cpp-useful-iterators/include/iterator/indexed_iterator.hpp

//
//  indexed_iterator.hpp
//  iterator
//
//  Created by Sam Jaffe on 3/5/17.
//

#pragma once

#include <iterator>

#include "iterator/facade.h"

namespace iterator {
  template <typename It>
  class indexed_iterator : public facade<indexed_iterator<It>> {
  public:
    using reference = std::pair<size_t, typename std::iterator_traits<It>::reference>;
    using difference_type = typename std::iterator_traits<It>::difference_type;
  public:
    indexed_iterator() = default;
    explicit indexed_iterator(It base, size_t idx = 0) : _base(base), _index(idx) {}

    template <typename O> indexed_iterator(indexed_iterator<O> const & oiter)
        : _base(oiter._base), _index(oiter._index) {}

    reference dereference() const { return {_index, *_base}; }
    
    void advance(difference_type off) {
      _base += off;
      _index += off;
    }

    // SFINAE means that if Iterator is not random access, then this still works
    // TODO: Investigate using _index for comparisons instead of _base
    bool equal_to(indexed_iterator const & other) const { return _base == other._base; }
    difference_type distance_to(indexed_iterator const & other) const {
      return other._base - _base;
    }

  private:
    template <typename O> friend class indexed_iterator;
    It _base;
    size_t _index{0};
  };
}

MAKE_ITERATOR_FACADE_TYPEDEFS_T(::iterator::indexed_iterator)