libcruft-util/cruft/util/alloc/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 2016-2018 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "direct.hpp"

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

#include <utility>


namespace cruft::alloc::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<u08*> _data, std::size_t _alignment, Args &&...args):
            m_successor (
                view<u08*> {
                    reinterpret_cast<u08*> (_alignment),
                    reinterpret_cast<u08*> (_alignment + _data.size ()),
                },
                _alignment,
                std::forward<Args> (args)...
            ),
            m_offset (_data.data () - m_successor.data ()),
            m_alignment (_alignment)
        { ; }


        cruft::view<u08*>
        allocate (std::size_t bytes, std::size_t alignment)
        {
            (void)alignment;
            auto inner = m_successor.allocate (bytes, m_alignment);
            return cruft::view { inner.begin () + m_offset, bytes };
        }


        cruft::view<u08*>
        allocate (std::size_t bytes)
        {
            auto res = m_successor.allocate (bytes);
            return { res.begin () + m_offset, bytes };
        }


        decltype(auto)
        deallocate (void *ptr, std::size_t bytes, std::size_t alignment)
        {
            CHECK_MOD (m_alignment, alignment);
            return m_successor.deallocate (reinterpret_cast<u08*> (ptr) - m_offset, bytes, alignment);
        }


        decltype(auto)
        deallocate (void *ptr, std::size_t bytes)
        {
            return m_successor.deallocate (reinterpret_cast<u08*> (ptr) - m_offset, bytes, m_alignment);
        }


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

        auto offset (const void *ptr) const
        {
            return m_successor.offset (
                reinterpret_cast<u08 const*> (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;
    };
}