using System.Diagnostics.CodeAnalysis;
using NewLife.Reflection;

namespace NewLife.Collections;

/// <summary>轻量级对象池。数组无锁实现，高性能</summary>
/// <remarks>
/// 文档 https://newlifex.com/core/object_pool
/// 内部 1+N 的存储结果，保留最热的一个对象在外层，便于快速存取。
/// 数组具有极快的查找速度，结构体确保没有GC操作。
/// </remarks>
/// <typeparam name="T"></typeparam>
public class Pool<T> : IPool<T> where T : class
{
    #region 属性
    /// <summary>对象池大小。默认CPU*2，初始化后改变无效</summary>
    public Int32 Max { get; set; }

    private Item[]? _items;
    private T? _current;

    struct Item
    {
        public T? Value;
    }
    #endregion

    #region 构造
    /// <summary>实例化对象池。默认大小CPU*2</summary>
    /// <param name="max"></param>
    public Pool(Int32 max = 0)
    {
        if (max <= 0) max = Environment.ProcessorCount * 2;

        Max = max;
    }

    [MemberNotNull(nameof(_items))]
    private void Init()
    {
        if (_items != null) return;
        lock (this)
        {
            if (_items != null) return;

            _items = new Item[Max - 1];
        }
    }
    #endregion

    #region 方法
    /// <summary>获取</summary>
    /// <returns></returns>
    public virtual T Get()
    {
        // 最热的一个对象在外层，便于快速存取
        var val = _current;
        if (val != null && Interlocked.CompareExchange(ref _current, null, val) == val) return val;

        Init();

        var items = _items;
        for (var i = 0; i < items.Length; i++)
        {
            val = items[i].Value;
            if (val != null && Interlocked.CompareExchange(ref items[i].Value, null, val) == val) return val;
        }

        var rs = OnCreate();
        if (rs == null) throw new InvalidOperationException($"Unable to create an instance of [{typeof(T).FullName}]");

        return rs;
    }

    /// <summary>归还</summary>
    /// <param name="value"></param>
    /// <returns></returns>
    public virtual Boolean Put(T value)
    {
        // 最热的一个对象在外层，便于快速存取
        if (_current == null && Interlocked.CompareExchange(ref _current, value, null) == null) return true;

        Init();

        var items = _items;
        for (var i = 0; i < items.Length; ++i)
        {
            if (Interlocked.CompareExchange(ref items[i].Value, value, null) == null) return true;
        }

        return false;
    }

    /// <summary>归还</summary>
    /// <param name="value"></param>
    /// <returns></returns>
    public virtual Boolean Return(T value)
    {
        // 最热的一个对象在外层，便于快速存取
        if (_current == null && Interlocked.CompareExchange(ref _current, value, null) == null) return true;

        Init();

        var items = _items;
        for (var i = 0; i < items.Length; ++i)
        {
            if (Interlocked.CompareExchange(ref items[i].Value, value, null) == null) return true;
        }

        return false;
    }

    /// <summary>清空</summary>
    /// <returns></returns>
    public virtual Int32 Clear()
    {
        var count = 0;

        if (_current != null)
        {
            _current = null;
            count++;
        }

        var items = _items;
        if (items == null) return count;

        for (var i = 0; i < items.Length; ++i)
        {
            if (items[i].Value != null)
            {
                items[i].Value = null;
                count++;
            }
        }
        _items = null;

        return count;
    }
    #endregion

    #region 重载
    /// <summary>创建实例</summary>
    /// <returns></returns>
    protected virtual T? OnCreate() => typeof(T).CreateInstance() as T;
    #endregion
}