enumerate.hpp

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#pragma once
 
/// @file userver/utils/enumerate.hpp
/// @brief @copybrief utils::enumerate
/// @ingroup userver_universal
 
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <type_traits>
#include <utility>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::impl {
 
template <typename Iter>
auto DetectEnumerateValueType()
    -> std::pair<const std::size_t, decltype(*std::declval<Iter>())>;
 
template <typename Iter, typename... Args>
auto DetectEnumerateValueType(Args&&...) -> void;
 
template <typename Iter>
struct IteratorWrapper {
  using difference_type = std::ptrdiff_t;
  using value_type = decltype(DetectEnumerateValueType<Iter>());
  using reference = value_type;
  using iterator_category = std::input_iterator_tag;
 
  Iter iterator;
  std::size_t pos{0};
 
  constexpr IteratorWrapper& operator++() {
    ++pos;
    ++iterator;
    return *this;
  }
 
  constexpr value_type operator*() const { return {pos, *iterator}; }
 
  template <typename OtherIter>
  constexpr bool operator==(const IteratorWrapper<OtherIter>& other) const {
    return iterator == other.iterator;
  }
 
  template <typename OtherIter>
  constexpr bool operator!=(const IteratorWrapper<OtherIter>& other) const {
    return !(iterator == other.iterator);
  }
};
 
template <typename Range>
using IteratorTypeOf = decltype(std::begin(std::declval<Range&>()));
 
template <typename Range>
using SentinelTypeOf = decltype(std::end(std::declval<Range&>()));
 
template <typename Container>
struct ContainerWrapper {
  constexpr IteratorWrapper<IteratorTypeOf<Container>> begin() {
    return {std::begin(container), 0};
  }
 
  constexpr IteratorWrapper<SentinelTypeOf<Container>> end() {
    return {std::end(container), 0};
  }
 
  constexpr IteratorWrapper<IteratorTypeOf<const Container>> begin() const {
    return {std::begin(container), 0};
  }
 
  constexpr IteratorWrapper<SentinelTypeOf<const Container>> end() const {
    return {std::end(container), 0};
  }
 
  Container container;
};
 
}  // namespace utils::impl
 
namespace utils {
 
/// @brief Implementation of python-style enumerate function for range-for loops
/// @param iterable: Container to iterate
/// @returns ContainerWrapper, which iterator after dereference returns pair
/// of index and (!!!)non-const reference to element(it seems impossible to make
/// this reference const). It can be used in "range based for loop" with
/// "structured binding" like this
/// @code
/// for (auto [pos, elem] : enumerate(someContainer)) {...}
/// @endcode
template <typename Container>
constexpr auto enumerate(Container&& iterable) {
  return impl::ContainerWrapper<Container>{std::forward<Container>(iterable)};
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END