPORT bool shouldCallDone(CallDoneFlags callDone, DoneWith result);

// Checks if Function may be invoked with Args and if Function's return type is Result.
template <typename Result, typename Function, typename ...Args,
          typename DecayedFunction = std::decay_t<Function>>
static constexpr bool isInvocable()
{
    // Note, that std::is_invocable_r_v doesn't check Result type properly.
    if constexpr (std::is_invocable_r_v<Result, DecayedFunction, Args...>)
        return std::is_same_v<Result, std::invoke_result_t<DecayedFunction, Args...>>;
    return false;
}

class LoopData;
class StorageData;
class TaskTreePrivate;

class TASKING_EXPORT TaskInterface : public QObject
{
    Q_OBJECT

public:
    void reportDone(DoneResult result);

Q_SIGNALS:
    void done(DoneResult result, QPrivateSignal);
};

class TASKING_EXPORT Loop
{
public:
    using Condition = std::function<bool(int)>; // Takes iteration, called prior to each iteration.
    using ValueGetter = std::function<const void *(int)>; // Takes iteration, returns ptr to ref.

    int iteration() const;

protected:
    Loop(); // LoopForever
    Loop(int count, const ValueGetter &valueGetter = {}); // LoopRepeat, LoopList
    Loop(const Condition &condition); // LoopUntil

    const void *valuePtr() const;

private:
    friend class ExecutionContextActivator;
    friend class TaskTreePrivate;
    std::shared_ptr<LoopData> m_loopData;
};

class TASKING_EXPORT LoopForever final : public Loop
{
public:
    LoopForever() : Loop() {}
};

class TASKING_EXPORT LoopRepeat final : public Loop
{
public:
    LoopRepeat(int count) : Loop(count) {}
};

class TASKING_EXPORT LoopUntil final : public Loop
{
public:
    LoopUntil(const Condition &condition) : Loop(condition) {}
};

template <typename T>
class LoopList final : public Loop
{
public:
    LoopList(const QList<T> &list) : Loop(list.size(), [list](int i) { return &list.at(i); }) {}
    const T *operator->() const { return static_cast<const T *>(valuePtr()); }
    const T &operator*() const { return *static_cast<const T *>(valuePtr()); }
};

class TASKING_EXPORT StorageBase
{
private:
    using StorageConstructor = std::function<void *(void)>;
    using StorageDestructor = std::function<void(void *)>;
    using StorageHandler = std::function<void(void *)>;

    StorageBase(const StorageConstructor &ctor, const StorageDestructor &dtor);

    void *activeStorageVoid() const;

    friend bool operator==(const StorageBase &first, const StorageBase &second)
    { return first.m_storageData == second.m_storageData; }

    friend bool operator!=(const StorageBase &first, const StorageBase &second)
    { return first.m_storageData != second.m_storageData; }

    friend size_t qHash(const StorageBase &storage, uint seed = 0)
    { return size_t(storage.m_storageData.get()) ^ seed; }

    std::shared_ptr<StorageData> m_storageData;

    template <typename StorageStruct> friend class Storage;
    friend class ExecutionContextActivator;
    friend class StorageData;
    friend class RuntimeContainer;
    friend class TaskTree;
    friend class TaskTreePrivate;
};

template <typename StorageStruct>
class Storage final : public StorageBase
{
public:
    Storage() : StorageBase(Storage::ctor(), Storage::dtor()) {}
#if __cplusplus >= 201803L // C++20: Allow pack expansion in lambda init-capture.
    template <typename ...Args>
    Storage(const Args &...args)
        : StorageBase([...args = args] { return new StorageStruct{args...}; }, Storage::dtor()) {}
#else // C++17
    template <typename ...Args>
    Storage(const Args &...args)
        : StorageBase([argsTuple = std::tuple(args...)] {
            return std::apply([](const Args &...arguments) { return new StorageStruct{arguments...}; }, argsTuple);
        }, Storage::dtor()) {}
#endif
    StorageStruct &operator*() const noexcept { return *activeStorage(); }
    StorageStruct *operator->() const noexcept { return activeStorage(); }
    StorageStruct *activeStorage() const {
        return static_cast<StorageStruct *>(activeStorageVoid());
    }

