path: root/third_party/nix/src/libutil/istringstream_nocopy.hh

/* This file provides a variant of std::istringstream that doesn't
   copy its string argument. This is useful for large strings. The
   caller must ensure that the string object is not destroyed while
   it's referenced by this object. */

#pragma once

#include <iostream>
#include <string>

template <class CharT, class Traits = std::char_traits<CharT>,
          class Allocator = std::allocator<CharT>>
class basic_istringbuf_nocopy : public std::basic_streambuf<CharT, Traits> {
 public:
  using string_type = std::basic_string<CharT, Traits, Allocator>;

  using off_type = typename std::basic_streambuf<CharT, Traits>::off_type;

  using pos_type = typename std::basic_streambuf<CharT, Traits>::pos_type;

  using int_type = typename std::basic_streambuf<CharT, Traits>::int_type;

  using traits_type = typename std::basic_streambuf<CharT, Traits>::traits_type;

 private:
  const string_type& s;

  off_type off;

 public:
  explicit basic_istringbuf_nocopy(const string_type& s) : s{s}, off{0} {}

 private:
  pos_type seekoff(off_type off, std::ios_base::seekdir dir,
                   std::ios_base::openmode which) {
    if (which & std::ios_base::in) {
      this->off =
          dir == std::ios_base::beg
              ? off
              : (dir == std::ios_base::end ? s.size() + off : this->off + off);
    }
    return pos_type(this->off);
  }

  pos_type seekpos(pos_type pos, std::ios_base::openmode which) {
    return seekoff(pos, std::ios_base::beg, which);
  }

  std::streamsize showmanyc() { return s.size() - off; }

  int_type underflow() {
    if (typename string_type::size_type(off) == s.size()) {
      return traits_type::eof();
    }
    return traits_type::to_int_type(s[off]);
  }

  int_type uflow() {
    if (typename string_type::size_type(off) == s.size()) {
      return traits_type::eof();
    }
    return traits_type::to_int_type(s[off++]);
  }

  int_type pbackfail(int_type ch) {
    if (off == 0 || (ch != traits_type::eof() && ch != s[off - 1])) {
      return traits_type::eof();
    }

    return traits_type::to_int_type(s[--off]);
  }
};

template <class CharT, class Traits = std::char_traits<CharT>,
          class Allocator = std::allocator<CharT>>
class basic_istringstream_nocopy : public std::basic_iostream<CharT, Traits> {
  using buf_type = basic_istringbuf_nocopy<CharT, Traits, Allocator>;
  buf_type buf;

 public:
  explicit basic_istringstream_nocopy(const typename buf_type::string_type& s)
      : std::basic_iostream<CharT, Traits>(&buf), buf(s){};
};

using istringstream_nocopy = basic_istringstream_nocopy<char>;