include/latch

// -*- C++ -*-
//===--------------------------- latch -----------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP_LATCH
#define _LIBCPP_LATCH

/*
    latch synopsis

namespace std
{

  class latch
  {
  public:
    constexpr explicit latch(ptrdiff_t __expected);
    ~latch();

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

    void count_down(ptrdiff_t __update = 1);
    bool try_wait() const noexcept;
    void wait() const;
    void arrive_and_wait(ptrdiff_t __update = 1);

  private:
    ptrdiff_t __counter; // exposition only
  };

}

*/

#include <__config>
#include <atomic>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

#ifdef _LIBCPP_HAS_NO_THREADS
# error <latch> is not supported on this single threaded system
#endif

#if _LIBCPP_STD_VER < 14
# error <latch> requires C++14 or later
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

class latch
{
    __atomic_base<ptrdiff_t> __a;

public:
    static constexpr ptrdiff_t max() noexcept {
        return numeric_limits<ptrdiff_t>::max();
    }

    inline _LIBCPP_INLINE_VISIBILITY
    constexpr explicit latch(ptrdiff_t __expected) : __a(__expected) { }

    ~latch() = default;
    latch(const latch&) = delete;
    latch& operator=(const latch&) = delete;

    inline _LIBCPP_INLINE_VISIBILITY
    void count_down(ptrdiff_t __update = 1)
    {
        auto const __old = __a.fetch_sub(__update, memory_order_release);
        if(__old == __update)
            __a.notify_all();
    }
    inline _LIBCPP_INLINE_VISIBILITY
    bool try_wait() const noexcept
    {
        return 0 == __a.load(memory_order_acquire);
    }
    inline _LIBCPP_INLINE_VISIBILITY
    void wait() const
    {
        auto const __test_fn = [=]() -> bool {
            return try_wait();
        };
        __cxx_atomic_wait(&__a.__a_, __test_fn);
    }
    inline _LIBCPP_INLINE_VISIBILITY
    void arrive_and_wait(ptrdiff_t __update = 1)
    {
        count_down(__update);
        wait();
    }
};

_LIBCPP_END_NAMESPACE_STD

#endif //_LIBCPP_LATCH