Crafter.DB/interfaces/Crafter.DB-Reflect.cppm

// SPDX-License-Identifier: LGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// Compile-time introspection of aggregates without P2996 (clang 23 has no
// <meta>). Three ingredients, all verified on the toolchain this library
// targets: structured-binding packs give member count and per-member access,
// std::source_location::function_name() leaks the member name when a pointer
// to a member of an extern object is used as a template argument, and address
// comparison on a constexpr instance maps a pointer-to-member to its index.
// When a clang ships <meta>, this partition is the only thing to replace.
export module Crafter.DB:Reflect;
import std;

namespace Crafter::DB::Reflect {
    template<class T>
    struct CountHelper {
        template<class U>
        static consteval auto Count(U& u) {
            auto&& [...m] = u;
            return std::integral_constant<std::size_t, sizeof...(m)>{};
        }
        static constexpr std::size_t value = decltype(Count(std::declval<T&>()))::value;
    };

    // A constant-initialised instance whose member addresses are turned into
    // text by function_name(). Only its address is ever used, in a constant
    // expression, so nothing reads or writes it at run time; no_destroy keeps
    // the compiler from registering a destructor for it, which lets the linker
    // drop the object entirely. (An undefined `extern` declaration would be
    // cheaper still, but clang warns about it at every instantiation.)
    template<class T>
    [[clang::no_destroy]] inline const T external{};

    template<auto P>
    consteval std::string_view RawName() {
        return std::source_location::current().function_name();
    }

    template<class T>
    consteval std::string_view RawTypeName() {
        return std::source_location::current().function_name();
    }
}

export namespace Crafter::DB {
    // True for class types whose non-static data members can be bound by a
    // structured binding: public members, no bases carrying members, no
    // user-declared constructors. Tables and nested column groups must be this.
    template<class T>
    concept Aggregate = std::is_aggregate_v<T> && !std::is_array_v<T> && requires(T& t) { Reflect::CountHelper<T>::value; };

    template<Aggregate T>
    inline constexpr std::size_t memberCount = Reflect::CountHelper<T>::value;

    // Reference to the I-th member of an aggregate lvalue.
    template<std::size_t I, class T>
    constexpr decltype(auto) Member(T&& t) noexcept {
        auto&& [...m] = t;
        return (m...[I]);
    }

    template<std::size_t I, class T>
    using MemberType = std::remove_cvref_t<decltype(Member<I>(std::declval<T&>()))>;

    // The declared name of the I-th member, e.g. "iban". Parsed out of
    // "std::string_view Crafter::DB::Reflect::RawName() [P = &external.iban]".
    template<std::size_t I, Aggregate T>
    consteval std::string_view MemberName() {
        constexpr std::string_view raw = Reflect::RawName<&Member<I>(Reflect::external<T>)>();
        const std::size_t end = raw.rfind(']');
        const std::size_t dot = raw.rfind('.', end);
        return raw.substr(dot + 1, end - dot - 1);
    }

    // The type's qualified name as the compiler spells it ("Nostro::Transaction").
    // Used in diagnostics only, so the exact spelling is not load-bearing.
    template<class T>
    consteval std::string_view TypeName() {
        constexpr std::string_view raw = Reflect::RawTypeName<T>();
        const std::size_t end = raw.rfind(']');
        const std::size_t eq = raw.rfind("T = ", end);
        return raw.substr(eq + 4, end - eq - 4);
    }

    template<class Ptr>
    struct MemberPointerTraits;

    template<class C, class M>
    struct MemberPointerTraits<M C::*> {
        using Class = C;
        using Type = M;
    };

    template<auto Ptr>
    using MemberClass = typename MemberPointerTraits<decltype(Ptr)>::Class;

    template<auto Ptr>
    using MemberTypeOf = typename MemberPointerTraits<decltype(Ptr)>::Type;

    inline constexpr std::size_t noMember = static_cast<std::size_t>(-1);

    // Equality of two pointers-to-member that may point into different member
    // types (comparing those directly is ill-formed): false unless the types
    // match and the pointers are equal.
    template<auto A, auto B>
    consteval bool SameMemberPointer() {
        if constexpr (std::is_same_v<std::remove_cv_t<decltype(A)>, std::remove_cv_t<decltype(B)>>) return A == B;
        else return false;
    }

    // Index of the member a pointer-to-member designates, or noMember. Works
    // by comparing the address of each bound member with the address the
    // pointer selects on the same constexpr instance.
    template<Aggregate T, auto Ptr>
    consteval std::size_t MemberIndex() {
        T t{};
        std::size_t index = noMember;
        [&]<std::size_t... I>(std::index_sequence<I...>) {
            ((void)[&] {
                if constexpr (std::is_same_v<MemberType<I, T>, std::remove_cvref_t<decltype(t.*Ptr)>>) {
                    if (std::addressof(Member<I>(t)) == std::addressof(t.*Ptr)) index = I;
                }
            }(), ...);
        }(std::make_index_sequence<memberCount<T>>{});
        return index;
    }

    // Runtime counterpart of MemberIndex for a pointer that is not a template
    // argument. Every call site passes a literal `&T::member`, and the probe
    // object is constant-initialised (no guard to check), so after inlining the
    // comparison chain folds to a constant.
    template<Aggregate T, class M>
    std::size_t MemberIndexOf(M T::* ptr) noexcept {
        const T& instance = Reflect::external<T>;
        std::size_t index = noMember;
        [&]<std::size_t... I>(std::index_sequence<I...>) {
            ((void)[&] {
                if constexpr (std::is_same_v<MemberType<I, T>, M>) {
                    if (std::addressof(Member<I>(instance)) == std::addressof(instance.*ptr)) index = I;
                }
            }(), ...);
        }(std::make_index_sequence<memberCount<T>>{});
        return index;
    }

    // Calls f.template operator()<I>(member) for every member, in order.
    template<Aggregate T, class F>
    constexpr void ForEachMember(T& t, F&& f) {
        [&]<std::size_t... I>(std::index_sequence<I...>) {
            (f.template operator()<I>(Member<I>(t)), ...);
        }(std::make_index_sequence<memberCount<T>>{});
    }
}

// Self-tests: a compiler whose function_name() text changes shape fails here,
// at the library's build, instead of producing garbage column names.
namespace Crafter::DB::Reflect {
    struct Probe {
        std::int32_t alpha;
        double beta;
        std::string gamma;
        std::chrono::system_clock::time_point delta;
    };
    static_assert(Aggregate<Probe>);
    static_assert(memberCount<Probe> == 4);
    static_assert(MemberName<0, Probe>() == "alpha");
    static_assert(MemberName<2, Probe>() == "gamma");
    static_assert(MemberName<3, Probe>() == "delta");
    static_assert(MemberIndex<Probe, &Probe::gamma>() == 2);
    static_assert(MemberIndex<Probe, &Probe::delta>() == 3);
    static_assert(std::is_same_v<MemberType<1, Probe>, double>);
    static_assert(std::is_same_v<MemberTypeOf<&Probe::gamma>, std::string>);
    static_assert(TypeName<Probe>().ends_with("Probe"));
}