/*************************************************************************
 * ModernUO                                                              *
 * Copyright 2019-2026 - ModernUO Development Team                       *
 * Email: hi@modernuo.com                                                *
 * File: SerializationThreadWorker.cs                                    *
 *                                                                       *
 * This program 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.                                   *
 *                                                                       *
 * You should have received a copy of the GNU General Public License     *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
 *************************************************************************/

using System;
using System.Runtime.CompilerServices;
using System.Threading;

namespace Server;

public class SerializationThreadWorker
{
    private const int MinHeapSize = 1024 * 1024; // 1MB
    private readonly int _index;
    private readonly Thread _thread;
    private readonly AutoResetEvent _startEvent; // Main thread tells the thread to start working
    private readonly AutoResetEvent _stopEvent; // Main thread waits for the worker finish draining
    private readonly SerializationChunkSource _chunkSource;
    private readonly int _heapSizeHint;
    private bool _pause;
    private bool _exit;
    private bool _exited;
    private byte[] _heap;
    private long _entitiesSerialized;
    private long _bytesSerialized;

    public SerializationThreadWorker(int index, SerializationChunkSource chunkSource, int heapSizeHint = 0)
        : this(index, chunkSource, heapSizeHint, inline: false)
    {
    }

    private SerializationThreadWorker(int index, SerializationChunkSource chunkSource, int heapSizeHint, bool inline)
    {
        _index = index;
        _chunkSource = chunkSource;
        _heapSizeHint = heapSizeHint;

        if (!inline)
        {
            _startEvent = new AutoResetEvent(false);
            _stopEvent = new AutoResetEvent(false);
            _thread = new Thread(Execute);
            _thread.Start(this);
        }
    }

    /// <summary>
    /// Creates a worker with no thread of its own. The owner drains chunks inline via
    /// <see cref="DrainInline"/> — used by the main thread to join the drain instead of
    /// idling while the thread workers finish.
    /// </summary>
    public static SerializationThreadWorker CreateInline(int index, SerializationChunkSource chunkSource, int heapSizeHint = 0) =>
        new(index, chunkSource, heapSizeHint, inline: true);

    // Stats from the most recent save, for diagnosing load balance.
    public long EntitiesSerialized => _entitiesSerialized;
    public long BytesSerialized => _bytesSerialized;

    public void Wake()
    {
        _startEvent.Set();
    }

    public void Sleep()
    {
        Volatile.Write(ref _pause, true);
        _stopEvent.WaitOne();
    }

    public void Exit()
    {
        if (_exited)
        {
            return;
        }

        _exited = true;
        _exit = true;
        Wake();
        Sleep();
    }

    // Sized from the previous world load so the first save doesn't pay copy-on-grow during the freeze.
    public void AllocateHeap() =>
        _heap ??= GC.AllocateUninitializedArray<byte>(Math.Max(MinHeapSize, _heapSizeHint));

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ReadOnlySpan<byte> GetHeap(int start, int length) => _heap.AsSpan(start, length);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    internal static void Serialize(IGenericSerializable e, BufferWriter writer, byte threadIndex)
    {
        e.SerializedThread = threadIndex;
        var start = e.SerializedPosition = (int)writer.Position;
        e.Serialize(writer);
        e.SerializedLength = (int)(writer.Position - start);
    }

    private static long ProcessChunk(
        in SerializationChunkSource.Chunk chunk, SerializationChunkSource chunkSource,
        BufferWriter writer, byte threadIndex
    )
    {
        if (chunk.Single != null)
        {
            Serialize(chunk.Single, writer, threadIndex);
            return 1;
        }

        if (chunk.Source != null)
        {
            return chunk.Source.SerializeRange(writer, threadIndex, chunk.Offset, chunk.Count);
        }

        var buffer = chunk.Buffer;
        var count = chunk.Count;
        for (var i = 0; i < count; i++)
        {
            Serialize(buffer[i], writer, threadIndex);
        }

        chunkSource.Return(buffer, count);
        return count;
    }

    /// <summary>
    /// Drains chunks on the calling thread until the queue is empty, then returns.
    /// Only valid on inline workers; the main thread calls this after publishing all work
    /// so it contributes drain throughput instead of idling.
    /// </summary>
    public void DrainInline()
    {
        var writer = new BufferWriter(_heap, true, World.SerializedTypes);
        var threadIndex = (byte)_index;
        var entities = 0L;

        while (_chunkSource.TryTake(out var chunk))
        {
            entities += ProcessChunk(in chunk, _chunkSource, writer, threadIndex);
        }

        _heap = writer.Buffer;
        _entitiesSerialized = entities;
        _bytesSerialized = writer.Position;

        writer.Close();
    }

    private static void Execute(object obj)
    {
        var worker = (SerializationThreadWorker)obj;
        var threadIndex = (byte)worker._index;

        var chunkSource = worker._chunkSource;
        var serializedTypes = World.SerializedTypes;

        while (worker._startEvent.WaitOne())
        {
            var writer = new BufferWriter(worker._heap, true, serializedTypes);
            var entities = 0L;
            var spinner = new SpinWait();

            while (true)
            {
                var pauseRequested = Volatile.Read(ref worker._pause);
                if (chunkSource.TryTake(out var chunk))
                {
                    spinner.Reset();
                    entities += ProcessChunk(in chunk, chunkSource, writer, threadIndex);
                }
                else if (pauseRequested) // Break when finished
                {
                    break;
                }
                else
                {
                    // Idle backoff instead of hammering the queue head while the producer works.
                    // sleep1Threshold: -1 keeps escalation at Yield/Sleep(0) and never Sleep(1),
                    // avoiding timer-resolution stalls at the end of the drain.
                    spinner.SpinOnce(-1);
                }
            }

            worker._heap = writer.Buffer;
            worker._entitiesSerialized = entities;
            worker._bytesSerialized = writer.Position;

            writer.Close();

            worker._stopEvent.Set(); // Allow the main thread to continue now that we are finished
            worker._pause = false;

            if (Core.Closing || worker._exit)
            {
                return;
            }
        }
    }
}