/*************************************************************************
 * ModernUO                                                              *
 * Copyright 2019-2021 - ModernUO Development Team                       *
 * Email: hi@modernuo.com                                                *
 * File: Timer.Pool.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.Threading;

namespace Server
{
    public partial class Timer
    {
        private const int _timerPoolDepletionThreshold = 128; // Maximum timers allocated in a single tick before we force adjust
        private static int _timerPoolDepletionAmount;         // Amount the pool has been depleted by
        private static int _maxPoolCapacity;
        private static int _poolCapacity;
        private static int _poolCount;
        private static DelayCallTimer _poolHead;

        public static void CheckTimerPool()
        {
            // Anything less than this threshold and we are ok with the number of allocations.
            if (_timerPoolDepletionAmount < _timerPoolDepletionThreshold)
            {
                _timerPoolDepletionAmount = 0;
                return;
            }

            var growthFactor = Math.DivRem(_timerPoolDepletionAmount, _poolCapacity, out var rem);
            var amountToGrow = _poolCapacity * (growthFactor + (rem > 0 ? 1 : 0));
            var amountToRefill = Math.Min(_maxPoolCapacity, amountToGrow);

            var maximumHit = amountToGrow > amountToRefill ? " Maximum pool size has been reached." : "";
            var warningMessage = $"Timer pool depleted by {{Amount}}. Refilling with {{AmountRefill}}.{maximumHit}";

            logger.Warning(warningMessage, _timerPoolDepletionAmount, amountToRefill);
            RefillPoolAsync(amountToRefill);
            _timerPoolDepletionAmount = 0;
        }

        public static void ConfigureTimerPool()
        {
            _poolCapacity = ServerConfiguration.GetOrUpdateSetting("timer.initialPoolCapacity", 1024);
            _maxPoolCapacity = ServerConfiguration.GetOrUpdateSetting("timer.maxPoolCapacity", _poolCapacity * 16);

            RefillPool(_poolCapacity, out var head, out var tail);
            ReturnToPool(_poolCapacity, head, tail);
        }

        private static void ReturnToPool(int amount, DelayCallTimer head, DelayCallTimer tail)
        {
            tail.Attach(_poolHead);
            _poolHead = head;
            _poolCount += amount;
#if DEBUG_TIMERS
            logger.Information("Returning to pool. ({Count} / {Capacity})", _poolCount, _poolCapacity);
#endif
        }

        private static DelayCallTimer GetFromPool()
        {
            if (_poolHead == null)
            {
                return null;
            }

            var timer = _poolHead;
            _poolHead = _poolHead._nextTimer as DelayCallTimer;
            timer.Detach();
            _poolCount--;

            return timer;
        }

        internal static void RefillPool(int amount, out DelayCallTimer head, out DelayCallTimer tail)
        {
#if DEBUG_TIMERS
            logger.Information("Filling pool with {Amount} timers.", amount);
#endif

            head = null;
            tail = null;

            for (var i = 0; i < amount; i++)
            {
                var timer = new DelayCallTimer(TimeSpan.Zero, TimeSpan.Zero, 0, null);
                timer.Attach(head);

                if (i == 0)
                {
                    tail = timer;
                }

                head = timer;
            }
        }

        internal static void RefillPoolAsync(int amountToRefill)
        {
            ThreadPool.UnsafeQueueUserWorkItem(
                static amount =>
                {
                    RefillPool(amount, out var head, out var tail);

                    // Run this on the core thread
                    Core.LoopContext.Post(
                        state =>
                        {
                            if (state == null)
                            {
                                return;
                            }

                            var (listHead, listTail) = ((DelayCallTimer, DelayCallTimer))state;
                            ReturnToPool(amount, listHead, listTail);
                            _poolCapacity = amount;
                        },
                        (head, tail)
                    );
                },
                amountToRefill,
                false
            );
        }
    }
}