//

// immer: immutable data structures for C++

// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente

//

// This software is distributed under the Boost Software License, Version 1.0.

// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt

//

#pragma once

#include <immer/heap/tags.hpp>

#include <atomic>

#include <memory>

#include <utility>

namespace immer {

/*!

 * Provides transience ownership tracking when a *tracing garbage

 * collector* is used instead of reference counting.

 *

 * @rst

 *

 * .. warning:: Using this policy without an allocation scheme that

 *    includes automatic tracing garbage collection may cause memory

 *    leaks.

 *

 * @endrst

 */

struct gc_transience_policy

{

    template <typename HeapPolicy>

    struct apply

    {

        struct type

        {

            using heap_ = typename HeapPolicy::type;

            struct edit

            {

                void* v;

                edit(void* v_)

                    : v{v_}

                {

                }

                edit() = delete;

                bool operator==(edit x) const { return v == x.v; }

                bool operator!=(edit x) const { return v != x.v; }

            };

            struct owner

            {

                void* make_token_()

                {

                    return heap_::allocate(1, norefs_tag{});

                };

                mutable std::atomic<void*> token_;

                operator edit() { return {token_}; }

                owner()

                    : token_{make_token_()}

                {

                }

                owner(const owner& o)

                    : token_{make_token_()}

                {

                    o.token_ = make_token_();

                }

                owner(owner&& o) noexcept

                    : token_{o.token_.load()}

                {

                }

                owner& operator=(const owner& o)

                {

                    o.token_ = make_token_();

                    token_   = make_token_();

                    return *this;

                }

                owner& operator=(owner&& o) noexcept

                {

                    token_ = o.token_.load();

                    return *this;

                }

            };

            struct ownee

            {

                edit token_{nullptr};

                ownee& operator=(edit e)

                {

                    assert(e != noone);

                    // This would be a nice safety plug but it sadly

                    // does not hold during transient concatenation.

                    // assert(token_ == e || token_ == edit{nullptr});

                    token_ = e;

                    return *this;

                }

                bool can_mutate(edit t) const { return token_ == t; }

                bool owned() const { return token_ != edit{nullptr}; }

            };

            static owner noone;

        };

    };

};

template <typename HP>

typename gc_transience_policy::apply<HP>::type::owner

    gc_transience_policy::apply<HP>::type::noone = {};

} // namespace immer