Program Listing for File partition.hpp

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#pragma once

#include "htlist.hpp"
#include "predicates.hpp"
#include <type_traits>


namespace ctql {

    namespace detail {

        // partition_by_impl<Pivot, Rel, HTList<...>>
        // Produces two buckets:
        //   - pass: elements where Rel<Pivot, Elem>::value is true
        //   - fail: the remaining elements
        template <typename Pivot, template <typename, typename> class Rel, typename List>
        struct partition_by_impl;

        // Empty list -> both buckets empty.
        template <typename Pivot, template <typename, typename> class Rel>
        struct partition_by_impl<Pivot, Rel, HTList<>> {
            using pass = HTList<>;
            using fail = HTList<>;
        };

        // Step: peel head H; recurse on tail Ts...
        template <typename Pivot,
                  template <typename, typename> class Rel,
                  typename H,
                  typename... Ts>
        struct partition_by_impl<Pivot, Rel, HTList<H, Ts...>> {
        private:
            using rest = partition_by_impl<Pivot, Rel, HTList<Ts...>>;

        public:
            using pass = std::conditional_t<
                Rel<Pivot, H>::value,
                typename append<HTList<H>, typename rest::pass>::type,
                typename rest::pass>;

            using fail = std::conditional_t<
                Rel<Pivot, H>::value,
                typename rest::fail,
                typename append<HTList<H>, typename rest::fail>::type>;
        };

    } // namespace detail

    template <typename Pivot, template <typename, typename> class Rel, typename... Ts>
    struct partition_by {
        using impl   = detail::partition_by_impl<Pivot, Rel, detail::HTList<Ts...>>;
        using pass   = typename impl::pass; // kept / left bucket
        using fail   = typename impl::fail; // rejected / right bucket
        using concat = typename detail::HTList<pass, fail>; // pair as a two-element typelist
    };

    template <typename Pivot, template <typename, typename> class Rel, typename... Ts>
    using filter_by = typename partition_by<Pivot, Rel, Ts...>::pass;

    template <typename Pivot, template <typename, typename> class Rel, typename... Ts>
    using reject_if_by = typename partition_by<Pivot, Rel, Ts...>::fail;

    template <typename Pivot,
              template <typename, typename> class Rel,
              template <typename> class KeyOf = Size,
              typename... Ts>
    using partition_by_key = partition_by<Pivot, Rel, KeyOf<Ts>...>;

} // namespace ctql