C#工具类之素数扩展类

Posted 2020-09-08

/// <summary>
    /// 素数帮忙类
    /// 本类是从.net源码 类 internal static class HashHelpers 类里抽取相应的代码
    /// https://referencesource.microsoft.com/#mscorlib/system/collections/hashtable.cs,e8668bf19da49963
    /// Hashtable.cs里
    /// </summary>
    public class PrimeHelper
    {
        /// <summary>
        /// 给定一些素数(除了2)
        /// </summary>
        public static readonly int[] Primes = {
            3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
            1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
            17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
            187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
            1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369};

        /// <summary>
        /// 判断是否是素数
        /// </summary>
        /// <param name="candidate"></param>
        /// <returns></returns>
        public static bool IsPrime(int candidate)
        {
            //偶数当然不是素数
            if ((candidate & 1) != 0)
            {
                int limit = (int)Math.Sqrt(candidate);
                for (int divisor = 3; divisor <= limit; divisor += 2)
                {
                    if ((candidate % divisor) == 0)
                        return false;
                }
                return true;
            }

            //2是素数
            return (candidate == 2);
        }

        /// <summary>
        /// 大于给定数的最小素数
        /// </summary>
        /// <param name="min"></param>
        /// <returns></returns>
        public static int GetPrime(int min)
        {
            if (min < 0)
            {
                throw new ArgumentException("给定数必须是自然数");
            }

            //先从给定数组里找
            foreach (int prime in Primes)
            {
                if (prime >= min)
                {
                    return prime;
                }
            }

            //给定数组里没有相应的素数,则循环找
            //如果min是偶数,则起始值为加1,否则起始值为本身
            for (int i = (min | 1); i < Int32.MaxValue; i += 2)
            {
                if (IsPrime(i) )
                    return i;
            }

            return min;
        }

        /// <summary>
        /// 获取最小素数(2除外)
        /// </summary>
        /// <returns></returns>
        public static int GetMinPrime()
        {
            return Primes[0];
        }

    }