integral_constant_like.hpp

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// SPDX-License-Identifier: BSL-1.0
// SPDX-FileCopyrightText: Copyright (C) 2024 Tobias Hienzsch
 
#ifndef TETL_MDSPAN_INTEGRAL_CONSTANT_LIKE_HPP
#define TETL_MDSPAN_INTEGRAL_CONSTANT_LIKE_HPP
 
#include <etl/_concepts/convertible_to.hpp>
#include <etl/_cstddef/size_t.hpp>
#include <etl/_mdspan/strided_slice.hpp>
#include <etl/_type_traits/always_false.hpp>
#include <etl/_type_traits/bool_constant.hpp>
#include <etl/_type_traits/is_integral.hpp>
#include <etl/_type_traits/is_same.hpp>
#include <etl/_type_traits/remove_const.hpp>
 
namespace etl::detail {
 
template <typename T>
concept pair_like = true;
 
template <typename T, typename IndexType>
concept index_pair_like = pair_like<T>
                      and etl::convertible_to<etl::tuple_element_t<0, T>, IndexType>
                      and etl::convertible_to<etl::tuple_element_t<1, T>, IndexType>;
 
template <typename T>
concept integral_constant_like = etl::is_integral_v<decltype(T::value)>
                             and not etl::is_same_v<bool, etl::remove_const_t<decltype(T::value)>>
                             and etl::convertible_to<T, decltype(T::value)>
                             /* and etl::equality_comparable_with<T, decltype(T::value)>   */
                             and etl::bool_constant<T() == T::value>::value
                             and etl::bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
 
template <typename T>
[[nodiscard]] constexpr auto de_ice(T val) -> T
{
    return val;
}
 
template <integral_constant_like T>
[[nodiscard]] constexpr auto de_ice(T /*unused*/)
{
    return T::value;
}
 
template <etl::size_t K>
[[nodiscard]] constexpr auto nth_slice_specifier(auto first, auto... rest)
{
    if constexpr (K == 0) {
        return first;
    } else {
        nth_slice_specifier<K - 1>(rest...);
    }
}
 
template <typename IndexType, etl::size_t K, typename... SliceSpecifiers>
[[nodiscard]] constexpr auto submdspan_first(SliceSpecifiers... slices) -> IndexType
{
    auto sk  = nth_slice_specifier<K>(slices...);
    using Sk = decltype(sk);
 
    if constexpr (etl::convertible_to<Sk, IndexType>) {
        return static_cast<IndexType>(sk);
    } else if constexpr (index_pair_like<Sk, IndexType>) {
        return static_cast<IndexType>(get<0>(sk));
    } else if constexpr (is_strided_slice<Sk>) {
        return static_cast<IndexType>(de_ice(sk.offset));
    } else {
        return static_cast<IndexType>(0);
    }
}
 
template <etl::size_t K, typename Extents, typename... SliceSpecifiers>
[[nodiscard]] constexpr auto submdspan_last(Extents const& src, SliceSpecifiers... slices)
{
    auto sk         = nth_slice_specifier<K>(slices...);
    using Sk        = decltype(sk);
    using IndexType = typename Extents::index_type;
 
    if constexpr (etl::convertible_to<Sk, IndexType>) {
        return static_cast<IndexType>(de_ice(sk)) + IndexType(1);
    } else if constexpr (index_pair_like<Sk, IndexType>) {
        return static_cast<IndexType>(get<1>(sk));
    } else if constexpr (is_strided_slice<Sk>) {
        return static_cast<IndexType>(de_ice(sk.offset) + de_ice(sk.extent));
    } else {
        return static_cast<IndexType>(src.extent(K));
    }
}
 
// template <typename IndexType, size_t N, typename... SliceSpecifiers>
// [[nodiscard]] constexpr auto src_indices(array<IndexType, N> const& indices, SliceSpecifiers...
// slices)
//     -> array<IndexType, sizeof...(SliceSpecifiers)>
// {
// }
 
} // namespace etl::detail
 
#endif // TETL_MDSPAN_INTEGRAL_CONSTANT_LIKE_HPP