libcruft-util/pool.hpp

/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Copyright 2011-2016 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_POOL_HPP
#define CRUFT_UTIL_POOL_HPP

#include "nocopy.hpp"
#include "debug.hpp"

#include <atomic>
#include <cstdlib>
#include <cstdint>

namespace util {
    /// a simple pre-allocated pool for storage of PODs.
    ///
    /// non-POD types can be stored, but there are no guarantees for calling
    /// item destructors at pool destruction time.
    template <typename T>
    class pool {
    protected:
        union node;

        union alignas (T) node {
            std::atomic<node*> next;
            char data[sizeof (T)];
        };

        static_assert (std::atomic<node*>::is_always_lock_free);

        // root address of allocation. used in deletion at destruction time.
        node* m_head;

        // the next available entry in the linked list
        std::atomic<node *> m_next;

        // the total number of items that could be stored
        const size_t m_capacity;

        // the number of items currently stored.
        std::atomic<size_t> m_size;

    public:
        pool (const pool&) = delete;
        pool& operator= (const pool&) = delete;

        pool (pool&&);
        pool& operator= (pool&&);

        explicit
        pool (unsigned int _capacity):
            m_capacity (_capacity),
            m_size (0u)
        {
            // allocate the memory and note the base address for deletion in destructor
            m_next = m_head = new node[m_capacity];

            // build out a complete singly linked list from all the nodes.
            for (size_t i = 0; i < m_capacity - 1; ++i)
                m_next[i].next = m_next + i + 1;
            m_next[m_capacity - 1].next = nullptr;
        }

        ~pool ()
        {
            // don't check if everything's been returned as pools are often used
            // for PODs which don't need to be destructed via calling release.
            delete [] m_head;
        }

        // Data management
        [[nodiscard]] T*
        allocate (void)
        {
            // double check we have enough capacity left
            if (!m_next)
                throw std::bad_alloc ();
            CHECK_LT (m_size, m_capacity);

            // unlink the current cursor
            do {
                node* curr = m_next;
                node* soon = curr->next;

                if (m_next.compare_exchange_weak (curr, soon)) {
                    ++m_size;
                    return std::launder (reinterpret_cast<T*> (curr));
                }
            } while (1);
        }

        void
        deallocate (T *base)
        {
            auto soon = reinterpret_cast<node*> (base);

            do {
                node *curr = m_next;
                soon->next = curr;
                if (m_next.compare_exchange_weak (curr, soon)) {
                    --m_size;
                    return;
                }
            } while (1);
        }


        template <typename ...Args>
        T*
        construct (Args &&...args)
        {
            auto ptr = allocate ();
            try {
                return new (ptr) T (std::forward<Args> (args)...);
            } catch (...) {
                deallocate (ptr);
                throw;
            }
        }


        void
        destroy (T *ptr)
        {
            ptr->~T ();
            deallocate (ptr);
        }


        size_t capacity (void) const
        {
            return m_capacity;
        }


        size_t size (void) const
        {
            return m_size;
        }


        bool empty (void) const
        {
            return m_size == m_capacity;
        }


        // Indexing
        size_t index (const T*) const;

        T& operator[] (size_t idx) &;

        const T& operator[] (size_t idx) const&;
    };
}

#endif