geode/loader/include/Geode/platform/ItaniumCast.hpp

#pragma once

namespace geode::cast {
    using uinthalf_t = uint32_t;
    using inthalf_t = int32_t;

    struct DummyClass {
        virtual ~DummyClass() {}
    };

    struct DummySingleClass : DummyClass {};

    struct DummyClass2 {};

    struct DummyMultipleClass : DummySingleClass, DummyClass2 {};

    struct ClassTypeinfoType {
        void** m_typeinfoVtable;
        char const* m_typeinfoName;
    };

    struct SingleClassTypeinfoType : ClassTypeinfoType {
        ClassTypeinfoType* m_baseClassTypeinfo;
    };

#pragma pack(push, 1)

    struct MultipleClassSingleEntryType {
        ClassTypeinfoType* m_baseClassTypeinfo;
        uint8_t m_visibilityFlag;
        inthalf_t m_offset;
        uint8_t m_padding[sizeof(inthalf_t) - 1];
    };

#pragma pack(pop)

    struct MultipleClassTypeinfoType : ClassTypeinfoType {
        uint32_t m_flags;
        uint32_t m_numBaseClass;
        MultipleClassSingleEntryType m_baseClasses[0x100];
    };

    struct VtableTypeinfoType {
        inthalf_t m_offset;
        ClassTypeinfoType* m_typeinfo;
    };

    struct VtableType {
        void* m_vtable[0x100];
    };

    struct CompleteVtableType : VtableTypeinfoType, VtableType {};

    inline void** typeinfoVtableOf(void* ptr) {
        auto vftable = *reinterpret_cast<VtableType**>(ptr);

        auto typeinfoPtr =
            static_cast<VtableTypeinfoType*>(static_cast<CompleteVtableType*>(vftable));

        return typeinfoPtr->m_typeinfo->m_typeinfoVtable;
    }

    inline void* traverseTypeinfoFor(
        void* ptr, ClassTypeinfoType const* typeinfo, char const* afterIdent
    ) {
        DummySingleClass dummySingleClass;
        DummyMultipleClass dummyMultipleClass;

        {
            auto optionIdent = typeinfo->m_typeinfoName;
            if (std::strcmp(optionIdent, afterIdent) == 0) {
                return ptr;
            }
        }
        if (typeinfo->m_typeinfoVtable == typeinfoVtableOf(&dummySingleClass)) {
            auto siTypeinfo = static_cast<SingleClassTypeinfoType const*>(typeinfo);
            return traverseTypeinfoFor(ptr, siTypeinfo->m_baseClassTypeinfo, afterIdent);
        }
        else if (typeinfo->m_typeinfoVtable == typeinfoVtableOf(&dummyMultipleClass)) {
            auto vmiTypeinfo = static_cast<MultipleClassTypeinfoType const*>(typeinfo);
            for (int i = 0; i < vmiTypeinfo->m_numBaseClass; ++i) {
                auto& entry = vmiTypeinfo->m_baseClasses[i];
                auto optionPtr = reinterpret_cast<std::byte*>(ptr) + entry.m_offset;
                auto ret = traverseTypeinfoFor(optionPtr, entry.m_baseClassTypeinfo, afterIdent);
                if (ret != nullptr) return ret;
            }
        }

        return nullptr;
    }

    inline void* typeinfoCastInternal(void* ptr, ClassTypeinfoType const* beforeTypeinfo, ClassTypeinfoType const* afterTypeinfo, size_t hint) {
        // we're not using either because uhhh idk
        // hint is for diamond inheritance iirc which is never 
        // used in gd, so should be pretty safe to ignore
        if (!ptr) {
            return nullptr;
        }

        (void)beforeTypeinfo;
        (void)hint;

        auto vftable = *reinterpret_cast<VtableType**>(ptr);
        auto dataPointer = static_cast<VtableTypeinfoType*>(static_cast<CompleteVtableType*>(vftable));
        auto typeinfo = dataPointer->m_typeinfo;
        auto basePtr = static_cast<std::byte*>(ptr) + dataPointer->m_offset;

        auto afterIdent = afterTypeinfo->m_typeinfoName;

        return traverseTypeinfoFor(basePtr, typeinfo, afterIdent);
    }

    template <class After, class Before>
    inline After typeinfo_cast(Before ptr) {
        static_assert(
            std::is_polymorphic_v<std::remove_pointer_t<Before>> && std::is_polymorphic_v<std::remove_pointer_t<After>>, 
            "Input is not a polymorphic type"
        );
        if (!ptr) {
            return static_cast<After>(nullptr);
        }

        auto beforeTypeinfo = reinterpret_cast<ClassTypeinfoType const*>(&typeid(std::remove_pointer_t<Before>));
        auto afterTypeinfo = reinterpret_cast<ClassTypeinfoType const*>(&typeid(std::remove_pointer_t<After>));
        return static_cast<After>(typeinfoCastInternal(ptr, beforeTypeinfo, afterTypeinfo, 0));
    }
}