using System;
using System.Threading;
using Microsoft.Extensions.ObjectPool;
using Robust.Shared.Configuration;
using Robust.Shared.IoC;
using Robust.Shared.Log;

namespace Robust.Shared.Threading;

[NotContentImplementable]
public interface IParallelManager
{
    event Action ParallelCountChanged;

    int ParallelProcessCount { get; }

    /// <summary>
    /// Add the delegate to <see cref="ParallelCountChanged"/> and immediately invoke it.
    /// </summary>
    void AddAndInvokeParallelCountChanged(Action changed);

    /// <summary>
    /// Takes in a job that gets flushed.
    /// </summary>
    /// <param name="job"></param>
    WaitHandle Process(IRobustJob job);

    public void ProcessNow(IRobustJob job);

    /// <summary>
    /// Takes in a parallel job and runs it the specified amount.
    /// </summary>
    void ProcessNow(IParallelRobustJob jobs, int amount);

    /// <summary>
    /// Processes a robust job sequentially if desired.
    /// </summary>
    void ProcessSerialNow(IParallelRobustJob jobs, int amount);

    /// <summary>
    /// Takes in a parallel job and runs it without blocking.
    /// </summary>
    WaitHandle Process(IParallelRobustJob jobs, int amount);

    /// <summary>
    /// Takes in a bulk parallel job and runs it the specified amount.
    /// </summary>
    /// <param name="jobs">The bulk parallel job to process.</param>
    /// <param name="amount">The total number of elements to process.</param>
    void ProcessNow(IParallelBulkRobustJob jobs, int amount);

    /// <summary>
    /// Processes a bulk robust job sequentially if desired.
    /// </summary>
    /// <param name="jobs">The bulk parallel job to process.</param>
    /// <param name="amount">The total number of elements to process.</param>
    void ProcessSerialNow(IParallelBulkRobustJob jobs, int amount);

    /// <summary>
    /// Takes in a bulk parallel job and runs it without blocking.
    /// </summary>
    /// <param name="jobs">The bulk parallel job to process.</param>
    /// <param name="amount">The total number of elements to process.</param>
    /// <returns>A wait handle that signals when the job is complete.</returns>
    WaitHandle Process(IParallelBulkRobustJob jobs, int amount);
}

internal interface IParallelManagerInternal : IParallelManager
{
    void Initialize();
}

internal sealed partial class ParallelManager : IParallelManagerInternal
{
    [Dependency] private IConfigurationManager _cfg = default!;
    [Dependency] private ILogManager _logs = default!;

    public event Action? ParallelCountChanged;
    public int ParallelProcessCount { get; private set; }

    public static readonly ManualResetEventSlim DummyResetEvent = new(true);

    private ISawmill _sawmill = default!;

    // Without pooling it's hard to keep task allocations down for classes
    // This lets us avoid re-allocating the ManualResetEventSlims constantly when we just need a way to signal job completion
    // and Parallel.For is really not built for running parallel tasks every tick.

    private readonly ObjectPool<InternalJob> _jobPool =
        new DefaultObjectPool<InternalJob>(new DefaultPooledObjectPolicy<InternalJob>(), 1024);

    private readonly ObjectPool<InternalParallelRangeJob> _parallelPool =
        new DefaultObjectPool<InternalParallelRangeJob>(new DefaultPooledObjectPolicy<InternalParallelRangeJob>(), 1024);

    /// <summary>
    /// Used internally for Parallel jobs, for external callers it gets garbage collected.
    /// </summary>
    private readonly ObjectPool<ParallelTracker> _trackerPool =
        new DefaultObjectPool<ParallelTracker>(new DefaultPooledObjectPolicy<ParallelTracker>(), 1024);

    public void Initialize()
    {
        _sawmill = _logs.GetSawmill("parallel");
        _cfg.OnValueChanged(CVars.ThreadParallelCount, UpdateCVar, true);
    }

    public void AddAndInvokeParallelCountChanged(Action changed)
    {
        ParallelCountChanged += changed;
        changed();
    }

    private InternalJob GetJob(IRobustJob job)
    {
        var robustJob = _jobPool.Get();
        robustJob.Event.Reset();
        robustJob.Set(_sawmill, job, _jobPool);
        return robustJob;
    }

    private InternalParallelRangeJob GetParallelJob(
        IParallelRangeRobustJob job,
        int start,
        int end,
        ParallelTracker tracker)
    {
        var internalJob = _parallelPool.Get();
        internalJob.Set(_sawmill, job, start, end, tracker, _parallelPool);
        return internalJob;
    }

    private void UpdateCVar(int value)
    {
        var oldCount = ParallelProcessCount;
        ThreadPool.GetAvailableThreads(out var oldWorker, out var oldCompletion);
        ParallelProcessCount = value == 0 ? oldWorker : value;

        if (oldCount != ParallelProcessCount)
        {
            ParallelCountChanged?.Invoke();
            ThreadPool.SetMaxThreads(ParallelProcessCount, oldCompletion);
        }
    }

