sparse_set.hpp

#ifndef ENTT_ENTITY_SPARSE_SET_HPP
#define ENTT_ENTITY_SPARSE_SET_HPP


#include <algorithm>
#include <iterator>
#include <utility>
#include <vector>
#include <memory>
#include <cstddef>
#include <numeric>
#include <type_traits>
#include "../config/config.h"
#include "../core/algorithm.hpp"
#include "entity.hpp"
#include "fwd.hpp"


namespace entt {


template<typename Entity>
class sparse_set {
    using traits_type = entt_traits<std::underlying_type_t<Entity>>;

    static_assert(ENTT_PAGE_SIZE && ((ENTT_PAGE_SIZE & (ENTT_PAGE_SIZE - 1)) == 0));
    static constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(typename traits_type::entity_type);

    class iterator {
        friend class sparse_set<Entity>;

        using direct_type = const std::vector<Entity>;
        using index_type = typename traits_type::difference_type;

        iterator(direct_type *ref, const index_type idx) ENTT_NOEXCEPT
            : direct{ref}, index{idx}
        {}

    public:
        using difference_type = index_type;
        using value_type = Entity;
        using pointer = const value_type *;
        using reference = const value_type &;
        using iterator_category = std::random_access_iterator_tag;

        iterator() ENTT_NOEXCEPT = default;

        iterator & operator++() ENTT_NOEXCEPT {
            return --index, *this;
        }

        iterator operator++(int) ENTT_NOEXCEPT {
            iterator orig = *this;
            return ++(*this), orig;
        }

        iterator & operator--() ENTT_NOEXCEPT {
            return ++index, *this;
        }

        iterator operator--(int) ENTT_NOEXCEPT {
            iterator orig = *this;
            return --(*this), orig;
        }

        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
            index -= value;
            return *this;
        }

        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
            return iterator{direct, index-value};
        }

        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
            return (*this += -value);
        }

        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
            return (*this + -value);
        }

        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
            return other.index - index;
        }

        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
            const auto pos = size_type(index-value-1);
            return (*direct)[pos];
        }

        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
            return other.index == index;
        }

        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
            return !(*this == other);
        }

        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
            return index > other.index;
        }

        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
            return index < other.index;
        }

        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
            return !(*this > other);
        }

        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
            return !(*this < other);
        }

        pointer operator->() const ENTT_NOEXCEPT {
            const auto pos = size_type(index-1);
            return &(*direct)[pos];
        }

        reference operator*() const ENTT_NOEXCEPT {
            return *operator->();
        }

    private:
        direct_type *direct;
        index_type index;
    };

    void assure(const std::size_t page) {
        if(!(page < reverse.size())) {
            reverse.resize(page+1);
        }

        if(!reverse[page]) {
            reverse[page] = std::make_unique<entity_type[]>(entt_per_page);
            // null is safe in all cases for our purposes
            std::fill_n(reverse[page].get(), entt_per_page, null);
        }
    }

    auto map(const Entity entt) const ENTT_NOEXCEPT {
        const auto identifier = to_integer(entt) & traits_type::entity_mask;
        const auto page = size_type(identifier / entt_per_page);
        const auto offset = size_type(identifier & (entt_per_page - 1));
        return std::make_pair(page, offset);
    }

public:
    using entity_type = Entity;
    using size_type = std::size_t;
    using iterator_type = iterator;

    sparse_set() = default;

    sparse_set(const sparse_set &other)
        : reverse{},
          direct{other.direct}
    {
        for(size_type pos{}, last = other.reverse.size(); pos < last; ++pos) {
            if(other.reverse[pos]) {
                assure(pos);
                std::copy_n(other.reverse[pos].get(), entt_per_page, reverse[pos].get());
            }
        }
    }

    sparse_set(sparse_set &&) = default;

    virtual ~sparse_set() ENTT_NOEXCEPT = default;

    sparse_set & operator=(const sparse_set &other) {
        if(&other != this) {
            auto tmp{other};
            *this = std::move(tmp);
        }

        return *this;
    }

    sparse_set & operator=(sparse_set &&) = default;

    void reserve(const size_type cap) {
        direct.reserve(cap);
    }

    size_type capacity() const ENTT_NOEXCEPT {
        return direct.capacity();
    }

    void shrink_to_fit() {
        // conservative approach
        if(direct.empty()) {
            reverse.clear();
        }

