fast_ostreambuf_iterator.hpp

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// SPDX-FileCopyrightText: 2006-2026 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2026 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: BSD-3-Clause
 
#pragma once
 
#include <seqan3/std/charconv>
#include <algorithm>
#include <array>
#include <cassert>
#include <iterator>
#include <ranges>
 
#include <seqan3/io/stream/detail/stream_buffer_exposer.hpp>
 
namespace seqan3::detail
{
template <typename char_t, typename traits_t = std::char_traits<char_t>>
class fast_ostreambuf_iterator
{
private:
    stream_buffer_exposer<char_t, traits_t> * stream_buf = nullptr;
 
public:
    using difference_type = ptrdiff_t;                  
    using value_type = char_t;                          
    using reference = char_t;                           
    using pointer = void;                               
    using iterator_category = std::output_iterator_tag; 
 
    fast_ostreambuf_iterator() noexcept = default;                                             
    fast_ostreambuf_iterator(fast_ostreambuf_iterator const &) noexcept = default;             
    fast_ostreambuf_iterator(fast_ostreambuf_iterator &&) noexcept = default;                  
    fast_ostreambuf_iterator & operator=(fast_ostreambuf_iterator const &) noexcept = default; 
    fast_ostreambuf_iterator & operator=(fast_ostreambuf_iterator &&) noexcept = default;      
    ~fast_ostreambuf_iterator() noexcept = default;                                            
 
    explicit fast_ostreambuf_iterator(std::basic_streambuf<char_t, traits_t> & ibuf) :
        stream_buf{reinterpret_cast<stream_buffer_exposer<char_t, traits_t> *>(&ibuf)}
    {
        assert(stream_buf != nullptr);
        if (stream_buf->pptr() == stream_buf->epptr())
            stream_buf->overflow(); // ensures that put area has space available
    }
 
    fast_ostreambuf_iterator & operator++()
    {
        return *this;
    }
    fast_ostreambuf_iterator & operator++(int)
    {
        return *this;
    }
 
    fast_ostreambuf_iterator & operator*()
    {
        return *this;
    }
 
    fast_ostreambuf_iterator & operator=(char_t const c)
    {
        assert(stream_buf != nullptr);
        if (stream_buf->pptr() == stream_buf->epptr())
        {
            if (stream_buf->sputc(c) == traits_t::eof()) // overflow() [virtual], then write character
            {
                // LCOV_EXCL_START
                throw std::ios_base::failure{"Cannot write to output stream (reached traits::eof() condition)."};
                // LCOV_EXCL_STOP
            }
        }
        else
        {
            *stream_buf->pptr() = c;
            stream_buf->pbump(1); // advance pptr() in put area without any checks
        }
        return *this;
    }
 
    bool failed() const noexcept
    {
        return stream_buf->overflow() == traits_t::eof();
    }
 
    template <std::ranges::forward_range range_type>
        requires std::ranges::borrowed_range<range_type>
    auto write_range(range_type && rng)
    {
        using sen_t = std::ranges::sentinel_t<range_type>;
        using it_t = std::ranges::iterator_t<range_type>;
 
        it_t it = std::ranges::begin(rng);
        sen_t end = std::ranges::end(rng);
 
        while (it != end)
        {
            size_t const buffer_space = stream_buf->epptr() - stream_buf->pptr();
 
            if constexpr (std::ranges::sized_range<range_type>)
            {
                size_t const characters_to_write = std::min<size_t>(std::ranges::distance(it, end), buffer_space);
                auto copy_res = std::ranges::copy_n(it, characters_to_write, stream_buf->pptr());
                it = copy_res.in;
                stream_buf->pbump(characters_to_write);
            }
            else
            {
                size_t i = 0;
                for (; it != end && i < buffer_space; ++it, ++i)
                    *stream_buf->pptr() = *it;
                stream_buf->pbump(i);
            }
 
            if (it == end) // no more characters to write
                return it;
 
            // Push one more character and flush
            if (stream_buf->overflow(*it) == traits_t::eof())
            {
                // LCOV_EXCL_START
                throw std::ios_base::failure{"Cannot write to output stream (reached traits::eof() condition)."};
                // LCOV_EXCL_STOP
            }
 
            ++it; // drop 1 character that has been written in overflow()
        }
 
        return it;
    }
 
    // overload for non-std::ranges::borrowed_range types that return void
    template <std::ranges::forward_range range_type>
    void write_range(range_type && rng)
    {
        write_range(rng); // lvalue is always a safe range. return value is ignored because iterator would be dangling
    }
 
    template <typename number_type>
        requires std::is_arithmetic_v<number_type>
    auto write_number(number_type num)
    {
        if (stream_buf->epptr() - stream_buf->pptr() > 300) // enough space for any number, should be likely
        {
            auto res = std::to_chars(stream_buf->pptr(), stream_buf->epptr(), num);
            stream_buf->pbump(res.ptr - stream_buf->pptr()); // advance pptr
        }
        else
        {
            std::array<char, 300> arithmetic_buffer{};
            auto res = std::to_chars(&arithmetic_buffer[0], &arithmetic_buffer[0] + sizeof(arithmetic_buffer), num);
            write_range(std::ranges::subrange<char *, char *>(&arithmetic_buffer[0], res.ptr));
        }
    }
 
    void write_end_of_line(bool const add_cr)
    {
        if (add_cr)
            *this = '\r';
        *this = '\n';
    }
};
 
} // namespace seqan3::detail