获取指定数量的有序列表

信息系统中经常用到排序、查找等简单的数据结构，结合大数据量的后台设计实践；

将插入排序、折半查找结合起来，实现一个经常用的场景：实时从海量数据中获取指定数量产品的有序列表

       /// <summary>
       /// 插入排序
       /// 获取前count个有序列表
       /// </summary>
       /// <param name="list"></param>
       /// <param name="count"></param>
       /// <returns></returns>
       public static List<decimal> GetSortCountList(List<decimal> list, int count = int.MaxValue)
       {
           var result = new List<decimal>();
           foreach (var item in list)
           {
               int index = GetOrderByIndex(result, item, count);
               if (index > -1)
               {
                   result.Insert(index, item);
                   if (result.Count > count)
                   {
                       result.RemoveAt(count);
                   }
               }
           }
           return result;
       }

        /// <summary>
        /// 插入排序
        /// 返回数值应该插入的index
        /// </summary>
        /// <param name="list"></param>
        /// <param name="price"></param>
        /// <returns></returns>
        public static int GetOrderByIndex(List<decimal> list, decimal price, int count = int.MaxValue)
        {
            int length = list.Count;
            if (length == 0)
            {
                return 0;
            }
            else
            {
                if (price >= list.Last())//大于等于最大
                {
                    return length < count ? length : -1;
                }
                else if (price <= list.First()) //小于等于最小
                {
                    return 0;
                }
                else//最小-->最大之间
                {
                    return GetOrderByIndex(list, price, 1, length - 1);
                }
            }
        }

        /// <summary>
        /// 折半查找
        /// 返回数值应该插入的index
        /// </summary>
        /// <param name="list"></param>
        /// <param name="item"></param>
        /// <param name="first"></param>
        /// <param name="last"></param>
        /// <returns></returns>
        public static int GetOrderByIndex(List<decimal> list, decimal item, int first, int last)
        {
            int middle = (last + first) / 2;
            var middlePrice = list[middle];
            if (last <= first)
            {
                return (item < middlePrice) ? middle : middle + 1;
            }
            else
            {
                if (item < middlePrice)
                {
                    return GetOrderByIndex(list, item, first, middle - 1);
                }
                else
                {
                    return GetOrderByIndex(list, item, middle + 1, last);
                }
            }
        }