// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "UObject/GCObject.h"
#include "Templates/EnableIf.h"
#include "Templates/PointerIsConvertibleFromTo.h"
#include "Templates/UniquePtr.h"

namespace UE4StrongObjectPtr_Private
{
class FInternalReferenceCollector: public FGCObject
{
public:
    explicit FInternalReferenceCollector(const volatile UObject* InObject)
        : Object(InObject)
    {
        check(IsInGameThread());
    }

    virtual ~FInternalReferenceCollector()
    {
        check(IsInGameThread() || IsInGarbageCollectorThread());
    }

    bool IsValid() const
    {
        return Object != nullptr;
    }

    template <typename UObjectType>
    FORCEINLINE UObjectType* GetAs() const
    {
        return (UObjectType*)Object;
    }

    FORCEINLINE void Set(const volatile UObject* InObject)
    {
        Object = InObject;
    }

    //~ FGCObject interface
    virtual void AddReferencedObjects(FReferenceCollector& Collector) override
    {
        Collector.AddReferencedObject(Object);
    }

    virtual FString GetReferencerName() const override
    {
        return "UE4StrongObjectPtr_Private::FInternalReferenceCollector";
    }

private:
    const volatile UObject* Object;
};
}  // namespace UE4StrongObjectPtr_Private

/**
 * Specific implementation of FGCObject that prevents a single UObject-based pointer from being GC'd while this guard is in scope.
 * @note This is the "full-fat" version of FGCObjectScopeGuard which uses a heap-allocated FGCObject so *can* safely be used with containers that treat types as trivially relocatable.
 */
template <typename ObjectType>
class TStrongObjectPtr
{
public:
    TStrongObjectPtr(TStrongObjectPtr&& InOther)            = default;
    TStrongObjectPtr& operator=(TStrongObjectPtr&& InOther) = default;
    ~TStrongObjectPtr()                                     = default;

    FORCEINLINE_DEBUGGABLE TStrongObjectPtr(TYPE_OF_NULLPTR = nullptr)
    {
        static_assert(TPointerIsConvertibleFromTo<ObjectType, const volatile UObject>::Value, "TStrongObjectPtr can only be constructed with UObject types");
    }

    FORCEINLINE_DEBUGGABLE explicit TStrongObjectPtr(ObjectType* InObject)
    {
        static_assert(TPointerIsConvertibleFromTo<ObjectType, const volatile UObject>::Value, "TStrongObjectPtr can only be constructed with UObject types");
        Reset(InObject);
    }

    FORCEINLINE_DEBUGGABLE TStrongObjectPtr(const TStrongObjectPtr& InOther)
    {
        Reset(InOther.Get());
    }

    template <
        typename OtherObjectType,
        typename = decltype(ImplicitConv<ObjectType*>((OtherObjectType*)nullptr))>
    FORCEINLINE_DEBUGGABLE TStrongObjectPtr(const TStrongObjectPtr<OtherObjectType>& InOther)
    {
        Reset(InOther.Get());
    }

    FORCEINLINE_DEBUGGABLE TStrongObjectPtr& operator=(const TStrongObjectPtr& InOther)
    {
        Reset(InOther.Get());
        return *this;
    }

    template <
        typename OtherObjectType,
        typename = decltype(ImplicitConv<ObjectType*>((OtherObjectType*)nullptr))>
    FORCEINLINE_DEBUGGABLE TStrongObjectPtr& operator=(const TStrongObjectPtr<OtherObjectType>& InOther)
    {
        Reset(InOther.Get());
        return *this;
    }

    FORCEINLINE_DEBUGGABLE ObjectType& operator*() const
    {
        check(IsValid());
        return *Get();
    }

    FORCEINLINE_DEBUGGABLE ObjectType* operator->() const
    {
        check(IsValid());
        return Get();
    }

    FORCEINLINE_DEBUGGABLE bool IsValid() const
    {
        return ReferenceCollector && ReferenceCollector->IsValid();
    }

    FORCEINLINE_DEBUGGABLE explicit operator bool() const
    {
        return IsValid();
    }

    FORCEINLINE_DEBUGGABLE ObjectType* Get() const
    {
        return ReferenceCollector ? ReferenceCollector->GetAs<ObjectType>() : nullptr;
    }

    FORCEINLINE_DEBUGGABLE void Reset(ObjectType* InNewObject = nullptr)
    {
        if (ReferenceCollector)
        {
            ReferenceCollector->Set(InNewObject);
        }
        else if (InNewObject)
        {
            ReferenceCollector = MakeUnique<UE4StrongObjectPtr_Private::FInternalReferenceCollector>(InNewObject);
        }
    }

    FORCEINLINE_DEBUGGABLE friend uint32 GetTypeHash(const TStrongObjectPtr& InStrongObjectPtr)
    {
        return GetTypeHash(InStrongObjectPtr.Get());
    }

private:
    TUniquePtr<UE4StrongObjectPtr_Private::FInternalReferenceCollector> ReferenceCollector;
};

template <typename LHSObjectType, typename RHSObjectType>
FORCEINLINE bool operator==(const TStrongObjectPtr<LHSObjectType>& InLHS, const TStrongObjectPtr<RHSObjectType>& InRHS)
{
    return InLHS.Get() == InRHS.Get();
}

template <typename LHSObjectType, typename RHSObjectType>
FORCEINLINE bool operator!=(const TStrongObjectPtr<LHSObjectType>& InLHS, const TStrongObjectPtr<RHSObjectType>& InRHS)
{
    return InLHS.Get() != InRHS.Get();
}