    /// <inheritdoc/>
    public WaitHandle Process(IRobustJob job)
    {
        var subJob = GetJob(job);
        // From what I can tell preferLocal is more of a !forceGlobal flag.
        // Also UnsafeQueue should be fine as long as we don't use async locals.
        ThreadPool.UnsafeQueueUserWorkItem(subJob, true);
        return subJob.Event.WaitHandle;
    }

    public void ProcessNow(IRobustJob job)
    {
        job.Execute();
    }

    public void ProcessNow(IParallelRobustJob jobs, int amount) =>
        ProcessNow((IParallelRangeRobustJob) jobs, amount);

    public void ProcessNow(IParallelBulkRobustJob jobs, int amount) =>
        ProcessNow((IParallelRangeRobustJob) jobs, amount);

    public void ProcessSerialNow(IParallelRobustJob jobs, int amount) =>
        ProcessSerialNow((IParallelRangeRobustJob) jobs, amount);

    public void ProcessSerialNow(IParallelBulkRobustJob jobs, int amount) =>
        ProcessSerialNow((IParallelRangeRobustJob) jobs, amount);

    public WaitHandle Process(IParallelRobustJob jobs, int amount) =>
        Process((IParallelRangeRobustJob) jobs, amount);

    public WaitHandle Process(IParallelBulkRobustJob jobs, int amount) =>
        Process((IParallelRangeRobustJob) jobs, amount);

    public void ProcessNow(IParallelRangeRobustJob job, int amount)
    {
        var batches = amount / (float) job.BatchSize;

        // Below the threshold so just do it now.
        if (batches <= job.MinimumBatchParallel)
        {
            ProcessSerialNow(job, amount);
            return;
        }

        var tracker = InternalProcess(job, amount);
        tracker.Event.WaitHandle.WaitOne();
        _trackerPool.Return(tracker);
    }

    public void ProcessSerialNow(IParallelRangeRobustJob jobs, int amount)
    {
        if (amount <= 0)
            return;

        jobs.ExecuteRange(0, amount);
    }

    public WaitHandle Process(IParallelRangeRobustJob job, int amount)
    {
        var tracker = InternalProcess(job, amount);
        return tracker.Event.WaitHandle;
    }

    /// <summary>
    /// Runs a parallel job internally. Used so we can pool the tracker task for ProcessParallelNow
    /// and not rely on external callers to return it where they don't want to wait.
    /// </summary>
    private ParallelTracker InternalProcess(IParallelRangeRobustJob job, int amount)
    {
        var batches = (int) MathF.Ceiling(amount / (float) job.BatchSize);
        var batchSize = job.BatchSize;
        var tracker = _trackerPool.Get();

        // Need to set this up front to avoid firing too early.
        tracker.Event.Reset();
        if (amount <= 0)
        {
            tracker.Event.Set();
            return tracker;
        }

        tracker.PendingTasks = batches;

        for (var i = 0; i < batches; i++)
        {
            var start = i * batchSize;
            var end = Math.Min(start + batchSize, amount);
            var subJob = GetParallelJob(job, start, end, tracker);

            // From what I can tell preferLocal is more of a !forceGlobal flag.
            // Also UnsafeQueue should be fine as long as we don't use async locals.
            ThreadPool.UnsafeQueueUserWorkItem(subJob, true);
        }

        return tracker;
    }

    #region Jobs

    /// <summary>
    /// Runs an <see cref="IRobustJob"/> and handles cleanup.
    /// </summary>
    private sealed class InternalJob : IRobustJob, IThreadPoolWorkItem
    {
        private ISawmill _sawmill = default!;
        private IRobustJob _robust = default!;

        public readonly ManualResetEventSlim Event = new();
        private ObjectPool<InternalJob> _parentPool = default!;

        public void Set(ISawmill sawmill, IRobustJob job, ObjectPool<InternalJob> parentPool)
        {
            _sawmill = sawmill;
            _robust = job;
            _parentPool = parentPool;
        }

        public void Execute()
        {
            try
            {
                _robust.Execute();
            }
            catch (Exception exc)
            {
                _sawmill.Error($"Exception in ParallelManager: {exc}");
            }
            finally
            {
                Event.Set();
                _parentPool.Return(this);
            }
        }
    }

    /// <summary>
    /// Runs a <see cref="IParallelRangeRobustJob"/> for a specified range and handles cleanup.
    /// This is so jobs that process per-element (<see cref="IParallelRobustJob"/>)
    /// and jobs that process in bulk (<see cref="IParallelBulkRobustJob"/>) can both use it.
    /// </summary>
    private sealed class InternalParallelRangeJob : IRobustJob, IThreadPoolWorkItem
    {
        private IParallelRangeRobustJob _robust = default!;
        private int _start;
        private int _end;

        private ISawmill _sawmill = default!;

        private ParallelTracker _tracker = default!;
        private ObjectPool<InternalParallelRangeJob> _parentPool = default!;

        public void Set(
            ISawmill sawmill,
            IParallelRangeRobustJob robust,
            int start,
            int end,
            ParallelTracker tracker,
            ObjectPool<InternalParallelRangeJob> parentPool)
        {
            _sawmill = sawmill;
            _robust = robust;
            _start = start;
            _end = end;
            _tracker = tracker;
            _parentPool = parentPool;
        }

        public void Execute()
        {
            try
            {
                _robust.ExecuteRange(_start, _end);
            }
            catch (Exception exc)
            {
                _sawmill.Error($"Exception in ParallelManager: {exc}");
            }
            finally
            {
                // Task is done, so tell the tracker that it has one less task to process.
                // And of course return the job to the pool.
                _tracker.Set();
                _parentPool.Return(this);
            }
        }
    }

    /// <summary>
    /// Tracks jobs internally. This is because WaitHandle has a max limit of 64 tasks.
    /// So we'll just decrement PendingTasks in lieu.
    /// </summary>
    private sealed class ParallelTracker
    {
        public readonly ManualResetEventSlim Event = new();
        public int PendingTasks;

        /// <summary>
        /// Marks the tracker as having 1 less pending task.
        /// </summary>
        public void Set()
        {
            // We should atomically get new value of PendingTasks
            // as the result of Decrement call and use it to prevent data race.
            if (Interlocked.Decrement(ref PendingTasks) <= 0)
                Event.Set();
        }
    }

    #endregion
}