private:
    static StorageConstructor ctor() { return [] { return new StorageStruct(); }; }
    static StorageDestructor dtor() {
        return [](void *storage) { delete static_cast<StorageStruct *>(storage); };
    }
};

class TASKING_EXPORT GroupItem
{
public:
    // Called when group entered, after group's storages are created
    using GroupSetupHandler = std::function<SetupResult()>;
    // Called when group done, before group's storages are deleted
    using GroupDoneHandler = std::function<DoneResult(DoneWith)>;

    template <typename StorageStruct>
    GroupItem(const Storage<StorageStruct> &storage)
        : m_type(Type::Storage)
        , m_storageList{storage} {}

    // TODO: Add tests.
    GroupItem(const GroupItems &children) : m_type(Type::List) { addChildren(children); }
    GroupItem(std::initializer_list<GroupItem> children) : m_type(Type::List) { addChildren(children); }

protected:
    GroupItem(const Loop &loop) : GroupItem(GroupData{{}, {}, {}, loop}) {}
    using TaskAdapterPtr = void *;
    // Internal, provided by QCustomTask
    using TaskAdapterConstructor = std::function<TaskAdapterPtr(void)>;
    // Internal, provided by QCustomTask
    using TaskAdapterDestructor = std::function<void(TaskAdapterPtr)>;
    //
    using TaskAdapterStarter = std::function<void(TaskAdapterPtr, TaskInterface *)>;
    // Called prior to task start, just after createHandler
    using TaskAdapterSetupHandler = std::function<SetupResult(TaskAdapterPtr)>;
    // Called on task done, just before deleteLater
    using TaskAdapterDoneHandler = std::function<DoneResult(TaskAdapterPtr, DoneWith)>;

    struct TaskHandler {
        TaskAdapterConstructor m_taskAdapterConstructor;
        TaskAdapterDestructor m_taskAdapterDestructor;
        TaskAdapterStarter m_taskAdapterStarter;
        TaskAdapterSetupHandler m_taskAdapterSetupHandler = {};
        TaskAdapterDoneHandler m_taskAdapterDoneHandler = {};
        CallDoneFlags m_callDoneFlags = CallDone::Always;
    };

    struct GroupHandler {
        GroupSetupHandler m_setupHandler;
        GroupDoneHandler m_doneHandler = {};
        CallDoneFlags m_callDoneFlags = CallDone::Always;
    };

    struct GroupData {
        GroupHandler m_groupHandler = {};
        std::optional<int> m_parallelLimit = {};
        std::optional<WorkflowPolicy> m_workflowPolicy = {};
        std::optional<Loop> m_loop = {};
    };

    enum class Type {
        List,
        Group,
        GroupData,
        Storage,
        TaskHandler
    };

    GroupItem() = default;
    GroupItem(Type type) : m_type(type) { }
    GroupItem(const GroupData &data)
        : m_type(Type::GroupData)
        , m_groupData(data) {}
    GroupItem(const TaskHandler &handler)
        : m_type(Type::TaskHandler)
        , m_taskHandler(handler) {}
    void addChildren(const GroupItems &children);

    static GroupItem groupHandler(const GroupHandler &handler) { return GroupItem({handler}); }

private:
    TASKING_EXPORT friend Group operator>>(const For &forItem, const Do &doItem);
    friend class ContainerNode;
    friend class TaskInterfaceAdapter;
    friend class TaskNode;
    friend class TaskTreePrivate;
    friend class When;
    Type m_type = Type::Group;
    GroupItems m_children;
    GroupData m_groupData;
    QList<StorageBase> m_storageList;
    TaskHandler m_taskHandler;
};

