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

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

#pragma once
#include <iterator>
#include <type_traits>

#include <iterator/concepts.h>
#include <iterator/detail/projection_tuple.h>
#include <iterator/end_aware_iterator.h>

namespace iterator::detail {
template <typename Proj, typename It, size_t I>
using projection_t = std::invoke_result_t<Proj, std::iter_reference_t<It>,
                                          std::integral_constant<size_t, I>>;

template <typename It, typename Projs, typename MaxDepth, size_t N = 0,
          typename V = std::decay_t<projection_t<Projs, It, N>>>
struct RecursiveExpander {
  using type = std::tuple<EndAwareIterator<It>>;
};

template <typename It, typename Projs, size_t N>
struct RecursiveExpander<It, Projs, bounded<N + 1>, N> {
  using type = std::tuple<EndAwareIterator<It>>;
};

template <typename It, typename Projs, typename MaxDepth, size_t N, Range V>
struct RecursiveExpander<It, Projs, MaxDepth, N, V> {
  using projected_value_type = projection_t<Projs, It, N>;

  using expand_next = RecursiveExpander<iterator_t<projected_value_type>, Projs,
                                        MaxDepth, N + 1>;

  using type =
      tuple_cat_t<std::tuple<EndAwareIterator<It>>, typename expand_next::type>;
};

template <typename It, typename Projs, typename MaxDepth, size_t N,
          AssocRange V>
struct RecursiveExpander<It, Projs, MaxDepth, N, V> {
  using projected_value_type =
      std::tuple_element_t<1,
                           std::remove_reference_t<projection_t<Projs, It, N>>>;

  using expand_next = RecursiveExpander<iterator_t<projected_value_type>, Projs,
                                        MaxDepth, N + 1>;

  using type =
      tuple_cat_t<std::tuple<EndAwareIterator<It>>, typename expand_next::type>;
};
}