PrimeNumberHelper.cs

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using System;

namespace OpenSim.Framework
{
    public static class PrimeNumberHelper
    {
        private static readonly int[] Primes = new int[]
                                                   {
                                                       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
                                                   };

        public static int GetPrime(int min)
        {
            if (min <= 2)
                return 2;

            if (Primes[ Primes.Length - 1 ] < min)
            {
                for (int i = min | 1 ; i < 0x7FFFFFFF ; i += 2)
                {
                    if (IsPrime(i))
                        return i;
                }

                return -1;
            }

            for (int i = Primes.Length - 2 ; i >= 0 ; i--)
            {
                if (min == Primes[ i ])
                    return min;

                if (min > Primes[ i ])
                    return Primes[ i + 1 ];
            }

            return 2;
        }

        public static bool IsPrime(int candinate)
        {
            if ((candinate & 1) == 0)

                // Even number - only prime if 2
                return candinate == 2;

            int upperBound = (int) Math.Sqrt(candinate);
            for (int i = 3 ; i < upperBound ; i += 2)
            {
                if (candinate % i == 0)
                    return false;
            }

            return true;
        }
    }
}