AuroraRuntime/Include/Aurora/Utility/DestructionWatch.hpp

/***
    Copyright (C) 2024 Jamie Reece Wilson (a/k/a "Reece"). All rights reserved.

    File: DestructionWatch.hpp
    Date: 2024-01-22
    Author: Reece
***/
#pragma once

namespace Aurora::Utility
{
    struct ADestructionWatcher;

    struct DestructionWatch
    {
        inline DestructionWatch();
        inline ~DestructionWatch();

        inline void RemoveAll();

    private:
        friend struct ADestructionWatcher;

        Threading::Waitables::FutexWaitable mutex;
        Threading::Waitables::FutexWaitable dtorSignal;
        ADestructionWatcher *pFirstWatcher {};
        AuList<ADestructionWatcher *> listpWatches;

        inline void RemoveWatcher(ADestructionWatcher *pWatcher);
        inline void RemoveWatcherCall(ADestructionWatcher *pWatcher);
    };

    struct ADestructionWatcher
    {
        inline ADestructionWatcher();
        inline ~ADestructionWatcher();

        inline void Observe(DestructionWatch *pWatch);
        inline void Observe(DestructionWatch &pWatch);
        inline bool IsObservedAlive();
        inline bool IsObservedDead();

    protected:
        inline virtual void OnDestroyedChild()
        {

        }

    private:
        friend struct DestructionWatch;
        friend class Threading::LockGuard<ADestructionWatcher *>;

        inline void OnSelfDTOR();
        inline void OnDTOR();
        inline void Lock();
        inline void Unlock();

    public:
        template <typename T, typename ...Args>
        AuConditional_t<AuIsSame_v<AuResultOf_t<T, Args...>, void>, bool, AuOptional<AuResultOf_t<T, Args...>>>
            DoUnderLock(const T &callable, Args && ...args)
        {
            AU_LOCK_GUARD(this);
            
            if (this->pOwnsLock)
            {
                if constexpr (AuIsSame_v<AuResultOf_t<T, Args...>, void>)
                {
                    callable(AuForward<Args &&>(args)...);
                    return true;
                }
                else
                {
                    return callable(AuForward<Args &&>(args)...);
                }
            }
            else
            {
                if constexpr (AuIsSame_v<AuResultOf_t<T, Args...>, void>)
                {
                    return false;
                }
                else
                {
                    return AuOptional<AuResultOf_t<T, Args...>> {};
                }
            }
        }

    private:
        Threading::Waitables::FutexWaitable selfLock;
        DestructionWatch *pParent {};
        Threading::Waitables::FutexWaitable *pOwnsLock {};
    };

    DestructionWatch::DestructionWatch()
    {

    }

    DestructionWatch::~DestructionWatch()
    {
        this->RemoveAll();
    }

    void DestructionWatch::RemoveAll()
    {
        {
            AU_LOCK_GUARD(this->mutex);

            if (auto pWatcher = AuExchange(this->pFirstWatcher, nullptr))
            {
                this->RemoveWatcherCall(pWatcher);
            }

            for (const auto pWatcher : AuExchange(this->listpWatches, {}))
            {
                this->RemoveWatcherCall(pWatcher);
            }
        }

        {
            AU_LOCK_GUARD(this->dtorSignal);
        }
    }

    void DestructionWatch::RemoveWatcher(ADestructionWatcher *pWatcher)
    {
        AU_LOCK_GUARD(this->mutex);

        if (this->pFirstWatcher == pWatcher)
        {
            this->pFirstWatcher = nullptr;
            return;
        }

        AuTryRemove(this->listpWatches, pWatcher);
    }

    void DestructionWatch::RemoveWatcherCall(ADestructionWatcher *pWatcher)
    {
        pWatcher->OnDTOR();
    }

    ADestructionWatcher::ADestructionWatcher()
    {

    }

    ADestructionWatcher::~ADestructionWatcher()
    {
        this->OnSelfDTOR();
    }

    void ADestructionWatcher::Observe(DestructionWatch *pWatch)
    {
        AU_LOCK_GUARD(pWatch->mutex);
        if (!pWatch->pFirstWatcher)
        {
            pWatch->pFirstWatcher = this;
        }
        else
        {
            pWatch->listpWatches.push_back(this);
        }
        this->pParent = pWatch;
    }

    void ADestructionWatcher::Observe(DestructionWatch &pWatch)
    {
        this->Observe(&pWatch);
    }

    bool ADestructionWatcher::IsObservedAlive()
    {
        return bool(this->pParent);
    }

    bool ADestructionWatcher::IsObservedDead()
    {
        return !bool(this->pParent);
    }
    
    void ADestructionWatcher::OnSelfDTOR()
    {
        AU_LOCK_GUARD(this->selfLock);

        if (auto pParent = AuExchange(this->pParent, nullptr))
        {
            pParent->RemoveWatcher(this);
        }
    }

    void ADestructionWatcher::OnDTOR()
    {
        {
            AU_LOCK_GUARD(this->selfLock);
            this->pParent = nullptr;
        }

        this->OnDestroyedChild();
    }

    void ADestructionWatcher::Lock()
    {
        AU_LOCK_GUARD(this->selfLock);

        this->pOwnsLock = nullptr;

        if (!this->pParent)
        {
            return;
        }

        this->pParent->dtorSignal.Lock();

        // noting that once we leave this scope, we could have our parent stolen from us (this->selfLock), but its' destructor will still be blocked until we finish
        // this is why we end up using a raw pointer here.
        this->pOwnsLock = &this->pParent->dtorSignal;
    }

    void ADestructionWatcher::Unlock()
    {
        if (this->pOwnsLock)
        {
            this->pOwnsLock->Unlock();
        }
    }
}