drop.hpp

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// -----------------------------------------------------------------------------------------------------
// Copyright (c) 2006-2020, Knut Reinert & Freie Universität Berlin
// Copyright (c) 2016-2020, Knut Reinert & MPI für molekulare Genetik
// This file may be used, modified and/or redistributed under the terms of the 3-clause BSD-License
// shipped with this file and also available at: https://github.com/seqan/seqan3/blob/master/LICENSE.md
// -----------------------------------------------------------------------------------------------------
 
#pragma once
 
#include <seqan3/std/concepts>
#include <seqan3/std/iterator>
#include <seqan3/std/ranges>
#include <seqan3/std/span>
#include <seqan3/std/type_traits>
 
#include <seqan3/core/detail/template_inspection.hpp>
#include <seqan3/io/exception.hpp>
#include <seqan3/range/concept.hpp>
#include <seqan3/range/views/detail.hpp>
 
namespace seqan3::detail
{
 
// ============================================================================
//  drop_fn (adaptor definition)
// ============================================================================
 
struct drop_fn
{
    constexpr auto operator()(size_t drop_size) const noexcept
    {
        return detail::adaptor_from_functor{*this, drop_size};
    }
 
    template <std::ranges::range urng_t>
    constexpr auto operator()(urng_t && urange, size_t drop_size) const
    {
        static_assert(std::ranges::viewable_range<urng_t>,
                      "The views::drop adaptor can only be passed viewable_ranges, i.e. Views or &-to-non-View.");
 
        [[maybe_unused]] size_t new_size = -1;
 
        // safeguard against wrong size
        if constexpr (std::ranges::sized_range<urng_t>)
        {
            size_t urange_size = std::ranges::size(urange);
            drop_size = std::min(drop_size, urange_size);
            new_size = urange_size - drop_size;
        }
 
        // string_view
        if constexpr (is_type_specialisation_of_v<std::remove_cvref_t<urng_t>, std::basic_string_view>)
        {
            return urange.substr(drop_size);
        }
        // string const &
        else if constexpr (is_type_specialisation_of_v<std::remove_cvref_t<urng_t>, std::basic_string> &&
                           std::is_const_v<std::remove_reference_t<urng_t>>)
        {
            return std::basic_string_view{std::ranges::data(urange) + drop_size, new_size};
        }
        // contiguous
        else if constexpr (std::ranges::borrowed_range<urng_t> &&
                           std::ranges::contiguous_range<urng_t> &&
                           std::ranges::sized_range<urng_t>)
        {
            return std::span{std::ranges::data(urange) + drop_size, new_size};
        }
        // random_access
        else if constexpr (std::ranges::borrowed_range<urng_t> &&
                           std::ranges::random_access_range<urng_t> &&
                           std::ranges::sized_range<urng_t>)
        {
            return std::ranges::subrange<std::ranges::iterator_t<urng_t>, std::ranges::iterator_t<urng_t>>
            {
                std::ranges::begin(urange) + drop_size,
                std::ranges::begin(urange) + drop_size + new_size,
                new_size
            };
        }
        // std::views::drop
        else
        {
            using drop_size_t = std::ranges::range_difference_t<urng_t>;
            // urange | std::views::drop(drop_size);
            return std::views::drop(std::forward<urng_t>(urange), static_cast<drop_size_t>(drop_size));
        }
    }
};
 
} // namespace seqan3::detail
 
// ============================================================================
//  views::drop (adaptor instance definition)
// ============================================================================
 
namespace seqan3::views
{
 
inline constexpr auto drop = detail::drop_fn{};
 
 
} // namespace seqan3::views