class TASKING_EXPORT ExecutableItem : public GroupItem
{
public:
    Group withTimeout(std::chrono::milliseconds timeout,
                      const std::function<void()> &handler = {}) const;
    Group withLog(const QString &logName) const;
    template <typename SenderSignalPairGetter>
    Group withCancel(SenderSignalPairGetter &&getter, std::initializer_list<GroupItem> postCancelRecipe = {}) const;
    template <typename SenderSignalPairGetter>
    Group withAccept(SenderSignalPairGetter &&getter) const;

protected:
    ExecutableItem() = default;
    ExecutableItem(const TaskHandler &handler) : GroupItem(handler) {}

private:
    TASKING_EXPORT friend Group operator!(const ExecutableItem &item);
    TASKING_EXPORT friend Group operator&&(const ExecutableItem &first,
                                           const ExecutableItem &second);
    TASKING_EXPORT friend Group operator||(const ExecutableItem &first,
                                           const ExecutableItem &second);
    TASKING_EXPORT friend Group operator&&(const ExecutableItem &item, DoneResult result);
    TASKING_EXPORT friend Group operator||(const ExecutableItem &item, DoneResult result);

    Group withCancelImpl(
        const std::function<void(QObject *, const std::function<void()> &)> &connectWrapper,
        const GroupItems &postCancelRecipe) const;
    Group withAcceptImpl(
        const std::function<void(QObject *, const std::function<void()> &)> &connectWrapper) const;
    friend class When;
};

class TASKING_EXPORT Group final : public ExecutableItem
{
public:
    Group(const GroupItems &children) { addChildren(children); }
    Group(std::initializer_list<GroupItem> children) { addChildren(children); }

    // GroupData related:
    template <typename Handler>
    static GroupItem onGroupSetup(Handler &&handler) {
        return groupHandler({wrapGroupSetup(std::forward<Handler>(handler))});
    }
    template <typename Handler>
    static GroupItem onGroupDone(Handler &&handler, CallDoneFlags callDone = CallDone::Always) {
        return groupHandler({{}, wrapGroupDone(std::forward<Handler>(handler)), callDone});
    }

private:
    template <typename Handler>
    static GroupSetupHandler wrapGroupSetup(Handler &&handler)
    {
        // R, V stands for: Setup[R]esult, [V]oid
        static constexpr bool isR = isInvocable<SetupResult, Handler>();
        static constexpr bool isV = isInvocable<void, Handler>();
        static_assert(isR || isV,
            "Group setup handler needs to take no arguments and has to return void or SetupResult. "
            "The passed handler doesn't fulfill these requirements.");
        return [handler = std::move(handler)] {
            if constexpr (isR)
                return std::invoke(handler);
            std::invoke(handler);
            return SetupResult::Continue;
        };
    }
    template <typename Handler>
    static GroupDoneHandler wrapGroupDone(Handler &&handler)
    {
        static constexpr bool isDoneResultType = std::is_same_v<std::decay_t<Handler>, DoneResult>;
        // R, B, V, D stands for: Done[R]esult, [B]ool, [V]oid, [D]oneWith
        static constexpr bool isRD = isInvocable<DoneResult, Handler, DoneWith>();
        static constexpr bool isR = isInvocable<DoneResult, Handler>();
        static constexpr bool isBD = isInvocable<bool, Handler, DoneWith>();
        static constexpr bool isB = isInvocable<bool, Handler>();
        static constexpr bool isVD = isInvocable<void, Handler, DoneWith>();
        static constexpr bool isV = isInvocable<void, Handler>();
        static_assert(isDoneResultType || isRD || isR || isBD || isB || isVD || isV,
            "Group done handler needs to take (DoneWith) or (void) as an argument and has to "
            "return void, bool or DoneResult. Alternatively, it may be of DoneResult type. "
            "The passed handler doesn't fulfill these requirements.");
        return [handler = std::move(handler)](DoneWith result) {
            if constexpr (isDoneResultType)
                return handler;
            if constexpr (isRD)
                return std::invoke(handler, result);
            if constexpr (isR)
                return std::invoke(handler);
            if constexpr (isBD)
                return toDoneResult(std::invoke(handler, result));
            if constexpr (isB)
                return toDoneResult(std::invoke(handler));
            if constexpr (isVD)
                std::invoke(handler, result);
            else if constexpr (isV)
                std::invoke(handler);
            return toDoneResult(result == DoneWith::Success);
        };
    }
};

