1.5.18 #

public class RandomBag<TItem> : IEnumerable<TItem>
{
    private TItem[] _bag;
    private int _count;

    /// <summary>
    /// 建立一个随机背包。
    /// </summary>
    public RandomBag()
    {
        _bag = new TItem[2];
        _count = 0;
    }

    /// <summary>
    /// 检查背包是否为空。
    /// </summary>
    /// <returns></returns>
    public bool IsEmpty()
    {
        return _count == 0;
    }

    /// <summary>
    /// 返回背包中元素的数量。
    /// </summary>
    /// <returns></returns>
    public int Size()
    {
        return _count;
    }

    /// <summary>
    /// 向背包中添加一个元素。
    /// </summary>
    /// <param name="item">要向背包中添加的元素。</param>
    public void Add(TItem item)
    {
        if (_count == _bag.Length)
        {
            Resize(_count * 2);
        }

        _bag[_count] = item;
        _count++;
    }

    /// <summary>
    /// 重新为背包分配内存空间。
    /// </summary>
    /// <param name="capacity"></param>
    private void Resize(int capacity)
    {
        if (capacity <= 0)
            throw new ArgumentException();
        var temp = new TItem[capacity];
        for (var i = 0; i < _count; i++)
        {
            temp[i] = _bag[i];
        }
        _bag = temp;
    }

    public IEnumerator<TItem> GetEnumerator()
    {
        return new RandomBagEnumerator(_bag, _count);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    private class RandomBagEnumerator : IEnumerator<TItem>
    {
        private TItem[] _bag;
        private int[] _sequence;
        private int _current;
        private readonly int _count;

        public RandomBagEnumerator(TItem[] bag, int count)
        {
            _bag = bag;
            _current = -1;
            _count = count;
            _sequence = new int[count];
            for (var i = 0; i < _count; i++)
            {
                _sequence[i] = i;
            }
            Shuffle(_sequence, DateTime.Now.Millisecond);
        }

        /// <summary>
        /// 随机打乱数组。
        /// </summary>
        /// <param name="a">需要打乱的数组。</param>
        /// <param name="seed">随机种子值。</param>
        private void Shuffle(int[] a, int seed)
        {
            var n = a.Length;
            var random = new Random(seed);
            for (var i = 0; i < n; i++)
            {
                var r = i + random.Next(n - i);
                var temp = a[i];
                a[i] = a[r];
                a[r] = temp;
            }
        }

        TItem IEnumerator<TItem>.Current => _bag[_sequence[_current]];

        object IEnumerator.Current => _bag[_sequence[_current]];

        void IDisposable.Dispose()
        {
            _bag = null;
            _sequence = null;
            _current = -1;
        }

        bool IEnumerator.MoveNext()
        {
            if (_current == _count - 1)
                return false;
            _current++;
            return true;
        }

        void IEnumerator.Reset()
        {
            _current = -1;
        }
    }
}