geode/loader/include/Geode/modify/Field.hpp

#pragma once

#include "../utils/MiniFunction.hpp"
#include "Traits.hpp"

#include <Geode/loader/Loader.hpp>
#include <cocos2d.h>
#include <vector>

namespace cocos2d {
    class CCNode;
}

namespace geode {
    template <class, class>
    class Modify;
}

namespace geode::modifier {
    class FieldContainer {
    private:
        std::vector<void*> m_containedFields;
        std::vector<utils::MiniFunction<void(void*)>> m_destructorFunctions;

    public:
        ~FieldContainer() {
            for (auto i = 0u; i < m_containedFields.size(); i++) {
                if (m_destructorFunctions[i] && m_containedFields[i]) {
                    m_destructorFunctions[i](m_containedFields[i]);
                    operator delete(m_containedFields[i]);
                }
            }
        }

        void* getField(size_t index) {
            while (m_containedFields.size() <= index) {
                m_containedFields.push_back(nullptr);
                m_destructorFunctions.push_back(nullptr);
            }
            return m_containedFields.at(index);
        }

        void* setField(size_t index, size_t size, utils::MiniFunction<void(void*)> destructor) {
            m_containedFields.at(index) = operator new(size);
            m_destructorFunctions.at(index) = destructor;
            return m_containedFields.at(index);
        }

        static FieldContainer* from(cocos2d::CCNode* node, char const* forClass) {
            return node->getFieldContainer(forClass);
        }
    };

    GEODE_DLL size_t getFieldIndexForClass(char const* name);

    template <class Parent>
    concept HasFields = requires {
        typename Parent::Fields;
    };

    template <class Parent, class Base>
    class FieldIntermediate {
        using Intermediate = Modify<Parent, Base>;
        // Padding used for guaranteeing any member of parents
        // will be in between sizeof(Intermediate) and sizeof(Parent)
        alignas(std::max(alignof(Base), alignof(uintptr_t))) uintptr_t m_padding;

    public:
        // the constructor that constructs the fields.
        // we construct the Parent first,
        static void fieldConstructor(void* offsetField) {
            if constexpr (HasFields<Parent>) {
                (void) new (offsetField) typename Parent::Fields();
            }
            else {
                std::array<std::byte, sizeof(Parent)> parentContainer;

                auto parent = new (parentContainer.data()) Parent();

                parent->Intermediate::~Intermediate();

                std::memcpy(
                    offsetField,
                    std::launder(&parentContainer[sizeof(Intermediate)]),
                    sizeof(Parent) - sizeof(Intermediate)
                );   
            }
        }

        static void fieldDestructor(void* offsetField) {
            if constexpr (HasFields<Parent>) {
                static_cast<typename Parent::Fields*>(offsetField)->~Fields();
            }
            else {
                std::array<std::byte, sizeof(Parent)> parentContainer;

                auto parent = new (parentContainer.data()) Intermediate();

                std::memcpy(
                    std::launder(&parentContainer[sizeof(Intermediate)]),
                    offsetField,
                    sizeof(Parent) - sizeof(Intermediate)
                );

                static_cast<Parent*>(parent)->Parent::~Parent();
            }
        }

        auto self() {
            if constexpr (HasFields<Parent>) {
                // get the this pointer of the base
                // field intermediate is the first member of Modify
                // meaning we canget the base from ourself
                auto node = reinterpret_cast<Parent*>(reinterpret_cast<std::byte*>(this) - sizeof(Base));
                static_assert(sizeof(Base) == offsetof(Parent, m_fields), "offsetof not correct");

                // generating the container if it doesn't exist
                auto container = FieldContainer::from(node, typeid(Base).name());

                // the index is global across all mods, so the
                // function is defined in the loader source
                static size_t index = getFieldIndexForClass(typeid(Base).name());

                // the fields are actually offset from their original
                // offset, this is done to save on allocation and space
                auto offsetField = container->getField(index);
                if (!offsetField) {
                    offsetField = container->setField(
                        index, sizeof(typename Parent::Fields), &FieldIntermediate::fieldDestructor
                    );

                    FieldIntermediate::fieldConstructor(offsetField);
                }

                return reinterpret_cast<typename Parent::Fields*>(offsetField);
            }
            else {
                static_assert(!HasFields<Parent>, "Parent must have a Fields struct");
            }
        }

        auto operator->() {
            // workaround for "static assertion is not an integral constant expression" in CLion
            // while the solution in https://github.com/microsoft/STL/issues/3311 works, you can't provide
            // cli args to clang-tidy in clion, so we use this workaround instead
            // https://youtrack.jetbrains.com/issue/CPP-27446/spurious-offsetof-in-staticassert-error-from-clangd#focus=Comments-27-8172811.0-0
            // update: that workaround didn't work,
            // undefining and re-defining offsetof caused another error further down
            // so we're doing this now
#ifdef __CLION_IDE__
            return reinterpret_cast<typename Parent::Fields*>(69420);
#else
            return this->self();
#endif
        }
    };

}