take_exactly_view.hpp

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// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: BSD-3-Clause
 
#pragma once
 
#include <algorithm>
#include <concepts>
#include <iterator>
#include <ranges>
#include <span>
#include <type_traits>
 
#include <seqan3/core/detail/iterator_traits.hpp>
#include <seqan3/core/detail/template_inspection.hpp>
#include <seqan3/core/range/detail/adaptor_from_functor.hpp>
#include <seqan3/core/range/detail/inherited_iterator_base.hpp>
#include <seqan3/core/range/type_traits.hpp>
#include <seqan3/io/exception.hpp>
#include <seqan3/utility/range/concept.hpp>
#include <seqan3/utility/type_traits/detail/transformation_trait_or.hpp>
 
namespace seqan3::detail
{
 
// ============================================================================
//  view_take_exactly
// ============================================================================
 
template <std::ranges::view urng_t, bool or_throw>
class view_take_exactly : public std::ranges::view_interface<view_take_exactly<urng_t, or_throw>>
{
private:
    urng_t urange;
 
    size_t target_size;
 
    template <bool const_range>
    class basic_iterator;
 
private:
    using iterator = basic_iterator<false>;
    using const_iterator = basic_iterator<true>;
 
public:
    view_take_exactly() = default;                                          
    view_take_exactly(view_take_exactly const & rhs) = default;             
    view_take_exactly(view_take_exactly && rhs) = default;                  
    view_take_exactly & operator=(view_take_exactly const & rhs) = default; 
    view_take_exactly & operator=(view_take_exactly && rhs) = default;      
    ~view_take_exactly() = default;                                         
 
    constexpr view_take_exactly(urng_t _urange, size_t const _size) : urange{std::move(_urange)}, target_size{_size}
    {
        if constexpr (std::ranges::sized_range<urng_t>)
        {
            if (std::ranges::size(urange) < target_size)
            {
                if constexpr (or_throw)
                {
                    throw std::invalid_argument{
                        "You are trying to construct a detail::take_exactly_or_throw from a range that is strictly "
                        "smaller."};
                }
                else
                {
                    target_size = std::ranges::size(urange);
                }
            }
        }
    }
 
    template <std::ranges::viewable_range rng_t>
        requires std::constructible_from<rng_t, std::views::all_t<rng_t>>
    constexpr view_take_exactly(rng_t && _urange, size_t const _size) :
        view_take_exactly{std::views::all(std::forward<rng_t>(_urange)), _size}
    {}
 
    constexpr auto begin() noexcept
    {
        if constexpr (std::ranges::random_access_range<urng_t> && std::ranges::sized_range<urng_t>)
            return std::ranges::begin(urange);
        else
            return iterator{std::ranges::begin(urange), 0, target_size, this};
    }
 
    constexpr auto begin() const noexcept
        requires const_iterable_range<urng_t> && std::ranges::forward_range<urng_t>
    {
        if constexpr (std::ranges::random_access_range<urng_t> && std::ranges::sized_range<urng_t>)
            return std::ranges::cbegin(urange);
        else
        {
            // const_iterator does not work if the underlying iterator is a std::input_iterator (it needs to access
            // target_size)
            return const_iterator{std::ranges::begin(urange), 0, target_size};
        }
    }
 
    constexpr auto end() noexcept
    {
        if constexpr (std::ranges::random_access_range<urng_t> && std::ranges::sized_range<urng_t>)
            return std::ranges::begin(urange) + target_size;
        else
            return std::ranges::end(urange);
    }
 
    constexpr auto end() const noexcept
        requires const_iterable_range<urng_t> && std::ranges::forward_range<urng_t>
    {
        if constexpr (std::ranges::random_access_range<urng_t> && std::ranges::sized_range<urng_t>)
            return std::ranges::cbegin(urange) + target_size;
        else
            return std::ranges::cend(urange);
    }
 
    constexpr auto size() const noexcept
    {
        return target_size;
    }
};
 
template <typename urng_t, bool or_throw = false>
view_take_exactly(urng_t &&, size_t) -> view_take_exactly<std::views::all_t<urng_t>, or_throw>;
 
