algorithm.hpp

#ifndef ENTT_CORE_ALGORITHM_HPP
#define ENTT_CORE_ALGORITHM_HPP

#include <algorithm>
#include <functional>
#include <iterator>
#include <utility>
#include <vector>
#include "utility.hpp"

namespace entt {

/**
 * @brief Function object to wrap `std::sort` in a class type.
 *
 * Unfortunately, `std::sort` cannot be passed as template argument to a class
 * template or a function template.<br/>
 * This class fills the gap by wrapping some flavors of `std::sort` in a
 * function object.
 */
struct std_sort {
    /**
     * @brief Sorts the elements in a range.
     *
     * Sorts the elements in a range using the given binary comparison function.
     *
     * @tparam It Type of random access iterator.
     * @tparam Compare Type of comparison function object.
     * @tparam Args Types of arguments to forward to the sort function.
     * @param first An iterator to the first element of the range to sort.
     * @param last An iterator past the last element of the range to sort.
     * @param compare A valid comparison function object.
     * @param args Arguments to forward to the sort function, if any.
     */
    template<typename It, typename Compare = std::less<>, typename... Args>
    void operator()(It first, It last, Compare compare = Compare{}, Args &&...args) const {
        std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
    }
};

/*! @brief Function object for performing insertion sort. */
struct insertion_sort {
    /**
     * @brief Sorts the elements in a range.
     *
     * Sorts the elements in a range using the given binary comparison function.
     *
     * @tparam It Type of random access iterator.
     * @tparam Compare Type of comparison function object.
     * @param first An iterator to the first element of the range to sort.
     * @param last An iterator past the last element of the range to sort.
     * @param compare A valid comparison function object.
     */
    template<typename It, typename Compare = std::less<>>
    void operator()(It first, It last, Compare compare = Compare{}) const {
        if(first < last) {
            for(auto it = first + 1; it < last; ++it) {
                auto value = std::move(*it);
                auto pre = it;

                for(; pre > first && compare(value, *(pre - 1)); --pre) {
                    *pre = std::move(*(pre - 1));
                }

                *pre = std::move(value);
            }
        }
    }
};

/**
 * @brief Function object for performing LSD radix sort.
 * @tparam Bit Number of bits processed per pass.
 * @tparam N Maximum number of bits to sort.
 */
template<std::size_t Bit, std::size_t N>
struct radix_sort {
    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");

    /**
     * @brief Sorts the elements in a range.
     *
     * Sorts the elements in a range using the given _getter_ to access the
     * actual data to be sorted.
     *
     * This implementation is inspired by the online book
     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
     *
     * @tparam It Type of random access iterator.
     * @tparam Getter Type of _getter_ function object.
     * @param first An iterator to the first element of the range to sort.
     * @param last An iterator past the last element of the range to sort.
     * @param getter A valid _getter_ function object.
     */
    template<typename It, typename Getter = identity>
    void operator()(It first, It last, Getter getter = Getter{}) const {
        if(first < last) {
            constexpr auto passes = N / Bit;

            using value_type = typename std::iterator_traits<It>::value_type;
            std::vector<value_type> aux(std::distance(first, last));

            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
                constexpr auto mask = (1 << Bit) - 1;
                constexpr auto buckets = 1 << Bit;

                std::size_t index[buckets]{};
                std::size_t count[buckets]{};

                for(auto it = from; it != to; ++it) {
                    ++count[(getter(*it) >> start) & mask];
                }

                for(std::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
                    index[pos + 1u] = index[pos] + count[pos];
                }

                for(auto it = from; it != to; ++it) {
                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
                }
            };

            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
                part(first, last, aux.begin(), pass * Bit);
                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
            }

            if constexpr(passes & 1) {
                part(first, last, aux.begin(), (passes - 1) * Bit);
                std::move(aux.begin(), aux.end(), first);
            }
        }
    }
};

} // namespace entt

#endif