template <typename SenderSignalPairGetter>
Group ExecutableItem::withCancel(SenderSignalPairGetter &&getter,
                                 std::initializer_list<GroupItem> postCancelRecipe) const
{
    const auto connectWrapper = [getter](QObject *guard, const std::function<void()> &trigger) {
        const auto senderSignalPair = getter();
        QObject::connect(senderSignalPair.first, senderSignalPair.second, guard, [trigger] {
            trigger();
        }, static_cast<Qt::ConnectionType>(Qt::QueuedConnection | Qt::SingleShotConnection));
    };
    return withCancelImpl(connectWrapper, postCancelRecipe);
}

template <typename SenderSignalPairGetter>
Group ExecutableItem::withAccept(SenderSignalPairGetter &&getter) const
{
    const auto connectWrapper = [getter](QObject *guard, const std::function<void()> &trigger) {
        const auto senderSignalPair = getter();
        QObject::connect(senderSignalPair.first, senderSignalPair.second, guard, [trigger] {
            trigger();
        }, static_cast<Qt::ConnectionType>(Qt::QueuedConnection | Qt::SingleShotConnection));
    };
    return withAcceptImpl(connectWrapper);
}

template <typename Handler>
static GroupItem onGroupSetup(Handler &&handler)
{
    return Group::onGroupSetup(std::forward<Handler>(handler));
}

template <typename Handler>
static GroupItem onGroupDone(Handler &&handler, CallDoneFlags callDone = CallDone::Always)
{
    return Group::onGroupDone(std::forward<Handler>(handler), callDone);
}

// Default: 1 (sequential). 0 means unlimited (parallel).
TASKING_EXPORT GroupItem parallelLimit(int limit);

// Default: WorkflowPolicy::StopOnError.
TASKING_EXPORT GroupItem workflowPolicy(WorkflowPolicy policy);

TASKING_EXPORT extern const GroupItem sequential;
TASKING_EXPORT extern const GroupItem parallel;
TASKING_EXPORT extern const GroupItem parallelIdealThreadCountLimit;

TASKING_EXPORT extern const GroupItem stopOnError;
TASKING_EXPORT extern const GroupItem continueOnError;
TASKING_EXPORT extern const GroupItem stopOnSuccess;
TASKING_EXPORT extern const GroupItem continueOnSuccess;
TASKING_EXPORT extern const GroupItem stopOnSuccessOrError;
TASKING_EXPORT extern const GroupItem finishAllAndSuccess;
TASKING_EXPORT extern const GroupItem finishAllAndError;

TASKING_EXPORT extern const GroupItem nullItem;
TASKING_EXPORT extern const ExecutableItem successItem;
TASKING_EXPORT extern const ExecutableItem errorItem;

class TASKING_EXPORT For final
{
public:
    explicit For(const Loop &loop) : m_loop(loop) {}

private:
    TASKING_EXPORT friend Group operator>>(const For &forItem, const Do &doItem);

    Loop m_loop;
};

class When;

class TASKING_EXPORT Do final
{
public:
    explicit Do(const GroupItems &children) : m_children(children) {}
    explicit Do(std::initializer_list<GroupItem> children) : m_children(children) {}

private:
    TASKING_EXPORT friend Group operator>>(const For &forItem, const Do &doItem);
    TASKING_EXPORT friend Group operator>>(const When &whenItem, const Do &doItem);

    GroupItem m_children;
};

class TASKING_EXPORT Forever final : public ExecutableItem
{
public:
    explicit Forever(const GroupItems &children)
        { addChildren({ For (LoopForever()) >> Do { children } } ); }
    explicit Forever(std::initializer_list<GroupItem> children)
        { addChildren({ For (LoopForever()) >> Do { children } } ); }
};