template <std::ranges::view urng_t, bool or_throw>
template <bool const_range>
class view_take_exactly<urng_t, or_throw>::basic_iterator :
    public inherited_iterator_base<basic_iterator<const_range>, maybe_const_iterator_t<const_range, urng_t>>
{
private:
    using base_base_t = maybe_const_iterator_t<const_range, urng_t>;
    using base_t = inherited_iterator_base<basic_iterator, maybe_const_iterator_t<const_range, urng_t>>;
 
    using sentinel_type = maybe_const_sentinel_t<const_range, urng_t>;
 
    size_t pos{};
 
    size_t max_pos{};
 
    std::conditional_t<!std::forward_iterator<base_base_t>, view_take_exactly *, detail::ignore_t> host_ptr;
 
public:
    basic_iterator() = default;                                       
    basic_iterator(basic_iterator const & rhs) = default;             
    basic_iterator(basic_iterator && rhs) = default;                  
    basic_iterator & operator=(basic_iterator const & rhs) = default; 
    basic_iterator & operator=(basic_iterator && rhs) = default;      
    ~basic_iterator() = default;                                      
 
    constexpr basic_iterator(base_base_t it) noexcept(noexcept(base_t{it})) : base_t{std::move(it)}
    {}
 
    constexpr basic_iterator(base_base_t it,
                             size_t const _pos,
                             size_t const _max_pos,
                             view_take_exactly * host = nullptr) noexcept(noexcept(base_t{it})) :
        base_t{std::move(it)},
        pos{_pos},
        max_pos(_max_pos)
    {
        host_ptr = host;
 
        if constexpr (!std::forward_iterator<base_base_t>)
        {
            assert(host_ptr != nullptr);
        }
    }
 
    using typename base_t::difference_type;
    using typename base_t::reference;
 
    constexpr basic_iterator & operator++() noexcept(noexcept(++std::declval<base_t &>()))
    {
        base_t::operator++();
        ++pos;
        if constexpr (!std::forward_iterator<base_base_t>)
            --host_ptr->target_size;
        return *this;
    }
 
    constexpr decltype(auto) operator++(int) noexcept(noexcept(++std::declval<basic_iterator &>())
                                                      && (std::same_as<decltype(std::declval<base_base_t &>()++), void>
                                                          || std::is_nothrow_copy_constructible_v<basic_iterator>))
    {
        // if underlying iterator is a C++20 input iterator (i.e. returns void), return void too.
        if constexpr (std::same_as<decltype(std::declval<base_base_t &>()++), void>)
        {
            ++(*this);
        }
        else
        {
            basic_iterator cpy{*this};
            ++(*this);
            return cpy;
        }
    }
 
    constexpr basic_iterator & operator--() noexcept(noexcept(--std::declval<base_base_t &>()))
        requires std::bidirectional_iterator<base_base_t>
    {
        base_t::operator--();
        --pos;
        return *this;
    }
 
    constexpr basic_iterator operator--(int)
        noexcept(noexcept(--std::declval<basic_iterator &>()) && std::is_nothrow_copy_constructible_v<basic_iterator>)
        requires std::bidirectional_iterator<base_base_t>
    {
        basic_iterator cpy{*this};
        --(*this);
        return cpy;
    }
 
    constexpr basic_iterator & operator+=(difference_type const skip)
        noexcept(noexcept(std::declval<base_t &>() += skip))
        requires std::random_access_iterator<base_base_t>
    {
        base_t::operator+=(skip);
        pos += skip;
        return *this;
    }
 
    constexpr basic_iterator & operator-=(difference_type const skip)
        noexcept(noexcept(std::declval<base_t &>() -= skip))
        requires std::random_access_iterator<base_base_t>
    {
        base_t::operator-=(skip);
        pos -= skip;
        return *this;
    }
 
    constexpr bool operator==(basic_iterator const & rhs) const
        noexcept(!or_throw && noexcept(std::declval<base_base_t &>() == std::declval<base_base_t &>()))
        requires std::forward_iterator<base_base_t>
    {
        return this->base() == rhs.base();
    }
 
