join.h

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#ifndef _utl_join_h_
#define _utl_join_h_

#include <boost/iostreams/categories.hpp>
#include <boost/iostreams/positioning.hpp>
#include <boost/iostreams/read.hpp>
#include <boost/shared_ptr.hpp>
#include <vector>


namespace utl {

  template<class ifstream>
  class join_source {
  public:
    typedef char char_type;
    struct category :
      boost::iostreams::input,
      boost::iostreams::device_tag,
      boost::iostreams::closable_tag
    { };

    join_source() { }

    join_source(const std::string& filename)
    { open(std::vector<std::string>(1, filename)); }

    join_source(const std::vector<std::string>& filenames)
    { open(filenames); }

    void open(const std::string& filename)
    { open(std::vector<std::string>(1, filename)); }

    void
    open(const std::vector<std::string>& filenames)
    {
      _ifstream.reset();
      _filenames = filenames;
      _current_file_index = 0;
      if (filenames.empty())
        return;
      _ifstream.reset(new ifstream(filenames.front()));
    }

    void
    close()
    {
      _ifstream.reset();
      _filenames.clear();
      _current_file_index = 0;
    }

    boost::iostreams::stream_offset
    get_current_file_index()
      const
    {
      if (_ifstream && _ifstream->is_open() &&_current_file_index < _filenames.size())
        return _current_file_index;
      return -1;
    }

    std::string
    get_current_filename()
      const
    {
      const auto i = get_current_file_index();
      return i >= 0 ? _filenames[i] : "";
    }

    std::streamsize
    read(char* const s, const std::streamsize n)
    {
      if (!(_ifstream && _ifstream->is_open()))
        return -1;
      const auto size = boost::iostreams::read(*_ifstream, s, n);
      if (size != -1)
        return size;
      next();
      return read(s, n);
    }

  private:
    void
    next()
    {
      _ifstream.reset();
      ++_current_file_index;
      if (_current_file_index < _filenames.size())
        _ifstream.reset(new ifstream(_filenames[_current_file_index]));
    }

    boost::shared_ptr<ifstream> _ifstream;
    std::vector<std::string> _filenames;
    std::size_t _current_file_index = 0;
  };


  /*
    \class join join.h "utl/join.h"

    \author Darko Veberic
    \date 21 Jan 2017
    \version $Id: join.h 531 2017-01-26 20:00:08Z darko $
  */

  template<class ifstream>
  class join : public boost::iostreams::stream<join_source<ifstream>> {
  private:
    typedef join_source<ifstream> source;
    typedef boost::iostreams::stream<source> base;

  public:
    join() : base(source()) { }

    join(const std::string& filename) : base(source(filename)) { }

    join(const std::vector<std::string>& filenames) : base(source(filenames)) { }

    void open(const std::string& filename) { (**this).open(filename); }

    void open(const std::vector<std::string>& filenames) { (**this).open(filenames); }
  };

}


#endif