scheduler.h

/* ScummVM - Graphic Adventure Engine
 *
 * ScummVM is the legal property of its developers, whose names
 * are too numerous to list here. Please refer to the COPYRIGHT
 * file distributed with this source distribution.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#ifndef ALCACHOFA_SCHEDULER_H
#define ALCACHOFA_SCHEDULER_H

#include "alcachofa/common.h"

#include "common/stack.h"
#include "common/str.h"
#include "common/type_traits.h"

namespace Alcachofa {

/* Tasks are generally written as coroutines however the common coroutines
 * cannot be used for two reasons:
 * 1. The scheduler is too limited in managing when to run what coroutines
 *    E.g. for the inventory/menu we need to pause a set of coroutines and
 *    continue them later on
 * 2. We need to save and load the state of coroutines
 *    For this we either write the state machine ourselves or we use
 *    the following careful macros where the state ID is explicitly written
 *    This way it is stable and if it has to change we can migrate
 *    savestates upon loading.
 *
 * Tasks are usually private, so in order to load them they:
 *   - need a constructor MyPrivateTask(Process &, Serializer &)
 *   - need call the macro DECLARE_TASK(MyPrivateTask)
 *   - they have to listed in tasks.h
 */

struct Task;
class Process;
class ObjectBase;

enum class TaskReturnType {
    Yield,
    Finished,
    Waiting
};

struct TaskReturn {
    static inline TaskReturn yield() { return {}; }
    static TaskReturn finish(int32 returnValue);
    static TaskReturn waitFor(Task *task);

    inline TaskReturnType type() const { return _type; }
    inline int32 returnValue() const {
        assert(_type == TaskReturnType::Finished);
        return _returnValue;
    }
    inline Task *taskToWaitFor() const {
        assert(_type == TaskReturnType::Waiting);
        return _taskToWaitFor;
    }

private:
    TaskReturn();
    TaskReturnType _type;
    union {
        int32 _returnValue;
        Task *_taskToWaitFor;
    };
};

struct Task {
    Task(Process &process);
    virtual ~Task() {}
    virtual TaskReturn run() = 0;
    virtual void debugPrint() = 0;
    virtual void syncGame(Common::Serializer &s);
    virtual const char *taskName() const = 0; // implemented by DECLARE_TASK

    inline Process &process() const { return _process; }

protected:
    Task *delay(uint32 millis);

    void syncObjectAsString(Common::Serializer &s, ObjectBase *&object, bool optional = false) const;
    template<class TObject>
    void syncObjectAsString(Common::Serializer &s, TObject *&object, bool optional = false) const {
        // We could add is_const and therefore true_type, false_type, integral_constant 
        // or we could just use const_cast and promise that we won't modify the object itself
        ObjectBase *base = const_cast<Common::remove_const_t<TObject> *>(object);
        syncObjectAsString(s, base, optional);
        object = dynamic_cast<TObject *>(base);
        if (object == nullptr && base != nullptr)
            errorForUnexpectedObjectType(base);
    }

    uint32 _stage = 0;
private:
    void errorForUnexpectedObjectType(const ObjectBase *base) const;

    Process &_process;
};

// implemented in alcachofa.cpp to prevent a compiler warning when
// the declaration of the construct function comes after the definition
struct DelayTask final : public Task {
    DelayTask(Process &process, uint32 millis);
    DelayTask(Process &process, Common::Serializer &s);
    TaskReturn run() override;
    void debugPrint() override;
    void syncGame(Common::Serializer &s) override;
    const char *taskName() const override;

private:
    uint32 _endTime = 0;
};

#define DECLARE_TASK(TaskName) \
    extern Task *constructTask_##TaskName(Process &process, Serializer &s) { \
        return new TaskName(process, s); \
    } \
    const char *TaskName::taskName() const { \
        return #TaskName; \
    }

#define TASK_BEGIN \
    switch(_stage) { \
    case 0:; \

#define TASK_END \
    TASK_RETURN(0); \
    default: assert(false && "Invalid line in task"); \
    } return TaskReturn::finish(0)

#define TASK_INTERNAL_BREAK(stage, ret) \
    do { \
        _stage = stage; \
        return ret; \
        case stage:; \
    } while(0)

#define TASK_YIELD(stage) TASK_INTERNAL_BREAK((stage), TaskReturn::yield())
#define TASK_WAIT(stage, task) TASK_INTERNAL_BREAK((stage), TaskReturn::waitFor(task))

#define TASK_RETURN(value) \
    do { \
        _stage = UINT_MAX; \
        return TaskReturn::finish(value); \
    } while(0)

using ProcessId = uint32;
class Process {
public:
    Process(ProcessId pid, MainCharacterKind characterKind);
    Process(Common::Serializer &s);
    ~Process();

    inline ProcessId pid() const { return _pid; }
    inline MainCharacterKind &character() { return _character; } // is changed in changeCharacter
    inline MainCharacterKind character() const { return _character; }
    inline int32 returnValue() const { return _lastReturnValue; }
    inline Common::String &name() { return _name; }
    bool isActiveForPlayer() const; 

    TaskReturnType run();
    void debugPrint();
    void syncGame(Common::Serializer &s);

private:
    friend class Scheduler;
    ProcessId _pid;
    MainCharacterKind _character;
    Common::Stack<Task *> _tasks;
    Common::String _name;
    int32 _lastReturnValue = 0;
};

class Scheduler {
public:
    ~Scheduler();

    void run();
    void backupContext();
    void restoreContext();
    void killAllProcesses();
    void killAllProcessesFor(MainCharacterKind characterKind);
    void killProcessByName(const Common::String &name);
    bool hasProcessWithName(const Common::String &name);
    void debugPrint();
    void prepareSyncGame(Common::Serializer &s);
    void syncGame(Common::Serializer &s);

    template<typename TTask, typename... TaskArgs>
    Process *createProcess(MainCharacterKind character, TaskArgs&&... args) {
        Process *process = createProcessInternal(character);
        process->_tasks.push(new TTask(*process, Common::forward<TaskArgs>(args)...));
        return process;
    }

private:
    Process *createProcessInternal(MainCharacterKind character);

    inline Common::Array<Process *> &processesToRun() { return _processArrays[_currentArrayI]; }
    inline Common::Array<Process *> &processesToRunNext() { return _processArrays[!_currentArrayI]; }
    Common::Array<Process *> _processArrays[2];
    Common::Array<Process *> _backupProcesses;
    uint8 _currentArrayI = 0;
    ProcessId _nextPid = 1;
    uint _currentProcessI = UINT_MAX;

};

}

#endif // ALCACHOFA_SCHEDULER_H