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

#pragma once

#include "recursive_iterator_base.hpp"
#include "recursive_iterator_traits.hpp"

namespace iterator { namespace detail {
  /**
   * @class recursive_iterator_layer
   * @brief A single layer for recursing down a nested collection. Represents
   * non-associative containers.
   *
   * Provides dispatch/overloading for types and functions of recursive_iterator
   * chains to resolve ambiguous typedefs and operators.
   *
   * @see recursive_iterator_impl
   * @see bounded_recursive_iterator_impl
   * @tparam Iterator The underlying iterator type of the layer
   * @tparam RecursiveIterator_NextLayer The next layer, either a
   * recursive_iterator_impl, or a bounded_recursive_iterator_impl
   */
  template <typename Iterator, typename RecursiveIterator_NextLayer>
  class recursive_iterator_layer : public recursive_iterator_base<Iterator>,
                                   public RecursiveIterator_NextLayer {
  public:
    using super = RecursiveIterator_NextLayer;
    using layer = recursive_iterator_base<Iterator>;

  protected:
    using recursive_category = continue_layer_tag_t;

  public:
    using reference = decltype(*std::declval<super>());
    using value_type = std::remove_cv_t<std::remove_reference_t<reference>>;
    using pointer = decltype(std::declval<super>().operator->());
    using difference_type = std::ptrdiff_t;
    using iterator_category = std::forward_iterator_tag;

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

    decltype(auto) operator*() const { return super::get(); }
    decltype(auto) operator->() const { return super::operator->(); }

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

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

    /**
     * Advance the iterator step. If the next layer has reached the end, then
     * we advance this iterator until it reaches either its own end, or a
     * non-empty subcollection to start iterating over.
     */
    void next() {
      super::next();
      update();
    }

    /**
     * Update the underlying iterator and propogate updates down the chain so
     * that if there is data available, the iterator is in a dereferencable
     * state.
     */
    void assign(layer v) {
      static_cast<layer &>(*this) = v;
      if (!v.at_end()) { super::assign(make_end_aware_iterator(*v)); }
    }

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

    using layer::at_end;
  };
}}