//

// 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 <atomic>

#include <thread>

// This has been shamelessly copied from boost...

#if defined(_MSC_VER) && _MSC_VER >= 1310 &&                                   \

    (defined(_M_IX86) || defined(_M_X64)) && !defined(__c2__)

extern "C" void _mm_pause();

#define IMMER_SMT_PAUSE _mm_pause()

#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))

#define IMMER_SMT_PAUSE __asm__ __volatile__("rep; nop" : : : "memory")

#endif

namespace immer {

// This is an atomic spinlock similar to the one used by boost to provide

// "atomic" shared_ptr operations.  It also does not differ much from the one

// from libc++ or libstdc++...

struct spinlock_policy

{

    std::atomic_flag v_{};

    bool try_lock() { return !v_.test_and_set(std::memory_order_acquire); }

    void lock()

    {

        for (auto k = 0u; !try_lock(); ++k) {

            if (k < 4)

                continue;

#ifdef IMMER_SMT_PAUSE

            else if (k < 16)

                IMMER_SMT_PAUSE;

#endif

            else

                std::this_thread::yield();

        }

    }

    void unlock() { v_.clear(std::memory_order_release); }

    struct scoped_lock

    {

        scoped_lock(const scoped_lock&)            = delete;

        scoped_lock& operator=(const scoped_lock&) = delete;

        explicit scoped_lock(spinlock_policy& sp)

            : sp_{sp}

        {

            sp.lock();

        }

        ~scoped_lock() { sp_.unlock(); }

    private:

        spinlock_policy& sp_;

    };

};

} // namespace immer