C#で組み合わせのパターンを求める(更新版)

pongeponge.hatenablog.jp
　前回、説明にもならない説明をポチポチ書いてて「elementsいらんくない？」と思ったので省いた。

using System.Numerics;

public partial class MathPlus
{
    public class Combination
    {
        /// <summary>
        /// 要素の個数
        /// </summary>
        private int _n;

        /// <summary>
        /// 選択する個数
        /// </summary>
        private int _k;

        private int[] _pattern;

        /// <summary>
        /// 組み合わせの数
        /// </summary>
        public BigInteger Count { get { return this.Combination_Count(); } }

        /// <summary>
        /// コンストラクタ
        /// </summary>
        /// <param name="n">要素の個数</param>
        /// <param name="k">選択する個数</param>
        public Combination(int n, int k)
        {
            this._n = n;
            this._k = k;
            this._pattern = Enumerable.Range(1, this._k).ToArray();
        }

        /// <summary>
        /// 組み合わせの数を求める
        /// </summary>
        /// <returns>組み合わせの数</returns>
        private BigInteger Combination_Count()
        {
            var numerator = new BigInteger(1);
            var denominator = new BigInteger(1);
            var n = this._n;
            var k = this._k;
            var tmp = 1L;

            if (this._k > this._n - this._k) k = this._n - this._k;

            for (int count = 1; count <= k; count++)
            {
                numerator *= n;
                n--;

                denominator *= tmp;
                tmp++;
            }

            return numerator / denominator;
        }

        /// <summary>
        /// 組み合わせパターンを得る
        /// </summary>
        /// <returns>組み合わせパターンの反復子</returns>
        public IEnumerable<int[]> GetEnumerable()
        {
            if (this._pattern.Length == 0) yield break;

            while (this._pattern[this._pattern.Length - 1] != this._n + 1)
            {
                yield return this._pattern;

                Update();
            }

            yield break;
        }

        /// <summary>
        /// 参照の更新
        /// </summary>
        private void Update()
        {
            for (int i = this._pattern.Length - 1; i >= 0; i--)
            {
                if (this._pattern[i] != this._n + 1)
                {
                    this._pattern[i]++;

                    for (int r = i + 1; r < this._pattern.Length; r++)
                    {
                        this._pattern[r] = this._pattern[r - 1] + 1;
                    }
                }

                if (this._pattern[this._pattern.Length - 1] != this._n + 1) break;
            }
        }
    }
}