fast_istreambuf_iterator.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 <cassert>
#include <iterator>
#include <string>
#include <vector>
 
#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_istreambuf_iterator
{
private:
    stream_buffer_exposer<char_t, traits_t> * stream_buf = nullptr;
 
    std::string overflow_buffer{};
 
public:
    using difference_type = ptrdiff_t;                 
    using value_type = char_t;                         
    using reference = char_t;                          
    using pointer = void;                              
    using iterator_category = std::input_iterator_tag; 
 
    fast_istreambuf_iterator() noexcept = default;                                             
    fast_istreambuf_iterator(fast_istreambuf_iterator const &) noexcept = default;             
    fast_istreambuf_iterator(fast_istreambuf_iterator &&) noexcept = default;                  
    fast_istreambuf_iterator & operator=(fast_istreambuf_iterator const &) noexcept = default; 
    fast_istreambuf_iterator & operator=(fast_istreambuf_iterator &&) noexcept = default;      
    ~fast_istreambuf_iterator() noexcept = default;                                            
 
    explicit fast_istreambuf_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->gptr() == stream_buf->egptr()) // If current get area is empty,
            stream_buf->underflow();                   // ensure the stream buffer has content on construction.
    }
 
    template <typename record_type>
        requires std::same_as<std::ranges::range_value_t<record_type>, std::string_view>
    void cache_record_into(char const record_end, char const field_sep, record_type & raw_record)
    {
        bool has_overflowed = false;
        size_t old_count = 0;
        char * data_begin = stream_buf->gptr(); // point into stream buffer by default
        size_t const number_of_fields = raw_record.size();
        size_t number_of_seen_fields = 0;
        std::vector<size_t> field_positions(number_of_fields, 0u);
 
        char const * ptr = stream_buf->gptr();
 
        auto overflow_into_buffer = [&]()
        {
            size_t count = stream_buf->egptr() - stream_buf->gptr();
            has_overflowed = true;
            overflow_buffer.resize(old_count + count);
            std::ranges::copy(stream_buf->gptr(), stream_buf->egptr(), overflow_buffer.data() + old_count);
 
            old_count += count;
            stream_buf->gbump(count);
            stream_buf->underflow();
        };
 
        while (number_of_seen_fields < number_of_fields - 1)
        {
            ptr = std::find(ptr, static_cast<char const *>(stream_buf->egptr()), field_sep);
 
            if (ptr != stream_buf->egptr()) // found an end of field
            {
                field_positions[number_of_seen_fields] = ptr - stream_buf->gptr() + old_count;
                ++ptr;
                ++number_of_seen_fields;
            }
            else
            {
                overflow_into_buffer();
                assert(stream_buf->gptr() != stream_buf->egptr()); // stream is not at end after overflow
                ptr = stream_buf->gptr();
            }
        }
 
        size_t count = 0;
 
        while (true) // Note: Might run idefinitely in release mode if no record_end is in input.
        {
            ptr = std::find(ptr, static_cast<char const *>(stream_buf->egptr()), record_end);
 
            if (ptr == stream_buf->egptr()) // stop_chr could not be found in current buffer
            {
                overflow_into_buffer();
                assert(stream_buf->gptr() != stream_buf->egptr()); // stream is not at end after overflow
                ptr = stream_buf->gptr();
            }
            else
            {
                count = ptr - stream_buf->gptr(); // processed characters until stop_chr has been found
                break;
            }
        }
 
        if (has_overflowed)
        {
            // need to copy last data
            overflow_buffer.resize(old_count + count);
            std::ranges::copy(stream_buf->gptr(), stream_buf->gptr() + count, overflow_buffer.data() + old_count);
 
            // make data pointer point into overflow
            data_begin = overflow_buffer.data();
        }
 
        stream_buf->gbump(count);
 
        // instantiate string_views in raw_record
        field_positions.back() = old_count + count;
        raw_record[0] = std::string_view{data_begin, field_positions[0]};
        for (size_t i = 1; i < number_of_fields; ++i)
            raw_record[i] = std::string_view{data_begin + field_positions[i - 1] + 1, data_begin + field_positions[i]};
    }
 
    std::string_view cache_bytes(int32_t const size)
    {
        std::string_view result;
 
        if (stream_buf->egptr() - stream_buf->gptr() >= size)
        {
            result = std::string_view{stream_buf->gptr(), stream_buf->gptr() + size};
            stream_buf->gbump(size);
        }
        else
        {
            overflow_buffer.resize(size);
 
            int32_t remaining_bytes{size};
 
            while (stream_buf->egptr() - stream_buf->gptr() < remaining_bytes) // still not fully in buffer
            {
                std::ranges::copy(stream_buf->gptr(),
                                  stream_buf->egptr(),
                                  overflow_buffer.data() + size - remaining_bytes);
                size_t const number_of_copied_bytes = stream_buf->egptr() - stream_buf->gptr();
                remaining_bytes -= number_of_copied_bytes;
                stream_buf->gbump(number_of_copied_bytes);
                stream_buf->underflow();
                assert((remaining_bytes == 0 || stream_buf->egptr() != stream_buf->gptr())
                       && "I still need to read characters but my stream is at end.");
            }
 
            if (remaining_bytes != 0) // In stream_buf but not yet copied to overflow_buffer
            {
                std::ranges::copy(stream_buf->gptr(),
                                  stream_buf->gptr() + remaining_bytes,
                                  overflow_buffer.data() + size - remaining_bytes);
 
                stream_buf->gbump(remaining_bytes);
            }
 
            result = {overflow_buffer.begin(), overflow_buffer.end()};
        }
 
        return result;
    }
 
    fast_istreambuf_iterator & operator++()
    {
        assert(stream_buf != nullptr);
 
        if ((stream_buf->gptr() + 1) == stream_buf->egptr())
            stream_buf->snextc(); // move right, then underflow()
        else
            stream_buf->gbump(1);
        return *this;
    }
 
    void operator++(int)
    {
        ++(*this);
    }
 
    reference operator*() const
    {
        assert(stream_buf != nullptr);
        assert(stream_buf->gptr() != stream_buf->egptr());
        return *stream_buf->gptr();
    }
 
    friend bool operator==(fast_istreambuf_iterator const & lhs, std::default_sentinel_t const &) noexcept
    {
        assert(lhs.stream_buf != nullptr);
        // compare size of remaining buffer; since ++ always resizes if possible, safe to compare pointers here
        return (lhs.stream_buf->gptr() == lhs.stream_buf->egptr());
    }
 
    friend bool operator!=(fast_istreambuf_iterator const & lhs, std::default_sentinel_t const &) noexcept
    {
        return !(lhs == std::default_sentinel);
    }
 
    friend bool operator==(std::default_sentinel_t const &, fast_istreambuf_iterator const & rhs) noexcept
    {
        return rhs == std::default_sentinel;
    }
 
    friend bool operator!=(std::default_sentinel_t const &, fast_istreambuf_iterator const & rhs) noexcept
    {
        return !(rhs == std::default_sentinel);
    }
};
 
} // namespace seqan3::detail