#pragma once

#include <algorithm>
#include <memory>
#include <numeric>

#define EQ_MEM(x) x == dynamic_cast<iterator const&>(other).x

#define DELEGATE_ITERATOR_IMPL_BASE(impl) \
super* clone() const override { return new iterator{*this}; } \
bool operator==(super const&other) const override { return EQ_MEM(impl); } \

#define DELEGATE_ITERATOR_IMPL(impl) \
super& operator++() override { ++impl; return *this; } \
DELEGATE_ITERATOR_IMPL_BASE(impl)

namespace stream {
  template <typename T>
  class iterator {
  public:
    explicit iterator(detail::iterator_impl<T>* impl) : impl_(impl) {}
    iterator(iterator const& other) : impl_(other.impl_->clone()) { }
    iterator(iterator&& other) : impl_(nullptr) { std::swap(impl_, other.impl_); }
    ~iterator() { if (impl_) delete impl_; }
    T operator*() const { return **impl_; }
    iterator& operator++() { ++(*impl_); return *this; }
    bool operator==(iterator const&other) const { return *impl_ == *(other.impl_); }
    bool operator!=(iterator const&other) const { return *impl_ != *(other.impl_); }
  private:
    detail::iterator_impl<T>* impl_;
  };
  
  namespace detail {
    template <typename T>
    stream_base<T, false> falsify(stream_base<T, true> const& in);
    
    template <typename T>
    class iterator_impl {
    public:
      virtual ~iterator_impl() {}
      virtual iterator_impl<T>* clone() const = 0;
      virtual T operator*() = 0;
      virtual iterator_impl<T>& operator++() = 0;
      virtual bool operator==(iterator_impl<T> const&other) const = 0;
      bool operator!=(iterator_impl<T> const&other) const {
        return !operator==(other);
      }
    };
    
    template <typename T>
    class stream_impl {
    public:
      virtual ~stream_impl() { }
      virtual ::stream::iterator<T> begin() = 0;
      virtual ::stream::iterator<T> end() = 0;
    };
    
    template <typename T, bool Own>
    class stream_base {
    private:
      template <typename F>
      using map_f = decltype(std::declval<F>()(std::declval<T>()));
      template <typename F>
      using flatmap_f = typename decltype(std::declval<F>()(std::declval<T>()))::value_type;
      
      using self = stream_base<T, Own>;
    public:
      explicit stream_base(stream_impl<T>* impl) : impl_(impl) {}
      stream_base(stream_base&&other) : impl_(nullptr) { std::swap(impl_, other.impl_); }
      ~stream_base() { if (Own && impl_) delete impl_; }
      ::stream::iterator<T> begin() const { return impl_->begin(); }
      ::stream::iterator<T> end  () const { return impl_->end  (); }
      
      std::vector<T> collect() const {
        std::vector<T> coll;
        collect(coll);
        return coll;
      }
      
      template <typename C>
      C& collect(typename std::enable_if<!
                 std::is_void<typename C::value_type>::value, C>::type& coll) const {
        std::copy(begin(), end(), std::inserter(coll.end(), coll));
        return coll;
      }
      
      template <typename F>
      T accumulate(F&& fold, T const& accum) {
        return std::accumulate(begin(), end(), accum, fold);
      }
      
      template <typename F>
      stream_base<map_f<F>, true> map(F&& func) const&;
      template <typename F>
      stream_base<map_f<F>, true> map(F&& func) &&;
      template <typename F>
      stream_base<T, true> filter(F&& func) const&;
      template <typename F>
      stream_base<T, true> filter(F&& func) &&;
      template <typename F>
      stream_base<flatmap_f<F>, true> flatmap(F&& func) const&;
      template <typename F>
      stream_base<flatmap_f<F>, true> flatmap(F&& func) &&;
    private:
      friend stream_base<T, false> falsify<T>(stream_base<T, true>const&);
      stream_impl<T>* impl_;
    };
    
    template <typename T>
    stream_base<T, false> const& falsify(stream_base<T, false> const& in) { return in; }
    template <typename T>
    stream_base<T, false> falsify(stream_base<T, true> const& in) {
      return stream_base<T, false>{in.impl_};
    }
  }
}