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

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

#pragma once

#include <iterator>

#include <iterator/facade.h>
#include <iterator/forwards.h>

#include <iterator/detail/macro.h>

namespace iterator {
// TODO: I should be respecting the underlying type
template <typename It>
class indexed_iterator
    : public facade<indexed_iterator<It>, category::random_access> {
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;
  indexed_iterator(It base) : base_(base) {}
  indexed_iterator(It base, size_t idx) : 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_;
  }
  SFINAE(detail::has_sentinel_type_v<It>) bool at_end() const {
    return base_ == typename It::sentinel_type();
  }

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

template <typename It> indexed_iterator(It) -> indexed_iterator<It>;
template <typename It> indexed_iterator(It, size_t) -> indexed_iterator<It>;
}

MAKE_ITERATOR_FACADE_TYPEDEFS_T(::iterator::indexed_iterator);