1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
|
// --------------------------------------------------------------------------------------------------------------------
// <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;
}
}
}
|
