cpp-useful-iterators/include/iterator/recursive/layer.hpp

#pragma once

#include "base.hpp"
#include "traits.hpp"

namespace iterator::recursive {
  /**
   * @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 Next The next layer, either a
   * recursive_iterator_impl, or a bounded_recursive_iterator_impl
   */
  template <typename Iterator, typename Next>
  class layer : public base<Iterator>, public Next {
  protected:
    using recursive_category = continue_layer_tag_t;

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

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

    bool operator==(layer const & other) const {
      return (static_cast<base<Iterator> const &>(*this) == other) &&
             (static_cast<Next 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() {
      Next::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(base<Iterator> v) {
      static_cast<base<Iterator> &>(*this) = v;
      if (!v.at_end()) { Next::assign(make_end_aware_iterator(*v)); }
    }

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

    using base<Iterator>::at_end;
  };
}