cpp-useful-iterators/include/iterator/detail/projection_tuple.h

//
//  projection_tuple.h
//  iterator
//
//  Created by Sam Jaffe on 9/23/25.
//  Copyright © 2025 Sam Jaffe. All rights reserved.
//

#pragma once

#include <functional>
#include <tuple>
#include <type_traits>

#include <iterator/concepts.h>
#include <iterator/detail/capture_fn.h>

namespace iterator::detail {
template <typename... Projs> class Projections {
private:
  std::tuple<Projs...> fns_;

public:
  Projections() = default;
  Projections(Projs... fns)
    requires(sizeof...(Projs) > 0)
      : fns_(fns...) {}
  Projections(std::tuple<Projs...> const & fns) : fns_(fns) {}

  template <size_t I, typename T>
  decltype(auto) operator()(T && arg,
                            std::integral_constant<size_t, I> = {}) const {
    if constexpr (I >= sizeof...(Projs)) {
      return std::forward<T>(arg);
    } else if constexpr (Assoc<std::decay_t<T>>) {
      return std::tie(arg.first, std::invoke(std::get<I>(fns_),
                                             std::forward<T>(arg).second));
    } else {
      return std::invoke(std::get<I>(fns_), std::forward<T>(arg));
    }
  }
};

template <typename It, typename Projs, size_t I = 0> struct ProjectionExpander {
  using type = std::tuple<>;
};

template <typename It, typename... Projs, size_t I>
  requires(I < sizeof...(Projs))
struct ProjectionExpander<It, Projections<Projs...>, I> {
  using Fn = std::tuple_element_t<I, std::tuple<Projs...>>;

  template <typename T> static decltype(auto) get(T && ref) {
    if constexpr (Assoc<std::decay_t<T>>) {
      return std::forward<T>(ref).second;
    } else {
      return std::forward<T>(ref);
    }
  }

  static std::iter_reference_t<It> _ref;

  using value_type = decltype(get(_ref));
  using result_type = std::invoke_result_t<Fn, value_type>;

  using type = tuple_cat_t<
      std::tuple<std::conditional_t<std::is_reference_v<result_type>, Fn,
                                    CaptureFn<Fn, value_type>>>,
      typename ProjectionExpander<iterator_t<result_type>,
                                  Projections<Projs...>, I + 1>::type>;
};
}