//////////////////////////////////////////////////////////////////////////////
//
// \(C\) Copyright Benedek Thaler 2015-2016
// \(C\) Copyright Ion Gaztanaga 2019-2020. Distributed under the Boost
// Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
// See http://erenon.hu/double_ended for documentation.
//
//////////////////////////////////////////////////////////////////////////////

/**
 * Emulates input iterator, validates algorithm
 * does a single pass only, removes visited elements.
 *
 * Container::erase(front_iterator) must not invalidate other iterators.
 *
 * The hack around `_erase_on_destroy` is required to make `*it++` work.
 */
template <typename Container>
class input_iterator
{
  typedef typename Container::iterator iterator;

  public:

  typedef std::input_iterator_tag iterator_category;
  typedef typename Container::value_type value_type;
  typedef typename Container::pointer pointer;
  typedef typename Container::reference reference;
  typedef typename Container::difference_type difference_type;

  struct erase_on_destroy {};

public:

   input_iterator()
      : _container()
      , _it()
      , _erase_on_destroy()
   {}

   input_iterator(Container& c, iterator it)
      :_container(&c)
      , _it(it)
      , _erase_on_destroy()
  {}

  input_iterator(const input_iterator& rhs)
    :_container(rhs._container),
     _it(rhs._it),
     _erase_on_destroy(rhs._erase_on_destroy)
  {
    rhs._erase_on_destroy = false;
  }

   input_iterator & operator=(const input_iterator& rhs)
   {
      _container = rhs._container;
      _it = rhs._it;
      _erase_on_destroy = rhs._erase_on_destroy;
      rhs._erase_on_destroy = false;
      return *this;
   }

  input_iterator(const input_iterator& rhs, erase_on_destroy)
    :_container(rhs._container),
     _it(rhs._it),
     _erase_on_destroy(true)
  {}

  ~input_iterator()
  {
    if (_erase_on_destroy)
    {
      _container->erase(_it); // must not invalidate other iterators
    }
  }

  const value_type& operator*()
  {
    return *_it;
  }

  input_iterator operator++()
  {
    _container->erase(_it);
    ++_it;
    return *this;
  }

  input_iterator operator++(int)
  {
    input_iterator old(*this, erase_on_destroy());
    ++_it;
    return old;
  }

  friend bool operator==(const input_iterator a, const input_iterator b)
  {
    return a._it == b._it;
  }

  friend bool operator!=(const input_iterator a, const input_iterator b)
  {
    return !(a == b);
  }

private:
  Container* _container;
  iterator _it;
  mutable bool _erase_on_destroy;
};

template <typename Container>
input_iterator<Container> make_input_iterator(Container& c, typename Container::iterator it)
{
  return input_iterator<Container>(c, it);
}