zip.hpp

#ifndef ITER_ZIP_HPP_
#define ITER_ZIP_HPP_

#include "internal/iter_tuples.hpp"
#include "internal/iterbase.hpp"

#include <algorithm>
#include <iterator>
#include <tuple>
#include <utility>

namespace iter {
  namespace impl {
    template <typename TupleType, std::size_t... Is>
    class Zipped;

    template <typename TupleType, std::size_t... Is>
    Zipped<TupleType, Is...> zip_impl(TupleType&&, std::index_sequence<Is...>);
  }

  template <typename... Containers>
  auto zip(Containers&&... containers);
}

template <typename TupleType, std::size_t... Is>
class iter::impl::Zipped {
 private:
  TupleType containers_;
  friend Zipped iter::impl::zip_impl<TupleType, Is...>(
      TupleType&&, std::index_sequence<Is...>);

  Zipped(TupleType&& containers) : containers_(std::move(containers)) {}

 public:
  Zipped(Zipped&&) = default;

  // template templates here because I need to defer evaluation in the const
  // iteration case for types that don't have non-const begin() and end(). If I
  // passed in the actual types of the tuples of iterators and the type for
  // deref they'd need to be known in the function declarations below.
  template <typename TupleTypeT, template <typename> class IteratorTuple,
      template <typename> class TupleDeref>
  class Iterator {
    // see gcc bug 87651
#if NO_GCC_FRIEND_ERROR
   private:
    template <typename, template <typename> class, template <typename> class>
    friend class Iterator;
#else
   public:
#endif
    IteratorTuple<TupleTypeT> iters_;

   public:
    using iterator_category = std::input_iterator_tag;
    using value_type = TupleDeref<TupleTypeT>;
    using difference_type = std::ptrdiff_t;
    using pointer = value_type*;
    using reference = value_type&;

    Iterator(IteratorTuple<TupleTypeT>&& iters) : iters_(std::move(iters)) {}

    Iterator& operator++() {
      absorb(++std::get<Is>(iters_)...);
      return *this;
    }

    Iterator operator++(int) {
      auto ret = *this;
      ++*this;
      return ret;
    }

    template <typename T, template <typename> class IT,
        template <typename> class TD>
    bool operator!=(const Iterator<T, IT, TD>& other) const {
      if constexpr (sizeof...(Is) == 0) {
        return false;
      } else {
        return (... && (std::get<Is>(iters_) != std::get<Is>(other.iters_)));
      }
    }

    template <typename T, template <typename> class IT,
        template <typename> class TD>
    bool operator==(const Iterator<T, IT, TD>& other) const {
      return !(*this != other);
    }

    TupleDeref<TupleTypeT> operator*() {
      return {(*std::get<Is>(iters_))...};
    }

    auto operator-> () -> ArrowProxy<decltype(**this)> {
      return {**this};
    }
  };

  Iterator<TupleType, iterator_tuple_type, iterator_deref_tuple> begin() {
    return {{get_begin(std::get<Is>(containers_))...}};
  }

  Iterator<TupleType, iterator_tuple_type, iterator_deref_tuple> end() {
    return {{get_end(std::get<Is>(containers_))...}};
  }

  Iterator<AsConst<TupleType>, const_iterator_tuple_type,
      const_iterator_deref_tuple>
  begin() const {
    return {{get_begin(std::as_const(std::get<Is>(containers_)))...}};
  }

  Iterator<AsConst<TupleType>, const_iterator_tuple_type,
      const_iterator_deref_tuple>
  end() const {
    return {{get_end(std::as_const(std::get<Is>(containers_)))...}};
  }
};

template <typename TupleType, std::size_t... Is>
iter::impl::Zipped<TupleType, Is...> iter::impl::zip_impl(
    TupleType&& containers, std::index_sequence<Is...>) {
  return {std::move(containers)};
}

template <typename... Containers>
auto iter::zip(Containers&&... containers) {
  return impl::zip_impl(
      std::tuple<Containers...>{std::forward<Containers>(containers)...},
      std::index_sequence_for<Containers...>{});
}

#endif