libcruft-util/alloc/raw/aligned/foreign.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 2016 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_ALLOC_RAW_ALIGNED_OFFSET_HPP
#define CRUFT_UTIL_ALLOC_RAW_ALIGNED_OFFSET_HPP

#include "direct.hpp"

#include "../../../cast.hpp"
#include "../../../pointer.hpp"
#include "../../../debug.hpp"

#include <cstddef>
#include <utility>

namespace cruft::alloc::raw::aligned {
    /// wraps a child allocator and enforces a fixed alignment that is
    /// independant of the alignment of the provided source buffer.
    ///
    /// we use the approach of:
    ///   * shifting the source buffer's alignment to satisfy the requested
    ///     alignment
    ///   * using a direct alignment child to service the requests
    ///   * then applying the reverse offset to values as we return the values
    ///
    /// This approach will explode if a child allocator wants to write to the
    /// range, so the user is mostly restricted to very simple allocators
    /// (like 'linear').
    ///
    /// We supply a proxy view at the (probably) invalid address:
    ///     (void*)alignment
    /// This removes any potential bias in the successor allocators. We very
    /// specifically cannot use nullptr because it breaks under optimisations
    /// due to undefined behaviour of null pointers.
    ///
    /// The proxy view will _probably_ result in a segfault if the successor
    /// allocator attempts to read/write directly to it given typical
    /// alignments will give addresses far below typical mappings.
    template <typename ChildT>
    class foreign {
    public:
        template <typename ...Args>
        foreign (cruft::view<std::byte*> _data, std::size_t _alignment, Args &&...args):
            m_successor (
                view<std::byte*> {
                    reinterpret_cast<std::byte*> (_alignment),
                    reinterpret_cast<std::byte*> (_alignment + _data.size ()),
                },
                _alignment,
                std::forward<Args> (args)...
            ),
            m_offset (_data.data () - m_successor.data ()),
            m_alignment (_alignment)
        { ; }


        template <typename T>
        cruft::view<T*>
        allocate (std::size_t count)
        {
            auto root = m_successor.template allocate<T> (count);
            auto base = root.template cast<char*> ().data ();

            // we can't use alignment cast here because it will almost
            // certainly fail the tests it performs.
            return { cruft::cast::ffs<T*> (base + m_offset), count };
        }

        template <typename T>
        auto
        deallocate (cruft::view<T*> ptr)
        {
            auto base = ptr.template cast<char*> ();
            auto next = base - m_offset;

            // we can't use alignment cast here because it will almost
            // certainly fail the tests it performs.
            return m_successor.template deallocate<T> (
                cruft::view<T*> { cruft::cast::ffs<T*> (next), ptr.size () }
            );
        }

        constexpr auto alignment (void) const noexcept { return m_alignment; }

        auto offset (const void *ptr) const
        {
            return m_successor.offset (
                reinterpret_cast<const std::byte*> (ptr) - m_offset
            );
        }

        auto data (void)       { return m_successor.data () + m_offset; }
        auto data (void) const { return m_successor.data () + m_offset; }

        auto begin (void)       { return m_successor.begin () + m_offset; }
        auto begin (void) const { return m_successor.begin () + m_offset; }

        auto end (void)       { return m_successor.end () + m_offset; }
        auto end (void) const { return m_successor.end () + m_offset; }

        auto reset (void) { return m_successor.reset (); }

        auto capacity (void) const { return m_successor.capacity (); }
        auto used (void) const { return m_successor.used (); }
        auto remain (void) const { return m_successor.remain (); }

    private:
        direct<ChildT> m_successor;
        std::ptrdiff_t m_offset;
        std::size_t m_alignment;
    };
}

#endif