id_manager.hpp

/*
 * mstd - Maipa's Standard Library
 *
 * Licensed under the BSD 3-Clause License with Attribution Requirement.
 * See the LICENSE file for details: https://github.com/MAIPA01/mstd/blob/main/LICENSE
 *
 * Copyright (c) 2025, Patryk Antosik (MAIPA01)
 */
 
#pragma once
#ifndef _MSTD_ID_MANAGER_HPP_
    #define _MSTD_ID_MANAGER_HPP_
 
    #include <mstd/config.hpp>
 
    #if !_MSTD_HAS_CXX17
_MSTD_WARNING("this is only available for c++17 and greater!");
    #else
 
        #include <mstd/types.hpp>
 
namespace mstd {
        #if _MSTD_HAS_CXX20
    template<unsigned_integral IdT>
        #else
    template<class IdT, std::enable_if_t<mstd::is_unsigned_integral_v<IdT>, bool> >
        #endif
    class base_id_manager {
    public:
        using id_type                            = IdT;
 
        static _MSTD_CONSTEXPR17 id_type max_ids = std::numeric_limits<id_type>::max();
 
    private:
        id_type _nextId               = 0;
        std::set<id_type> _removedIds = {};
 
        _MSTD_CONSTEXPR20 void _update_removed_ids() {
                if (_removedIds.empty()) { return; }
 
            auto last = std::prev(_removedIds.end());
                while (*last == _nextId - 1) {
                    --_nextId;
                    _removedIds.erase(*last);
 
                        if (_removedIds.empty()) { return; }
 
                    last = std::prev(_removedIds.end());
                }
        }
 
    public:
        _MSTD_CONSTEXPR20 base_id_manager() noexcept                                        = default;
        _MSTD_CONSTEXPR20 base_id_manager(const base_id_manager& other) noexcept            = default;
        _MSTD_CONSTEXPR20 base_id_manager(base_id_manager&& other) noexcept                 = default;
        _MSTD_CONSTEXPR20 ~base_id_manager() noexcept                                       = default;
 
        _MSTD_CONSTEXPR20 base_id_manager& operator=(const base_id_manager& other) noexcept = default;
        _MSTD_CONSTEXPR20 base_id_manager& operator=(base_id_manager&& other) noexcept      = default;
 
        [[nodiscard]] _MSTD_CONSTEXPR20 id_type get_next_id() {
                if (!_removedIds.empty()) {
                    const id_type id = *_removedIds.begin();
                    _removedIds.erase(id);
                    return id;
                }
 
                if (_nextId == max_ids) { return bad_id(); }
 
            const id_type id = _nextId;
            ++_nextId;
            return id;
        }
 
        _MSTD_CONSTEXPR20 bool return_id(id_type id) {
        #if _MSTD_HAS_CXX20
                if (id == bad_id() || id >= _nextId || _removedIds.contains(id)) {
        #else
                if (id == bad_id() || id >= _nextId || _removedIds.find(id) != _removedIds.end()) {
        #endif
                    return false;
                }
 
            _removedIds.insert(id);
            _update_removed_ids();
            return true;
        }
 
        _MSTD_CONSTEXPR20 void reset() noexcept {
            _nextId = 0;
            _removedIds.clear();
        }
 
        [[nodiscard]] static _MSTD_CONSTEXPR20 id_type bad_id() noexcept { return ~static_cast<id_type>(0); }
 
        [[nodiscard]] static _MSTD_CONSTEXPR20 id_type last_id() noexcept { return max_ids - 1; }
    };
} // namespace mstd
    #endif
#endif