ClickHouse/libs/libcommon/include/ext/enumerate.hpp

#pragma once

#include <ext/size.hpp>
#include <type_traits>
#include <utility>
#include <iterator>


/** \brief Provides a wrapper view around a container, allowing to iterate over it's elements and indices.
  *	Allow writing code like shown below:
  *
  *		std::vector<T> v = getVector();
  *		for (const std::pair<const std::size_t, T &> index_and_value : ext::enumerate(v))
  *			std::cout << "element " << index_and_value.first << " is " << index_and_value.second << std::endl;
  */
namespace ext
{
	template <typename It> struct enumerate_iterator
	{
		using traits = typename std::iterator_traits<It>;
		using iterator_category = typename traits::iterator_category;
		using value_type = std::pair<const std::size_t, typename traits::value_type>;
		using difference_type = typename traits::difference_type;
		using reference = std::pair<const std::size_t, typename traits::reference>;

		std::size_t idx;
		It it;

		enumerate_iterator(const std::size_t idx, It it) : idx{idx}, it{it} {}

		auto operator*() const { return reference(idx, *it); }

		bool operator!=(const enumerate_iterator & other) const { return it != other.it; }

		enumerate_iterator & operator++() { return ++idx, ++it, *this; }
	};

	template <typename Collection> struct enumerate_wrapper
	{
		using underlying_iterator = decltype(std::begin(std::declval<Collection &>()));
		using iterator = enumerate_iterator<underlying_iterator>;

		Collection & collection;

		enumerate_wrapper(Collection & collection) : collection(collection) {}

		auto begin() { return iterator(0, std::begin(collection)); }
		auto end() { return iterator(ext::size(collection), std::end(collection)); }
	};

	template <typename Collection> auto enumerate(Collection & collection)
	{
		return enumerate_wrapper<Collection>{collection};
	}

	template <typename Collection> auto enumerate(const Collection & collection)
	{
		return enumerate_wrapper<const Collection>{collection};
	}
}