latch.hpp

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// SPDX-FileCopyrightText: 2006-2024 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2024 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: BSD-3-Clause
 
#pragma once
 
#include <atomic>
#include <cassert>
#include <seqan3/std/new>
 
#include <seqan3/utility/parallel/detail/spin_delay.hpp>
 
namespace seqan3::detail
{
 
class latch
{
public:
    latch() = delete;                          
    latch(latch const &) = delete;             
    latch(latch &&) = delete;                  
    latch & operator=(latch const &) = delete; 
    latch & operator=(latch &&) = delete;      
 
    ~latch()
    {
        spin_delay delay{};
        while (num_waiting.load(std::memory_order_acquire) > 0)
            delay.wait(); // LCOV_EXCL_LINE
    }
 
    explicit latch(ptrdiff_t const expected) : counter{expected}
    {
        assert(expected >= 0);
        num_waiting.store(0, std::memory_order_relaxed);
    }
 
    void arrive(ptrdiff_t n = 1) noexcept
    {
        assert(counter.load(std::memory_order_acquire) >= n);
        assert(counter.load(std::memory_order_acquire) >= 0);
 
        counter.fetch_sub(n, std::memory_order_acq_rel);
    }
 
    void arrive_and_wait(ptrdiff_t n = 1) noexcept
    {
        ++num_waiting; // ensure that destructor is not finished in-between the arrive and wait call.
        arrive(n);
        wait();
        --num_waiting;
    }
 
    bool try_wait() const noexcept
    {
        return counter.load(std::memory_order_acquire) == 0;
    }
 
    void wait() const
    {
        ++num_waiting; // register waiting thread to synchronise with destructor.
        spin_delay delay{};
        while (counter.load(std::memory_order_acquire) > 0)
            delay.wait();
        --num_waiting;
    }
 
private:
    alignas(std::hardware_destructive_interference_size) std::atomic<std::ptrdiff_t> counter;
    alignas(std::hardware_destructive_interference_size) mutable std::atomic<std::ptrdiff_t> num_waiting;
};
 
} // namespace seqan3::detail