libstdc++-v3/include/std/latch

// <latch> -*- C++ -*-

// Copyright (C) 2020-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/latch
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_LATCH
#define _GLIBCXX_LATCH 1

#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif

#include <bits/requires_hosted.h> // concurrency

#define __glibcxx_want_latch
#include <bits/version.h>

#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
#include <bits/atomic_base.h>
#include <ext/numeric_traits.h>
#include <utility> // cmp_equal, cmp_less_equal, etc.

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  class latch
  {
  public:
    static constexpr ptrdiff_t
    max() noexcept
    {
      using __gnu_cxx::__int_traits;
      constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
      if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
	return __max;
      return __PTRDIFF_MAX__;
    }

    constexpr explicit
    latch(ptrdiff_t __expected) noexcept
    : _M_a(__expected)
    { __glibcxx_assert(__expected >= 0 && __expected <= max()); }

    ~latch() = default;

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

    _GLIBCXX_ALWAYS_INLINE void
    count_down(ptrdiff_t __update = 1)
    {
      __glibcxx_assert(__update >= 0 && __update <= max());
      auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,
						  memory_order::release);
      if (std::cmp_equal(__old, __update))
	__atomic_impl::notify_all(&_M_a);
      else
	__glibcxx_assert(std::cmp_less(__update, __old));
    }

    _GLIBCXX_ALWAYS_INLINE bool
    try_wait() const noexcept
    { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }

    _GLIBCXX_ALWAYS_INLINE void
    wait() const noexcept
    {
      auto const __pred = [this] { return this->try_wait(); };
      std::__atomic_wait_address(&_M_a, __pred);
    }

    _GLIBCXX_ALWAYS_INLINE void
    arrive_and_wait(ptrdiff_t __update = 1) noexcept
    {
      count_down(__update);
      wait();
    }

  private:
    alignas(__detail::__platform_wait_alignment)
      __detail::__platform_wait_t _M_a;
  };
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // __cpp_lib_latch
#endif // _GLIBCXX_LATCH