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/// ------------------------------------------------------
/// RandomOps - (Pseudo) Random Number Generator For C#
/// Copyright (C) 2003-2010 Magnus Erik Hvass Pedersen.
/// Please see the file license.txt for license details.
/// RandomOps on the internet: http://www.Hvass-Labs.org/
/// ------------------------------------------------------
using System;
using System.Diagnostics;
namespace RandomOps
{
/// <remarks>
/// Various methods for creating random integers.
/// </remarks>
public abstract partial class Random
{
/// <summary>
/// Draw a random integer from the set {0, .., n-1} with uniform probability.
/// Thread-safe if Uniform() is thread-safe.
/// </summary>
/// <remarks>
/// The default implementation uses Uniform() to generate an integer.
/// This is because many PRNGs must use division instead of modulo
/// arithmetics due to the lower-order bits having little randomness.
/// Assume Uniform cannot generate an exact value of one,
/// otherwise this must have been taken into account to ensure
/// uniform probability of choosing the different indices.
/// </remarks>
public virtual int Index(int n)
{
Debug.Assert(n >= 1);
double indexD = Uniform(0, n);
Debug.Assert(indexD > 0 && indexD < n);
int index = (int)Math.Truncate(indexD);
Debug.Assert(index >= 0 && index <= n - 1);
return index;
}
/// <summary>
/// Draw two distinct integers from the set {0, .., n-1} with equal probability.
/// Thread-safe if Uniform() is thread-safe.
/// </summary>
/// <param name="index1">Reference to the first integer drawn.</param>
/// <param name="index2">Reference to the second integer drawn.</param>
public virtual void Index2(int n, out int index1, out int index2)
{
Debug.Assert(n >= 2);
int r1, r2;
r1 = Index(n);
r2 = Index(n - 1);
int i1 = r1;
int i2 = (r1 + r2 + 1) % n;
Debug.Assert(i1 != i2);
index1 = i1;
index2 = i2;
}
/// <summary>
/// Draw a random integer from the set {0, .., n-1} according
/// to the supplied probability distribution. Probabilities are
/// assumed to sum to one. Time-complexity is O(n),
/// where n is the length of the probabilities-array.
/// Use this function if you only need to draw a few random numbers,
/// otherwise create an IndexDistribution-object.
/// Thread-safe if Uniform() is thread-safe.
/// </summary>
/// <param name="probabilities">Probability distribution.</param>
public int Index(double[] probabilities)
{
double sum = 0;
double r = Uniform();
int i;
for (i = 0; sum < r && i < probabilities.Length; i++)
{
sum += probabilities[i];
}
return i - 1;
}
}
}
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