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

#pragma once

#include <functional>

#include <iterator/proxy.h>
#include <stream/detail/traits.h>
#include <stream/forward.h>

#define FWD(x) std::forward<decltype(x)>(x)

namespace stream::ranges {
template <typename It, typename Proj>
class transform_iterator : public proxy<It, transform_iterator<It, Proj>> {
private:
  using super_t = proxy<It, transform_iterator<It, Proj>>;

private:
  Proj projection_;

public:
  transform_iterator() = default;
  transform_iterator(It iter, Proj projection)
      : super_t(std::move(iter)), projection_(projection) {}

  decltype(auto) dereference() const {
    return projection_(super_t::dereference());
  }

  bool at_end() const { return super_t::impl().at_end(); }
};

template <typename S, typename Proj> class transform_view {
private:
private:
  S stream_;
  Proj projection_;

public:
  transform_view(S && stream, Proj projection)
      : stream_(FWD(stream)), projection_(projection) {}

  auto begin() const { return transform_iterator(stream_.begin(), invoke()); }
  
  auto end() const {
    if constexpr (traits::is_sentinal_v<S>) {
      return stream_.end();
    } else {
      return transform_iterator(stream_.end(), invoke());
    }
  }
  
  auto empty() const {
    if constexpr (traits::has_empty_v<S>) {
      return stream_.empty();
    }
  }
  
  auto size() {
    if constexpr (traits::has_size_v<S>) {
      return stream_.size();
    }
  }

private:
  auto invoke() const {
    return std::function([this](traits::cref_t<S> t) -> decltype(auto) {
      return std::invoke(projection_, t);
    });
  }
};

template <typename S, typename Proj>
transform_view(S &&, Proj) -> transform_view<S, Proj>;
}

MAKE_ITERATOR_FACADE_TYPEDEFS_T(stream::ranges::transform_iterator);

namespace stream::ranges::views {
template <typename Proj> class transform {
public:
  transform(Proj const & projection) : projection_(projection) {}

  template <typename Stream>
  friend auto operator|(Stream && stream, transform map) {
    // TODO: if constexpr transform_view
    return transform_view(FWD(stream), std::move(map.projection_));
  }

private:
  Proj projection_;
};
}

#undef FWD