OpenSimMirror/OpenSim/Framework/PrimeNumberHelper.cs

// --------------------------------------------------------------------------------------------------------------------
// <copyright company="" file="PrimeNumberHelper.cs">
//   
// </copyright>
// <summary>
//   
// </summary>
// 
// --------------------------------------------------------------------------------------------------------------------

using System;

namespace OpenSim.Framework
{
    /// <summary>
    /// Utility class that is used to find small prime numbers and test is number prime number.
    /// </summary>
    public static class PrimeNumberHelper
    {
        /// <summary>
        /// Precalculated prime numbers. 
        /// </summary>
        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
                                                   };

        /// <summary>
        /// Get prime number that is equal or larger than <see cref="min"/>.
        /// </summary>
        /// <param name="min">
        /// Minimal returned prime number.
        /// </param>
        /// <returns>
        /// Primer number that is equal or larger than <see cref="min"/>. If <see cref="min"/> is too large, return -1.
        /// </returns>
        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;
        }

        /// <summary>
        /// Just basic Sieve of Eratosthenes prime number test.
        /// </summary>
        /// <param name="candinate">
        /// Number that is tested.
        /// </param>
        /// <returns>
        /// true, if <see cref="candinate"/> is prime number; otherwise false.
        /// </returns>
        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;
        }
    }
}