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 <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 : public nocopy {
    protected:
        union alignas (T) node {
            node *_node;
            char _data[sizeof (T)];
        };

        node *m_head; // root address of allocation
        node *m_next; // next available entry in the linked list

        const size_t m_capacity;
        size_t m_size;

    public:
        explicit
        pool (unsigned int _capacity):
            m_capacity (_capacity),
            m_size     (0u)
        {
            static_assert (sizeof (T) >= sizeof (uintptr_t),
                           "pool<T>'s chained block system requires that T be at least pointer sized");

            // allocate the memory and note the base address for deletion in destructor
            m_next = m_head = new node[m_capacity]; // static_cast<node *> (operator new (sizeof (T) * m_capacity));

            // initialise the linked list of nodes
            for (unsigned int i = 0; i < m_capacity - 1; ++i)
                m_next[i]._node = m_next + i + 1;
            m_next[m_capacity - 1]._node = 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
        template <typename ...Args>
        T*   acquire (Args&... args)
        {
            // double check we have enough capacity left
            if (!m_next)
                throw std::bad_alloc ();
            CHECK_LT (m_size, m_capacity);

            // save what will become the next node shortly. it could be overwritten
            // in the constructor we're about to call.
            node *newnext = m_next->_node;
            T *data = reinterpret_cast<T*> (m_next);

            // try to construct the returnable object.
            try {
                new (data) T (args...);
            } catch (...) {
                // the constructor may have overwritten the node linkages before
                // throwing. fix this up before forwarding the exception.
                m_next->_node = newnext;
                throw;
            }

            // the object is valid. save the new linked list head and bump the
            // stats for availability.
            m_next  = newnext;
            m_size++;

            return data;
        }

        void release (T *data)
        {
            CHECK_NEZ (m_size);

            data->~T();
            node *newnode = reinterpret_cast<node *> (data);

            newnode->_node = m_next;
            m_next = newnode;
            m_size--;
        }

        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