n4317.cc

// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4317.pdf

#include <iterator>

namespace n4317
{
  namespace internal
  {
    template <typename RanIter, typename Dist>
    void advance_helper(RanIter & i, RanIter e, Dist n, std::random_access_iterator_tag)
    {
      auto maxd = e - i;
      if ((n > 0 && maxd < n) || (n < 0 && maxd > n)) i = e;
      else i += n;
    }
 
    template <typename InpIter, typename Dist>
    void advance_helper(InpIter & i, InpIter e, Dist n, std::input_iterator_tag)
    {
      for (; 0 < n && i != e; --n) ++i;
    }
 
    template <typename BiIter, typename Dist>
    void advance_helper(BiIter & i, BiIter e, Dist n, std::bidirectional_iterator_tag)
    {
      for (; 0 < n && i != e; --n) ++i;
      for (; n < 0 && i != e; ++n) --i;
    }
  }

  template <typename Iter>
  void advance(Iter & i, Iter e, typename std::iterator_traits<Iter>::difference_type n)
  {
    typedef typename std::iterator_traits<Iter>::iterator_category Cat;
    internal::advance_helper(i, e, n, Cat{});
  }

  template <typename Iter>
  Iter next(Iter i, Iter e, typename std::iterator_traits<Iter>::difference_type n = 1)
  {
    advance(i, e, n);
    return i;
  }

  template <typename Iter>
  Iter prev(Iter i, Iter e, typename std::iterator_traits<Iter>::difference_type n = 1)
  {
    advance(i, e, -n);
    return i;
  }
}