// Synchronous invocation. Similarly to Group - isn't counted as a task inside taskCount()
class TASKING_EXPORT Sync final : public ExecutableItem
{
public:
    template <typename Handler>
    Sync(Handler &&handler) {
        addChildren({ onGroupDone(wrapHandler(std::forward<Handler>(handler))) });
    }

private:
    template <typename Handler>
    static auto wrapHandler(Handler &&handler) {
        // R, B, V stands for: Done[R]esult, [B]ool, [V]oid
        static constexpr bool isR = isInvocable<DoneResult, Handler>();
        static constexpr bool isB = isInvocable<bool, Handler>();
        static constexpr bool isV = isInvocable<void, Handler>();
        static_assert(isR || isB || isV,
            "Sync handler needs to take no arguments and has to return void, bool or DoneResult. "
            "The passed handler doesn't fulfill these requirements.");
        return handler;
    }
};

// A convenient default helper, when:
// 1. Task is derived from QObject.
// 2. Task::start() method starts the task.
// 3. Task::done(DoneResult) signal is emitted when the task is finished.
template <typename Task>
class DefaultTaskAdapter
{
public:
    void operator()(Task *task, TaskInterface *iface) {
        QObject::connect(task, &Task::done, iface, &TaskInterface::reportDone,
                         Qt::SingleShotConnection);
        task->start();
    }

private:
    template <typename, typename = void>
    struct has_start : std::false_type {};
    template <typename T>
    struct has_start<T, std::void_t<decltype(std::declval<T>().start())>> : std::true_type {};
    template <typename T>
    static inline constexpr bool has_start_v = has_start<T>::value;

    static_assert(std::is_base_of_v<QObject, Task>,
                  "DefaultTaskAdapter<Task>: The Task type needs to be derived from QObject.");
    static_assert(has_start_v<Task>,
                  "DefaultTaskAdapter<Task>: The Task type needs to specify public start() method.");
};

// TODO: Allow Task = void?
template <typename Task, typename Adapter = DefaultTaskAdapter<Task>,
          typename Deleter = std::default_delete<Task>>
class CustomTask final : public ExecutableItem
{
public:
    using TaskSetupHandler = std::function<SetupResult(Task &)>;
    using TaskDoneHandler = std::function<DoneResult(const Task &, DoneWith)>;

    template <typename SetupHandler = TaskSetupHandler, typename DoneHandler = TaskDoneHandler>
    CustomTask(SetupHandler &&setup = TaskSetupHandler(), DoneHandler &&done = TaskDoneHandler(),
               CallDoneFlags callDone = CallDone::Always)
        : ExecutableItem({&taskAdapterConstructor, &taskAdapterDestructor, &taskAdapterStarter,
                          wrapSetup(std::forward<SetupHandler>(setup)),
                          wrapDone(std::forward<DoneHandler>(done)), callDone})
    {}

private:
    static_assert(std::is_default_constructible_v<Task>,
                  "CustomTask<Type, Adapter, Deleter>: The Task type needs to "
                  "be default constructible.");
    static_assert(std::is_default_constructible_v<Adapter>,
                  "CustomTask<Type, Adapter, Deleter>: The Adapter type needs to "
                  "be default constructible.");
    static_assert(std::is_invocable_v<Adapter, Task *, TaskInterface *>,
                  "CustomTask<Type, Adapter, Deleter>: The Adapter type needs to "
                  "implement public \"void operator()(Task *task, QTaskInterface *iface);\" "
                  "method.");

    friend class When;

    struct TaskAdapter {
        TaskAdapter() : task(new Task()) {}
        std::unique_ptr<Task, Deleter> task;
        Adapter adapter;
    };

    static TaskAdapter *taskAdapterConstructor() { return new TaskAdapter; }

    static void taskAdapterDestructor(TaskAdapterPtr voidAdapter) {
        delete static_cast<TaskAdapter *>(voidAdapter);
    }

