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#pragma once
// ctql/match.hpp

#include <type_traits>


namespace ctql {

    template <auto K, class T>
    struct case_ {
        static constexpr auto key = K;
        using type                = T;
    };

    template <class T>
    struct default_ {
        using type = T;
    };

    namespace detail {
        // -------- matcher core (first-match wins; default if no match) --------
        template <auto Key, class Default, bool Matched, class... Alts>
        struct match_impl;

        // End of list -> yield accumulated Default.
        template <auto Key, class Default, bool Matched>
        struct match_impl<Key, Default, Matched> {
            using type = Default;
        };

        // case_ step: if not matched yet and Key == K, lock in T; otherwise keep going.
        template <auto Key, class Default, bool Matched, auto K, class T, class... Rest>
        struct match_impl<Key, Default, Matched, case_<K, T>, Rest...>
            : match_impl<
                  Key,
                  std::conditional_t<!Matched && (Key == K), T, Default>,
                  (Matched || (Key == K)),
                  Rest...> { };

        // default_ step: if not matched yet, update fallback; otherwise ignore.
        template <auto Key, class Default, bool Matched, class T, class... Rest>
        struct match_impl<Key, Default, Matched, default_<T>, Rest...>
            : match_impl<Key, std::conditional_t<Matched, Default, T>, Matched, Rest...> { };

        // Public interface wrapper.
        template <auto Key, class... Alts>
        struct match {
            using type = typename match_impl<Key, void, false, Alts...>::type;
        };
    } // namespace detail

    template <auto Key, class... Alts>
    using match_t = typename detail::match<Key, Alts...>::type;

} // namespace ctql