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

#pragma once

#include <iterator>
#include <tuple>

#include "proxy.h"

namespace iterator::zip {
  template <typename... Iters> struct impl {
  public:
    using difference_type = std::common_type_t<
        typename std::iterator_traits<Iters>::difference_type...>;

  public:
    std::tuple<Iters...> data;

  public:
    impl() = default;
    impl(Iters &&... iters) : data(iters...) {}

    auto operator*() const {
      return std::make_tuple(*std::get<Iters>(data)...);
    }

    void operator++(int) {
      [[maybe_unused]] auto l = {(++(std::get<Iters>(data)), 0)...};
    }

    void operator--(int) {
      [[maybe_unused]] auto l = {(--(std::get<Iters>(data)), 0)...};
    }

    void operator+=(difference_type d) {
      [[maybe_unused]] auto l = {((std::get<Iters>(data) += d), 0)...};
    }

    bool operator==(impl const & other) const { return data == other.data; }

    auto operator-(impl const & other) const {
      return std::get<0>(data) - std::get<0>(other.data);
    }
  };
}

namespace std {
  template <typename... Iters>
  struct iterator_traits<::iterator::zip::impl<Iters...>> {
    using difference_type =
        common_type_t<typename iterator_traits<Iters>::difference_type...>;
    using iterator_category =
        common_type_t<typename iterator_traits<Iters>::iterator_category...>;
  };
}

namespace iterator {
  template <typename... Iters>
  class zip_iterator
      : public proxy<zip::impl<Iters...>, zip_iterator<Iters...>> {
  private:
    using super = proxy<zip::impl<Iters...>, zip_iterator<Iters...>>;

  public:
    zip_iterator() = default;
    zip_iterator(Iters &&... iters) : super(std::forward<Iters>(iters)...) {}
  };
}

namespace std {
  template <typename... T>
  struct iterator_traits<::iterator::zip_iterator<T...>>
      : std::iterator_traits<
            ::iterator::facade<::iterator::zip_iterator<T...>>> {
    // This shouldn't need to be implemented, but for some reason my traits
    // are not correctly deducing here.
    using iterator_category =
        common_type_t<typename iterator_traits<T>::iterator_category...>;
  };
}

template <typename... Is>
iterator::zip_iterator<Is...> make_zip_iterator(Is &&... iters) {
  return {std::forward<Is>(iters)...};
}