concat.h

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#pragma once
 
#include "di/container/view/concat_view.h"
#include "di/container/view/empty.h"
#include "di/function/tag_invoke.h"
 
namespace di::container::view {
namespace detail {
    struct ConcatFunction;
 
    template<typename... Cons>
    concept CustomConcat = (sizeof...(Cons) > 1) && concepts::TagInvocable<ConcatFunction, Cons...>;
 
    template<typename... Cons>
    concept AllConcat = sizeof...(Cons) == 1;
 
    template<typename... Cons>
    concept ViewConcat = requires(Cons&&... containers) { ConcatView { util::forward<Cons>(containers)... }; };
 
    struct ConcatFunction {
        template<concepts::ViewableContainer... Cons>
        requires(CustomConcat<Cons...> || AllConcat<Cons...> || ViewConcat<Cons...>)
        constexpr /* concepts::View */ auto operator()(Cons&&... containers) const {
            if constexpr (CustomConcat<Cons...>) {
                return function::tag_invoke(*this, util::forward<Cons>(containers)...);
            } else if constexpr (AllConcat<Cons...>) {
                return all(util::forward<Cons>(containers)...);
            } else {
                return ConcatView { util::forward<Cons>(containers)... };
            }
        }
    };
}
 
constexpr inline auto concat = detail::ConcatFunction {};
}
 
namespace di {
using view::concat;
}