cpp-useful-iterators/include/iterator/detail/flatten_iterator_layer.hpp

#pragma once

#include "arrow_proxy.h"
#include "recursive_iterator_base.hpp"
#include "recursive_iterator_traits.hpp"

namespace iterator { namespace detail {

  /**
   * @class flatten_iterator_layer
   * @brief A single layer for recursing down a nested collection. Represents
   * associative containers.
   *
   * @copydoc recursive_iterator_layer
   */
  template <typename Iterator, typename RecursiveIterator_NextLayer>
  class flatten_iterator_layer : public recursive_iterator_base<Iterator>,
                                 public RecursiveIterator_NextLayer {
  public:
    using super = RecursiveIterator_NextLayer;
    using layer = recursive_iterator_base<Iterator>;
    using key_type =
        typename std::tuple_element<0, typename layer::value_type>::type;

  protected:
    using recursive_category = continue_layer_tag_t;

  public:
    flatten_iterator_layer() = default;
    explicit flatten_iterator_layer(layer v) : flatten_iterator_layer() {
      assign(v);
      update();
    }
    template <typename OIter, typename Rec>
    flatten_iterator_layer(flatten_iterator_layer<OIter, Rec> const & other)
        : layer(static_cast<recursive_iterator_base<OIter> const &>(other)),
          super(static_cast<Rec const &>(other)) {}

    /**
     * @brief Concatenate the key in this layer, with the dereferenced data from
     * the next.
     *
     * Due to the use of the next_layer_type metaprogramming, a type such as
     * std::map<K, std::vector<std::tuple<T1, T2, T3>>> would return a reference
     * of type std::tuple<K const &, std::tuple<T1, T2, T3>&>, preserving
     * sub-aggregates of pair/tuple type. Similarly, forward_as_tuple means
     * even a key-type of pair/tuple will not be unwrapped.
     */
    decltype(auto) operator*() const {
      using next_t =
          typename next_layer_type<decltype(super::get()),
                                   typename super::recursive_category>::type;
      return std::tuple_cat(std::forward_as_tuple(std::get<0>(layer::get())),
                            next_t(super::get()));
    }

    /**
     * Unimplemented because we return an inline constructed type, and tuple
     * can only be accessed through std::get anyway.
     */
    //    auto operator->() const { return detail::arrow_proxy(**this); }
    void operator->() const;

    bool operator==(flatten_iterator_layer const & other) const {
      return (static_cast<layer const &>(*this) == other) &&
             (static_cast<super const &>(*this) == other);
    }

  protected:
    decltype(auto) get() const { return **this; }

    /**
     * @copydoc recursive_iterator_layer::next
     */
    void next() {
      super::next();
      update();
    }

    void update() {
      layer & self = static_cast<layer &>(*this);
      while (super::at_end() && !(++self).at_end()) {
        super::assign(make_end_aware_iterator(self->second));
      }
    }

    /**
     * @copydoc recursive_iterator_layer::assign
     */
    void assign(layer v) {
      static_cast<layer &>(*this) = v;
      if (!v.at_end()) { super::assign(make_end_aware_iterator(v->second)); }
    }

    using layer::at_end;
  };
}}