ranges.h

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/*
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  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
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*/
 
#ifndef MYSQL_HARNESS_STDX_RANGES_ENUMERATE_H
#define MYSQL_HARNESS_STDX_RANGES_ENUMERATE_H
 
//
// From C++23's P2164
//
// http://wg21.link/p2164
 
#include <iterator>
#include <tuple>
#include <utility>  // declval, forward
 
#include "mysql/harness/stdx/iterator.h"  // iter_reference, iter_value
 
namespace stdx::ranges {
 
template <class T>
using iterator_t = decltype(std::begin(std::declval<T &>()));
 
template <class R>
using range_value_t = stdx::iter_value_t<ranges::iterator_t<R>>;
 
template <class R>
using range_reference_t = stdx::iter_reference_t<ranges::iterator_t<R>>;
 
/**
 * enumerate_view over a range.
 *
 * @note only implements the const-iterator parts.
 *
 * @tparam V a range to enumerate
 */
template <class V>
class enumerate_view {
 private:
  using Base = V;
 
  Base base_ = {};
 
  template <bool>
  class iterator;
 
 public:
  using value_type = stdx::iter_value_t<iterator<true>>;
 
  constexpr enumerate_view() = default;
 
  constexpr enumerate_view(V base) : base_(std::forward<Base>(base)) {}
 
  constexpr auto begin() const { return iterator<true>{std::begin(base_), 0}; }
 
  constexpr auto end() const { return iterator<true>{std::end(base_), 0}; }
};
 
template <class R>
enumerate_view(R &&) -> enumerate_view<R>;
 
template <class V>
template <bool Const>
class enumerate_view<V>::iterator {
 private:
  using Base = std::conditional_t<Const, const V, V>;
 
 public:
  using iterator_category = std::input_iterator_tag;
 
  using index_type = size_t;
  using reference = std::tuple<index_type, range_reference_t<Base>>;
  using value_type = std::tuple<index_type, range_value_t<Base>>;
 
  constexpr explicit iterator(iterator_t<Base> current, index_type pos)
      : pos_{pos}, current_{std::move(current)} {}
 
  constexpr bool operator!=(const iterator &other) const {
    return current_ != other.current_;
  }
 
  constexpr iterator &operator++() {
    ++pos_;
    ++current_;
 
    return *this;
  }
 
  constexpr decltype(auto) operator*() const {
    return reference{pos_, *current_};
  }
 
 private:
  index_type pos_;
 
  iterator_t<Base> current_;
};
 
namespace views {
/*
 * an iterator that wraps an iterable and returns a counter and the
 * deref'ed wrapped iterable.
 *
 * @tparam T a iterable
 *
 * @code
 * for (auto [ndx, vc]: enumerate(std::vector<int>{1, 23, 42})) {
 *   std::cerr << "[" << ndx << "] " << v << "\n";
 * }
 *
 * // [0] 1
 * // [1] 23
 * // [2] 42
 * @endcode
 *
 * modelled after P2164 from C++23, but implemented for C++17 (aka without
 * ranges and views)
 */
template <class T, class TIter = decltype(std::begin(std::declval<T>())),
          class = decltype(std::end(std::declval<T>()))>
constexpr auto enumerate(T &&iterable) {
  return enumerate_view{std::forward<T>(iterable)};
}
}  // namespace views
}  // namespace stdx::ranges
 
namespace stdx {
namespace views = ranges::views;
}
#endif