    static void taskAdapterStarter(TaskAdapterPtr voidAdapter, TaskInterface *iface) {
        TaskAdapter *taskAdapter = static_cast<TaskAdapter *>(voidAdapter);
        std::invoke(taskAdapter->adapter, taskAdapter->task.get(), iface);
    }

    template <typename Handler>
    static TaskAdapterSetupHandler wrapSetup(Handler &&handler) {
        if constexpr (std::is_same_v<std::decay_t<Handler>, TaskSetupHandler>) {
            if (!handler)
                return {}; // When user passed {} for the setup handler.
        }
        // R, V stands for: Setup[R]esult, [V]oid
        static constexpr bool isR = isInvocable<SetupResult, Handler, Task &>();
        static constexpr bool isV = isInvocable<void, Handler, Task &>();
        static_assert(isR || isV,
            "Task setup handler needs to take (Task &) as an argument and has to return void or "
            "SetupResult. The passed handler doesn't fulfill these requirements.");
        return [handler = std::move(handler)](TaskAdapterPtr voidAdapter) {
            Task *task = static_cast<TaskAdapter *>(voidAdapter)->task.get();
            if constexpr (isR)
                return std::invoke(handler, *task);
            std::invoke(handler, *task);
            return SetupResult::Continue;
        };
    }

    template <typename Handler>
    static TaskAdapterDoneHandler wrapDone(Handler &&handler) {
        if constexpr (std::is_same_v<std::decay_t<Handler>, TaskDoneHandler>) {
            if (!handler)
                return {}; // User passed {} for the done handler.
        }
        static constexpr bool isDoneResultType = std::is_same_v<std::decay_t<Handler>, DoneResult>;
        // R, B, V, T, D stands for: Done[R]esult, [B]ool, [V]oid, [T]ask, [D]oneWith
        static constexpr bool isRTD = isInvocable<DoneResult, Handler, const Task &, DoneWith>();
        static constexpr bool isRT = isInvocable<DoneResult, Handler, const Task &>();
        static constexpr bool isRD = isInvocable<DoneResult, Handler, DoneWith>();
        static constexpr bool isR = isInvocable<DoneResult, Handler>();
        static constexpr bool isBTD = isInvocable<bool, Handler, const Task &, DoneWith>();
        static constexpr bool isBT = isInvocable<bool, Handler, const Task &>();
        static constexpr bool isBD = isInvocable<bool, Handler, DoneWith>();
        static constexpr bool isB = isInvocable<bool, Handler>();
        static constexpr bool isVTD = isInvocable<void, Handler, const Task &, DoneWith>();
        static constexpr bool isVT = isInvocable<void, Handler, const Task &>();
        static constexpr bool isVD = isInvocable<void, Handler, DoneWith>();
        static constexpr bool isV = isInvocable<void, Handler>();
        static_assert(isDoneResultType || isRTD || isRT || isRD || isR
                                       || isBTD || isBT || isBD || isB
                                       || isVTD || isVT || isVD || isV,
            "Task done handler needs to take (const Task &, DoneWith), (const Task &), "
            "(DoneWith) or (void) as arguments and has to return void, bool or DoneResult. "
            "Alternatively, it may be of DoneResult type. "
            "The passed handler doesn't fulfill these requirements.");
        return [handler = std::move(handler)](TaskAdapterPtr voidAdapter, DoneWith result) {
            if constexpr (isDoneResultType)
                return handler;
            Task *task = static_cast<TaskAdapter *>(voidAdapter)->task.get();
            if constexpr (isRTD)
                return std::invoke(handler, *task, result);
            if constexpr (isRT)
                return std::invoke(handler, *task);
            if constexpr (isRD)
                return std::invoke(handler, result);
            if constexpr (isR)
                return std::invoke(handler);
            if constexpr (isBTD)
                return toDoneResult(std::invoke(handler, *task, result));
            if constexpr (isBT)
                return toDoneResult(std::invoke(handler, *task));
            if constexpr (isBD)
                return toDoneResult(std::invoke(handler, result));
            if constexpr (isB)
                return toDoneResult(std::invoke(handler));
            if constexpr (isVTD)
                std::invoke(handler, *task, result);
            else if constexpr (isVT)
                std::invoke(handler, *task);
            else if constexpr (isVD)
                std::invoke(handler, result);
            else if constexpr (isV)
                std::invoke(handler);
            return toDoneResult(result == DoneWith::Success);
        };
    }
};