    constexpr bool operator==(sentinel_type const & rhs) const
        noexcept(!or_throw && noexcept(std::declval<base_base_t const &>() == std::declval<sentinel_type const &>()))
    {
        if (pos >= max_pos)
            return true;
 
        if (this->base() == rhs)
        {
            if constexpr (or_throw)
                throw unexpected_end_of_input{"Reached end of input before designated size."};
 
            return true;
        }
        else
        {
            return false;
        }
    }
 
    constexpr friend bool operator==(sentinel_type const & lhs, basic_iterator const & rhs)
        noexcept(noexcept(rhs == lhs))
    {
        return rhs == lhs;
    }
 
    constexpr bool operator!=(sentinel_type const & rhs) const
        noexcept(noexcept(std::declval<basic_iterator &>() == rhs))
    {
        return !(*this == rhs);
    }
 
    constexpr bool operator!=(basic_iterator const & rhs) const
        noexcept(noexcept(std::declval<basic_iterator &>() == rhs))
        requires std::forward_iterator<base_base_t>
    {
        return !(*this == rhs);
    }
 
    constexpr friend bool operator!=(sentinel_type const & lhs, basic_iterator const & rhs)
        noexcept(noexcept(rhs != lhs))
    {
        return rhs != lhs;
    }
 
    constexpr reference operator[](std::make_unsigned_t<difference_type> const n) const
        noexcept(noexcept(std::declval<base_base_t &>()[0]))
        requires std::random_access_iterator<base_base_t>
    {
        return base_t::operator[](n);
    }
};
 
// ============================================================================
//  take_fn (adaptor definition)
// ============================================================================
 
template <bool or_throw>
struct take_exactly_fn
{
    constexpr auto operator()(size_t const size) const
    {
        return adaptor_from_functor{*this, size};
    }
 
    template <std::ranges::range urng_t>
    constexpr auto operator()(urng_t && urange, size_t target_size) const
    {
        static_assert(std::ranges::viewable_range<urng_t>,
                      "The views::take adaptor can only be passed viewable_ranges, i.e. Views or &-to-non-View.");
 
        // safeguard against wrong size
        if constexpr (std::ranges::sized_range<urng_t>)
        {
            if constexpr (or_throw)
            {
                if (target_size > std::ranges::size(urange))
                {
                    throw std::invalid_argument{"You are trying to construct a detail::take_exactly_or_throw from a "
                                                "range that is strictly smaller."};
                }
            }
            else
            {
                target_size = std::min<size_t>(target_size, std::ranges::size(urange));
            }
        }
 
        // string_view
        if constexpr (is_type_specialisation_of_v<std::remove_cvref_t<urng_t>, std::basic_string_view>)
        {
            // in standard
            return urange.substr(0, target_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>>)
        {
            // not in standard
            // seqan3::views::type_reduce does this too
            return std::basic_string_view{std::ranges::data(urange), target_size};
        }
        // contiguous
        else if constexpr (std::ranges::borrowed_range<urng_t> && std::ranges::contiguous_range<urng_t>
                           && std::ranges::sized_range<urng_t>)
        {
            // not in standard (special case for std::span in standard)
            // seqan3::views::type_reduce does this too
            return std::span{std::ranges::data(urange), target_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>)
        {
            // not in standard
            // seqan3::views::type_reduce does this too
            return std::ranges::subrange<std::ranges::iterator_t<urng_t>, std::ranges::iterator_t<urng_t>>{
                std::ranges::begin(urange),
                std::ranges::begin(urange) + target_size,
                target_size};
        }
        // our type
        else
        {
            return view_take_exactly<std::views::all_t<urng_t>, or_throw>{std::forward<urng_t>(urange), target_size};
        }
    }
};
 
} // namespace seqan3::detail
 
// ============================================================================
//  detail::take_exactly (adaptor instance definition)
// ============================================================================
 
namespace seqan3::detail
{
inline constexpr auto take_exactly = take_exactly_fn<false>{};
 
// ============================================================================
//  detail::take_exactly_or_throw (adaptor instance definition)
// ============================================================================
 
inline constexpr auto take_exactly_or_throw = take_exactly_fn<true>{};
} // namespace seqan3::detail