zip.h

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#pragma once
 
#include "di/container/view/empty.h"
#include "di/container/view/zip_view.h"
#include "di/function/tag_invoke.h"
 
namespace di::container::view {
namespace detail {
    struct ZipFunction;
 
    template<typename... Cons>
    concept CustomZip = concepts::TagInvocable<ZipFunction, Cons...>;
 
    template<typename... Cons>
    concept EmptyZip = sizeof...(Cons) == 0;
 
    template<typename... Cons>
    concept ViewZip =
        requires(Cons&&... containers) { ZipView<meta::AsView<Cons>...>(util::forward<Cons>(containers)...); };
    concept ViewZip = {…}
 
    struct ZipFunction {
        template<concepts::ViewableContainer... Cons>
        requires(CustomZip<Cons...> || EmptyZip<Cons...> || ViewZip<Cons...>)
        constexpr auto operator()(Cons&&... containers) const -> concepts::View auto {
            if constexpr (CustomZip<Cons...>) {
                return function::tag_invoke(*this, util::forward<Cons>(containers)...);
            } else if constexpr (EmptyZip<Cons...>) {
                return empty<Tuple<>>;
            } else {
                return ZipView<meta::AsView<Cons>...>(util::forward<Cons>(containers)...);
            }
        }
        constexpr auto operator()(Cons&&... containers) const -> concepts::View auto {…}
    };
    struct ZipFunction {…};
}
 
constexpr inline auto zip = detail::ZipFunction {};
}
 
namespace di {
using view::zip;
}