using System;
using System.Diagnostics;
using System.Threading;

namespace ZeroLevel.Services.Pools
{
    /// <summary>
    /// Steal from Roslyn
    /// https://github.com/dotnet/roslyn/blob/master/src/Dependencies/PooledObjects/ObjectPool%601.cs
    /// </summary>
    public class ObjectPool<T> where T : class
    {
        [DebuggerDisplay("{Value,nq}")]
        private struct Element
        {
            internal T Value;
        }

        /// <remarks>
        /// Not using System.Func{T} because this file is linked into the (debugger) Formatter,
        /// which does not have that type (since it compiles against .NET 2.0).
        /// </remarks>
        public delegate T Factory();

        // Storage for the pool objects. The first item is stored in a dedicated field because we
        // expect to be able to satisfy most requests from it.
        private T _firstItem;

        private readonly Element[] _items;

        // factory is stored for the lifetime of the pool. We will call this only when pool needs to
        // expand. compared to "new T()", Func gives more flexibility to implementers and faster
        // than "new T()".
        private readonly Factory _factory;

        public ObjectPool(Factory factory)
            : this(factory, Environment.ProcessorCount * 2)
        { }

        public ObjectPool(Factory factory, int size)
        {
            Debug.Assert(size >= 1);
            _factory = factory;
            _items = new Element[size - 1];
        }

        private T CreateInstance()
        {
            var inst = _factory();
            return inst;
        }

        /// <summary>
        /// Produces an instance.
        /// </summary>
        /// <remarks>
        /// Search strategy is a simple linear probing which is chosen for it cache-friendliness.
        /// Note that Free will try to store recycled objects close to the start thus statistically
        /// reducing how far we will typically search.
        /// </remarks>
        public T Allocate()
        {
            // PERF: Examine the first element. If that fails, AllocateSlow will look at the remaining elements.
            // Note that the initial read is optimistically not synchronized. That is intentional.
            // We will interlock only when we have a candidate. in a worst case we may miss some
            // recently returned objects. Not a big deal.
            T inst = _firstItem;
            if (inst == null || inst != Interlocked.CompareExchange(ref _firstItem, null, inst))
            {
                inst = AllocateSlow();
            }
            return inst;
        }

        private T AllocateSlow()
        {
            var items = _items;

            for (int i = 0; i < items.Length; i++)
            {
                // Note that the initial read is optimistically not synchronized. That is intentional.
                // We will interlock only when we have a candidate. in a worst case we may miss some
                // recently returned objects. Not a big deal.
                T inst = items[i].Value;
                if (inst != null)
                {
                    if (inst == Interlocked.CompareExchange(ref items[i].Value, null, inst))
                    {
                        return inst;
                    }
                }
            }

            return CreateInstance();
        }

        /// <summary>
        /// Returns objects to the pool.
        /// </summary>
        /// <remarks>
        /// Search strategy is a simple linear probing which is chosen for it cache-friendliness.
        /// Note that Free will try to store recycled objects close to the start thus statistically
        /// reducing how far we will typically search in Allocate.
        /// </remarks>
        public void Free(T obj)
        {
            if (!Validate(obj))
            {
                return;
            }
            if (_firstItem == null)
            {
                // Intentionally not using interlocked here.
                // In a worst case scenario two objects may be stored into same slot.
                // It is very unlikely to happen and will only mean that one of the objects will get collected.
                _firstItem = obj;
            }
            else
            {
                FreeSlow(obj);
            }
        }

        private void FreeSlow(T obj)
        {
            var items = _items;
            for (int i = 0; i < items.Length; i++)
            {
                if (items[i].Value == null)
                {
                    // Intentionally not using interlocked here.
                    // In a worst case scenario two objects may be stored into same slot.
                    // It is very unlikely to happen and will only mean that one of the objects will get collected.
                    items[i].Value = obj;
                    break;
                }
            }
        }

        private bool Validate(object obj)
        {
            if (obj == null) return false;
            if (_firstItem == obj) return false;
            var items = _items;
            for (int i = 0; i < items.Length; i++)
            {
                var value = items[i].Value;
                if (value == null)
                {
                    return true;
                }
                if (value == obj) return false;
            }
            return true;
        }
    }

    /*

    Alternate
    https://stackoverflow.com/questions/1698738/objectpoolt-or-similar-for-net-already-in-a-library

     */
}