cpp-lazy-stream/include/stream/view.h

//
//  view.h
//  stream
//
//  Created by Sam Jaffe on 3/29/23.
//

#pragma once

namespace stream::ranges {

template <typename T>
class view_iterator : public facade<view_iterator<T>> {
private:
  T (*dereference_)(void *){nullptr};
  bool (*equal_to_)(void *, void *){nullptr};
  void (*increment_)(void *){nullptr};
  std::shared_ptr<void> impl_{nullptr};

public:
  template <typename It>
  view_iterator(It impl)
      : dereference_([](void * p) -> T { return **((It *)p); }),
        equal_to_(
            [](void * l, void * r) { return *((It *)l) == *((It *)r); }),
        increment_([](void * p) { ++(*(It *)(p)); }),
        impl_(std::make_shared<It>(impl)) {}

  T dereference() const { return dereference_(impl_.get()); }
  
  void increment() { increment_(impl_.get()); }
  
  bool equal_to(view_iterator const & other) const {
    return impl_ == other.impl_ ||
           (impl_ && other.impl_ &&
            equal_to_(impl_.get(), other.impl_.get()));
  }
};

template <typename T> class view {
private:
  using make_iter_t = view_iterator<T>(*)(void*);
private:
  std::shared_ptr<void> impl_{nullptr};
  make_iter_t begin_{nullptr};
  make_iter_t end_{nullptr};

public:
  template <typename S>
  view(std::shared_ptr<S> impl)
      : impl_(impl), begin_(begin_function<S>()), end_(end_function<S>()) {}

  template <typename S>
  view(S & impl) : view(std::shared_ptr<S>(&impl, [](void *) {})) {}

  template <typename S>
  view(S const & impl) : view(std::shared_ptr<S const>(&impl, [](void *) {})) {}

  template <typename S>
  view(S && impl) : view(std::make_shared<S>(std::forward<S>(impl))) {}

  auto begin() const { return begin_(impl_.get()); }
  auto end() const { return end_(impl_.get()); }
  
private:
  template <typename S>
  static make_iter_t begin_function() {
    if constexpr (traits::is_sentinal_v<S>) {
      return [](void *p) -> view_iterator<T> {
        return iterator::sentinel_iterator(static_cast<S *>(p)->begin());
      };
    } else {
      return [](void * p) -> view_iterator<T> {
        return static_cast<S *>(p)->begin();
      };
    }
  }
  
  template <typename S>
  static make_iter_t end_function() {
    if constexpr (traits::is_sentinal_v<S>) {
      return [](void *p) -> view_iterator<T> {
        return iterator::sentinel_iterator<iter<S>>();
      };
    } else {
      return [](void * p) -> view_iterator<T> {
        return static_cast<S *>(p)->end();
      };
    }
  }

};

template <typename S> view(std::shared_ptr<S>) -> view<traits::value_type<S>>;
template <typename S> view(S &) -> view<traits::value_type<S>>;
template <typename S> view(S const &) -> view<traits::value_type<S>>;
template <typename S> view(S &&) -> view<traits::value_type<S>>;

}