/*
 * Copyright (c) 2023 Proton AG
 *
 * This file is part of ProtonVPN.
 *
 * ProtonVPN 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 of the License, or
 * (at your option) any later version.
 *
 * ProtonVPN 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.
 *
 * You should have received a copy of the GNU General Public License
 * along with ProtonVPN.  If not, see <https://www.gnu.org/licenses/>.
 */

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ProtonVPN.Common.Legacy.Threading;

/// <summary>
/// Implements consumer in a simple producer/consumer pattern. Multiple producers are producing work,
/// and a single consumer processes the work.
/// </summary>
public class CoalescingAction
{
    private readonly Func<CancellationToken, Task> _action;
    private readonly CancellationHandle _cancellationHandle;

    private volatile Task _currentTask = Task.CompletedTask;
    private volatile int _workRequested;

    public CoalescingAction(Action action) : this(ct =>
    {
        action();
        return Task.CompletedTask;
    })
    { }

    public CoalescingAction(Func<Task> action) : this(ct => action())
    { }

    public CoalescingAction(Func<CancellationToken, Task> action)
    {
        _action = action;

        _cancellationHandle = new CancellationHandle();
    }

    public event EventHandler<TaskCompletedEventArgs> Completed;

    public bool Running { get; private set; }

    public void Run()
    {
        if (Interlocked.Exchange(ref _workRequested, 1) != 0)
        {
            return;
        }

        Running = true;

        TaskCompletionSource<object> taskCompletion = new TaskCompletionSource<object>();
        Task<object> newTask = taskCompletion.Task;
        CancellationToken cancellationToken = _cancellationHandle.Token;

        while (true)
        {
            Task expectedTask = _currentTask;
            Task previousTask = Interlocked.CompareExchange(ref _currentTask, newTask, expectedTask);
            if (previousTask != expectedTask)
            {
                continue;
            }

            // ReSharper disable once PossibleNullReferenceException
            Task task = previousTask.IsCompleted
                ? Task.Run(() => Run(cancellationToken), cancellationToken)
                : previousTask.ContinueWith(_ => Run(cancellationToken), cancellationToken, TaskContinuationOptions.LazyCancellation, TaskScheduler.Current).Unwrap();

            task.ContinueWith(t => OnCompleted(t, taskCompletion), TaskContinuationOptions.ExecuteSynchronously);

            return;
        }
    }

    public void Cancel()
    {
        _cancellationHandle.Cancel();
        _workRequested = 0;
    }

    private Task Run(CancellationToken cancellationToken)
    {
        Running = true;
        if (Interlocked.Exchange(ref _workRequested, 0) == 0)
        {
            return Task.CompletedTask;
        }

        return _action(cancellationToken);
    }

    private void OnCompleted(Task task, TaskCompletionSource<object> taskCompletion)
    {
        Running = _workRequested != 0;
        Completed?.Invoke(this, new TaskCompletedEventArgs(task));
        taskCompletion.SetResult(null);
    }
}