        reverse.shrink_to_fit();
        direct.shrink_to_fit();
    }

    size_type extent() const ENTT_NOEXCEPT {
        return reverse.size() * entt_per_page;
    }

    size_type size() const ENTT_NOEXCEPT {
        return direct.size();
    }

    bool empty() const ENTT_NOEXCEPT {
        return direct.empty();
    }

    const entity_type * data() const ENTT_NOEXCEPT {
        return direct.data();
    }

    iterator_type begin() const ENTT_NOEXCEPT {
        const typename traits_type::difference_type pos = direct.size();
        return iterator_type{&direct, pos};
    }

    iterator_type end() const ENTT_NOEXCEPT {
        return iterator_type{&direct, {}};
    }

    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
        return has(entt) ? --(end() - index(entt)) : end();
    }

    bool has(const entity_type entt) const ENTT_NOEXCEPT {
        auto [page, offset] = map(entt);
        // testing against null permits to avoid accessing the direct vector
        return (page < reverse.size() && reverse[page] && reverse[page][offset] != null);
    }

    size_type index(const entity_type entt) const ENTT_NOEXCEPT {
        ENTT_ASSERT(has(entt));
        auto [page, offset] = map(entt);
        return size_type(reverse[page][offset]);
    }

    void construct(const entity_type entt) {
        ENTT_ASSERT(!has(entt));
        auto [page, offset] = map(entt);
        assure(page);
        reverse[page][offset] = entity_type(direct.size());
        direct.push_back(entt);
    }

    template<typename It>
    void batch(It first, It last) {
        std::for_each(std::make_reverse_iterator(last), std::make_reverse_iterator(first), [this, next = direct.size()](const auto entt) mutable {
            ENTT_ASSERT(!has(entt));
            auto [page, offset] = map(entt);
            assure(page);
            reverse[page][offset] = entity_type(next++);
        });

        direct.insert(direct.end(), first, last);
    }

    void destroy(const entity_type entt) {
        ENTT_ASSERT(has(entt));
        auto [from_page, from_offset] = map(entt);
        auto [to_page, to_offset] = map(direct.back());
        direct[size_type(reverse[from_page][from_offset])] = entity_type(direct.back());
        reverse[to_page][to_offset] = reverse[from_page][from_offset];
        reverse[from_page][from_offset] = null;
        direct.pop_back();
    }

    virtual void swap(const size_type lhs, const size_type rhs) ENTT_NOEXCEPT {
        ENTT_ASSERT(lhs < direct.size());
        ENTT_ASSERT(rhs < direct.size());
        auto [src_page, src_offset] = map(direct[lhs]);
        auto [dst_page, dst_offset] = map(direct[rhs]);
        std::swap(reverse[src_page][src_offset], reverse[dst_page][dst_offset]);
        std::swap(direct[lhs], direct[rhs]);
    }

    template<typename Compare, typename Sort = std_sort, typename... Args>
    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
        ENTT_ASSERT(!(first > last));

        std::vector<size_type> copy(last - first);
        const auto offset = std::distance(last, end());
        std::iota(copy.begin(), copy.end(), size_type{});

        algo(copy.rbegin(), copy.rend(), [this, offset, compare = std::move(compare)](const auto lhs, const auto rhs) {
            return compare(std::as_const(direct[lhs+offset]), std::as_const(direct[rhs+offset]));
        }, std::forward<Args>(args)...);

        for(size_type pos{}, length = copy.size(); pos < length; ++pos) {
            auto curr = pos;
            auto next = copy[curr];

            while(curr != next) {
                swap(copy[curr] + offset, copy[next] + offset);
                copy[curr] = curr;
                curr = next;
                next = copy[curr];
            }
        }
    }

    void respect(const sparse_set &other) ENTT_NOEXCEPT {
        const auto to = other.end();
        auto from = other.begin();

        size_type pos = direct.size() - 1;

        while(pos && from != to) {
            if(has(*from)) {
                if(*from != direct[pos]) {
                    swap(pos, index(*from));
                }

                --pos;
            }

            ++from;
        }
    }

    void reset() {
        reverse.clear();
        direct.clear();
    }

private:
    std::vector<std::unique_ptr<entity_type[]>> reverse;
    std::vector<entity_type> direct;
};


}


#endif // ENTT_ENTITY_SPARSE_SET_HPP