class TASKING_EXPORT TaskTree final : public QObject
{
    Q_OBJECT

public:
    TaskTree(QObject *parent = nullptr);
    TaskTree(const Group &recipe, QObject *parent = nullptr);
    ~TaskTree();

    void setRecipe(const Group &recipe);

    void start();
    void cancel();
    bool isRunning() const;

    // Helper methods. They execute a local event loop with ExcludeUserInputEvents.
    // The passed future is used for listening to the cancel event.
    // Don't use it in main thread. To be used in non-main threads or in auto tests.
    DoneWith runBlocking();
    DoneWith runBlocking(const QFuture<void> &future);
    static DoneWith runBlocking(const Group &recipe);
    static DoneWith runBlocking(const Group &recipe, const QFuture<void> &future);

    int asyncCount() const;
    int taskCount() const;
    int progressMaximum() const { return taskCount(); }
    int progressValue() const; // all finished / skipped / stopped tasks, groups itself excluded

    template <typename StorageStruct, typename Handler>
    void onStorageSetup(const Storage<StorageStruct> &storage, Handler &&handler) {
        static_assert(std::is_invocable_v<std::decay_t<Handler>, StorageStruct &>,
                      "Storage setup handler needs to take (Storage &) as an argument. "
                      "The passed handler doesn't fulfill this requirement.");
        setupStorageHandler(storage,
                            wrapHandler<StorageStruct>(std::forward<Handler>(handler)), {});
    }
    template <typename StorageStruct, typename Handler>
    void onStorageDone(const Storage<StorageStruct> &storage, Handler &&handler) {
        static_assert(std::is_invocable_v<std::decay_t<Handler>, const StorageStruct &>,
                      "Storage done handler needs to take (const Storage &) as an argument. "
                      "The passed handler doesn't fulfill this requirement.");
        setupStorageHandler(storage, {},
                            wrapHandler<const StorageStruct>(std::forward<Handler>(handler)));
    }

Q_SIGNALS:
    void started();
    void done(DoneWith result);
    void asyncCountChanged(int count);
    void progressValueChanged(int value); // updated whenever task finished / skipped / stopped

private:
    void setupStorageHandler(const StorageBase &storage,
                             const StorageBase::StorageHandler &setupHandler,
                             const StorageBase::StorageHandler &doneHandler);
    template <typename StorageStruct, typename Handler>
    StorageBase::StorageHandler wrapHandler(Handler &&handler) {
        return [handler](void *voidStruct) {
            auto *storageStruct = static_cast<StorageStruct *>(voidStruct);
            std::invoke(handler, *storageStruct);
        };
    }

    TaskTreePrivate *d;
};

class TaskTreeTaskAdapter final
{
public:
    TASKING_EXPORT void operator()(TaskTree *task, TaskInterface *iface);
};

class TimeoutTaskAdapter final
{
public:
    TASKING_EXPORT ~TimeoutTaskAdapter();
    TASKING_EXPORT void operator()(std::chrono::milliseconds *task, TaskInterface *iface);
private:
    std::optional<int> m_timerId;
};

using TaskTreeTask = CustomTask<TaskTree, TaskTreeTaskAdapter>;
using TimeoutTask = CustomTask<std::chrono::milliseconds, TimeoutTaskAdapter>;

TASKING_EXPORT ExecutableItem timeoutTask(const std::chrono::milliseconds &timeout,
                                          DoneResult result = DoneResult::Error);

} // namespace Tasking

QT_END_NAMESPACE

#endif // TASKING_TASKTREE_H
