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

#pragma once

#include <type_traits>
#include <utility>
#include <iterator>

/** \brief Numeric range iterator, used to represent a half-closed interval [begin, end).
 *    In conjunction with std::reverse_iterator allows for forward and backward iteration
 *    over corresponding interval. */
namespace ext
{
    /// @todo check whether difference_type should be signed (make_signed_t<T>)
    template<typename T> struct range_iterator : std::iterator<
        std::bidirectional_iterator_tag, T, T, void, T>
    {
        T current{};

        range_iterator() = default;
        range_iterator(const T t) : current(t) {}

        T operator*() const { return current; }

        range_iterator & operator++() { return ++current, *this; }
        range_iterator & operator--() { return --current, *this; }

        bool operator==(const range_iterator & other) const { return current == other.current; }
        bool operator!=(const range_iterator & other) const { return current != other.current; }
    };

    template<typename T> using reverse_range_iterator = std::reverse_iterator<range_iterator<T>>;

    /** \brief Range-based for loop adapter for (reverse_)range_iterator.
     *  By and large should be in conjunction with ext::range and ext::reverse_range. */
    template<typename T, bool forward> struct range_wrapper
    {
        using value_type = typename std::remove_reference<T>::type;
        using range_iterator_t = ext::range_iterator<value_type>;
        using iterator = typename std::conditional<forward,
            range_iterator_t,
            ext::reverse_range_iterator<value_type>>::type;

        value_type begin_, end_;

        iterator begin() const { return iterator(range_iterator_t{begin_}); }
        iterator end() const { return iterator(range_iterator_t{end_}); }
    };

    /** \brief Constructs range_wrapper for forward-iteration over [begin, end) in range-based for loop.
     *  Usage example:
     *      for (const auto i : ext::range(0, 4)) print(i);
     *  Output:
     *      0 1 2 3 */
    template<typename T1, typename T2>
    inline ext::range_wrapper<typename std::common_type<T1, T2>::type, true> range(T1 begin, T2 end)
    {
        using common_type = typename std::common_type<T1, T2>::type;
        return { static_cast<common_type>(begin), static_cast<common_type>(end) };
    }

    /** \brief Constructs range_wrapper for backward-iteration over [begin, end) in range-based for loop.
     *  Usage example:
     *      for (const auto i : ext::reverse_range(0, 4)) print(i);
     *  Output:
     *      3 2 1 0 */
    template<typename T1, typename T2>
    inline ext::range_wrapper<typename std::common_type<T1, T2>::type, false> reverse_range(T1 begin, T2 end)
    {
        using common_type = typename std::common_type<T1, T2>::type;
        return { static_cast<common_type>(end), static_cast<common_type>(begin) };
    }

    template<typename T>
    inline ext::range_iterator<T> make_range_iterator(const T value)
    {
        return { value };
    }
}