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Filename : array.hpp
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/*!
@file
Adapts `std::array` for use with Hana.

@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 */

#ifndef BOOST_HANA_EXT_STD_ARRAY_HPP
#define BOOST_HANA_EXT_STD_ARRAY_HPP

#include <boost/hana/bool.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/detail/algorithm.hpp>
#include <boost/hana/fwd/at.hpp>
#include <boost/hana/fwd/core/tag_of.hpp>
#include <boost/hana/fwd/drop_front.hpp>
#include <boost/hana/fwd/equal.hpp>
#include <boost/hana/fwd/is_empty.hpp>
#include <boost/hana/fwd/length.hpp>
#include <boost/hana/fwd/less.hpp>
#include <boost/hana/integral_constant.hpp>

#include <array>
#include <cstddef>
#include <type_traits>
#include <utility>


#ifdef BOOST_HANA_DOXYGEN_INVOKED
namespace std {
    //! @ingroup group-ext-std
    //! Adaptation of `std::array` for Hana.
    //!
    //!
    //!
    //! Modeled concepts
    //! ----------------
    //! 1. `Comparable`\n
    //! `std::array`s are compared as per `std::equal`, except that two arrays
    //! with different sizes compare unequal instead of triggering an error
    //! and the result of the comparison is `constexpr` if both arrays are
    //! `constexpr`.
    //! @include example/ext/std/array/comparable.cpp
    //!
    //! 2. `Orderable`\n
    //! `std::array`s are ordered with the usual lexicographical ordering,
    //! except that two arrays with different size can be ordered instead
    //! of triggering an error and the result of the comparison is `constexpr`
    //! if both arrays are `constexpr`.
    //! @include example/ext/std/array/orderable.cpp
    //!
    //! 3. `Foldable`\n
    //! Folding an array from the left is equivalent to calling
    //! `std::accumulate` on it, except it can be `constexpr`.
    //! @include example/ext/std/array/foldable.cpp
    //!
    //! 4. `Iterable`\n
    //! Iterating over a `std::array` is equivalent to iterating over it with
    //! a normal `for` loop.
    //! @include example/ext/std/array/iterable.cpp
    template <typename T, std::size_t N>
    struct array { };
}
#endif


BOOST_HANA_NAMESPACE_BEGIN
    namespace ext { namespace std { struct array_tag; }}

    template <typename T, std::size_t N>
    struct tag_of<std::array<T, N>> {
        using type = ext::std::array_tag;
    };

    //////////////////////////////////////////////////////////////////////////
    // Foldable
    //////////////////////////////////////////////////////////////////////////
    template <>
    struct length_impl<ext::std::array_tag> {
        template <typename Xs>
        static constexpr auto apply(Xs const&) {
            return hana::size_c<std::tuple_size<Xs>::type::value>;
        }
    };

    //////////////////////////////////////////////////////////////////////////
    // Iterable
    //////////////////////////////////////////////////////////////////////////
    template <>
    struct at_impl<ext::std::array_tag> {
        template <typename Xs, typename N>
        static constexpr decltype(auto) apply(Xs&& xs, N const&) {
            constexpr std::size_t n = N::value;
            return std::get<n>(static_cast<Xs&&>(xs));
        }
    };

    template <>
    struct drop_front_impl<ext::std::array_tag> {
        template <std::size_t n, typename Xs, std::size_t ...i>
        static constexpr auto drop_front_helper(Xs&& xs, std::index_sequence<i...>) {
            using T = typename std::remove_reference<Xs>::type::value_type;
            return std::array<T, sizeof...(i)>{{static_cast<Xs&&>(xs)[n + i]...}};
        }

        template <typename Xs, typename N>
        static constexpr auto apply(Xs&& xs, N const&) {
            constexpr std::size_t n = N::value;
            constexpr std::size_t len = std::tuple_size<
                typename std::remove_cv<
                    typename std::remove_reference<Xs>::type
                >::type
            >::value;
            return drop_front_helper<n>(static_cast<Xs&&>(xs),
                    std::make_index_sequence<(n < len ? len - n : 0)>{});
        }
    };

    template <>
    struct is_empty_impl<ext::std::array_tag> {
        template <typename T, std::size_t N>
        static constexpr auto apply(std::array<T, N> const&) {
            return hana::bool_c<N == 0>;
        }
    };

    //////////////////////////////////////////////////////////////////////////
    // Comparable
    //////////////////////////////////////////////////////////////////////////
    template <>
    struct equal_impl<ext::std::array_tag, ext::std::array_tag> {
        template <typename T, std::size_t n, typename U>
        static constexpr bool apply(std::array<T, n> const& xs, std::array<U, n> const& ys)
        { return detail::equal(&xs[0], &xs[0] + n, &ys[0], &ys[0] + n); }

        template <typename T, typename U>
        static constexpr auto apply(std::array<T, 0> const&, std::array<U, 0> const&)
        { return hana::true_c; }

        template <typename T, std::size_t n, typename U, std::size_t m>
        static constexpr auto apply(std::array<T, n> const&, std::array<U, m> const&)
        { return hana::false_c; }
    };

    //////////////////////////////////////////////////////////////////////////
    // Orderable
    //////////////////////////////////////////////////////////////////////////
    template <>
    struct less_impl<ext::std::array_tag, ext::std::array_tag> {
        template <typename T, std::size_t n, typename U, std::size_t m>
        static constexpr auto apply(std::array<T, n> const& xs, std::array<U, m> const& ys) {
            // This logic is more complex than it needs to be because we can't
            // use `.begin()` and `.end()`, which are not constexpr in C++14,
            // and because `&arr[0]` is UB when the array is empty.
            if (xs.empty()) {
                return !ys.empty();
            } else {
                if (ys.empty()) {
                    return false;
                } else {
                    return detail::lexicographical_compare(&xs[0], &xs[0] + n,
                                                           &ys[0], &ys[0] + m);
                }
            }
        }
    };
BOOST_HANA_NAMESPACE_END

#endif // !BOOST_HANA_EXT_STD_ARRAY_HPP