libcruft-util/pool.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2011-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "cast.hpp"
#include "debug/assert.hpp"
#include "parallel/stack.hpp"
#include "view.hpp"

#include <atomic>
#include <new>

#include <cstddef>
#include <cstdint>

namespace cruft {
    /// A simple, thread safe, pre-allocated pool allocator.
    template <typename T>
    class pool {
    private:
        /// A collection of all unallocated slots.
        parallel::stack<void*> m_available;

        /// A pointer to the start of the allocated region.
        void *m_store;

        /// The total number of items for which storage has been allocated.
        std::size_t m_capacity;

        // We used to use std::aligned_storage_t and arrays/vectors proper for
        // data storage. But this requires that the user has already defined
        // ValueT ahead of time (given we need to call sizeof outside a deduced
        // context).
        //
        // We tried a strategy where nodes were a union of ValueT and a
        // linked-list. However this proved heinously expensive to traverse to
        // find allocated objects that need to be destroyed when our destructor
        // is called.

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


        //---------------------------------------------------------------------
        pool (pool &&rhs) noexcept
            : m_available (0)
            , m_capacity (0)
        {
            std::swap (m_available, rhs.m_available);
            std::swap (m_store,     rhs.m_store);
            std::swap (m_capacity,  rhs.m_capacity);
        }


        //---------------------------------------------------------------------
        pool& operator= (pool&&);


        //---------------------------------------------------------------------
        explicit
        pool (std::size_t _capacity)
            : m_available (_capacity)
            , m_store     (::operator new[] (_capacity * sizeof (T), std::align_val_t {alignof (T)}))
            , m_capacity  (_capacity)
        {
            if (!m_store)
                throw std::bad_alloc ();

            T* elements = reinterpret_cast<T*> (m_store);
            for (size_t i = 0; i < m_capacity; ++i)
                m_available.push (elements + i);
        }


        //---------------------------------------------------------------------
        ~pool ()
        {
            clear ();

            ::operator delete[] (
                m_store,
                m_capacity * sizeof (T),
                std::align_val_t {alignof (T)}
            );
        }


        ///////////////////////////////////////////////////////////////////////
        [[nodiscard]] T*
        allocate [[gnu::malloc]] [[gnu::returns_nonnull]] (void)
        {
            void *raw;
            if (!m_available.pop (&raw))
                throw std::bad_alloc ();
            return reinterpret_cast<T*> (raw);
        }


        //---------------------------------------------------------------------
        void
        deallocate (T *cooked)
        {
            void *raw = reinterpret_cast<void*> (cooked);
            if (unlikely (!m_available.push (raw)))
                panic (__FUNCTION__);
        }


        ///////////////////////////////////////////////////////////////////////
        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);
        }


        //---------------------------------------------------------------------
        void destroy (size_t idx)
        {
            return destroy (&(*this)[idx]);
        }


        ///////////////////////////////////////////////////////////////////////
        auto capacity (void) const { return m_capacity; }
        auto size     (void) const { return capacity () - m_available.size (); }
        bool empty    (void) const { return size () == 0; }
        bool full     (void) const { return size () == capacity (); }


        /// Destroys all objects that have been allocated, frees the
        /// associated memory, and then rebuilds the free node list ready for
        /// allocations again.
        ///
        /// NOTE: All bets are off if any object throws an exception out of
        /// their destructor. We provide no exception guarantees.
        ///
        /// This call is NOT thread safe. No users should be accessing this
        /// object for the duration of this call.
        void clear (void)
        {
            auto const valid_queue = m_available.store (
                decltype(m_available)::contract::I_HAVE_LOCKED_THIS_STRUCTURE
            );
            std::sort (valid_queue.begin (), valid_queue.end ());

            // Now that we've ordered the nodes we can walk the list from
            // start to finish and find nodes that aren't in the free list.
            // Call the destructors on the data contained in these.
            auto node_cursor = valid_queue.begin ();
            auto data_cursor = reinterpret_cast<T*> (m_store);
            auto const data_end = data_cursor + m_capacity;

            while (node_cursor != valid_queue.end ()) {
                while (&*data_cursor < *node_cursor) {
                    reinterpret_cast<T*> (&*data_cursor)->~T ();
                    ++data_cursor;
                }

                ++node_cursor;
                ++data_cursor;
            }

            while (data_cursor != data_end) {
                reinterpret_cast<T*> (&*data_cursor)->~T ();
                ++data_cursor;
            }

            m_available.clear ();
            T* elements = reinterpret_cast<T*> (m_store);
            for (size_t i = 0; i < m_capacity; ++i)
                m_available.push (elements + i);
        }


        ///////////////////////////////////////////////////////////////////////
        size_t index (T const *ptr) const
        {
            return ptr - reinterpret_cast<T*> (m_store);
        }


        /// returns the base address of the allocation.
        ///
        /// guaranteed to point to the first _possible_ allocated value;
        /// however it may not be _live_ at any given moment.
        ///
        /// DO NOT use this pointer for indexing as you _may_ be unable to
        /// account for internal node sizes, alignment, or padding. This is
        /// why the return type is void.
        ///
        /// We may be using one particular representation at the moment but
        /// stability is not guaranteed at this point.
        void      * base (void)      & { return m_store; }
        void const* base (void) const& { return m_store; }


        ///////////////////////////////////////////////////////////////////////
        T& operator[] (size_t idx) &
        {
            return reinterpret_cast<T*> (m_store) [idx];
        }


        //---------------------------------------------------------------------
        T const& operator[] (size_t idx) const&
        {
            return reinterpret_cast<T*> (m_store) [idx];
        }
    };
}