Bias Random Number Generator

import java.util.*; 



// Bias Random number generator

// see static function test() for example use -



// johnny Wilson john66@talk21.com



public class BiasRandom

{

	static Random	DEFAULTRNG	=	new Random(1235) ;



	ArrayList prob = new ArrayList();	

	double	sum				=	0  ;

	Random	random			=	new Random(1235) ;

	

	public BiasRandom()

	{

		this(BiasRandom.DEFAULTRNG) ;

	}



	// Allows you to define uniform Random number generator

	// to be used.

	

	public BiasRandom(Random rng)

	{

		random	=	rng ;

	}



	public BiasRandom(Random rng,double [] numbers)

	{

		random	=	rng ;

		defineDistArray(numbers) ;

	}

	

	public BiasRandom(double [] numbers)

	{

		this(BiasRandom.DEFAULTRNG,numbers) ;

	}

	

	

	

	public void addValue(double num)

	{

		sum	=	sum + num ;

		prob.add(new Double(num)) ;

	}

	

	public void defineDist(double... numbers)

	{

		prob 	= 	new ArrayList();	

		sum		=	0 ;

		for(double num : numbers)

		{

			addValue(num) ;

		}

	}

	

	public void defineDistArray(double[] numbers)

	{

		prob 	= 	new ArrayList();	

		sum		=	0 ;

		for (int i = 0; i < numbers.length ; i++)

		{

			addValue(numbers[i]) ;

		}

		

	}

	

	public double getSum()

	{

		return sum ;

	}

	

	// selectEvent 

	// Given a List of % probabilities of a particular event

	// Returns - a Random index - weighted by that table

	// Eg. int [] eventdistTable = {50,25,10,15}

	// Gives 10% chance of returning 0 

	// Gives 25% chance of returning 1

	// Gives 50% chance of returning 2

	// Gives 15% chance of returning 3

	

	// NB: Sum of eventdistTable elements must work out to be 100

	//  If it doesn't then % chances are worked out

	//  over the sum of elements in eventdistTable



	// Probability table is set up by the helper functions

	// defineDist(double... numbers) or defineDistArray(double[] numbers)

	// See test() for example

	

	

	public int	selectEvent()

	{

		int index 		= 0 ;

		double cummSum	= 0.0 ;

		double rnd = (double)(1 + random.nextInt((int)sum)) ;

		while(true)

		{

			cummSum = cummSum + prob.get(index) ;

			if (rnd <= cummSum) {

				return index ;

			} 

			index = index + 1 ;

		}

	}

	



	public static void test()

	{

			double []dist = {10,15,50,25} ;



			BiasRandom br 	= 	new BiasRandom() ;

			br.defineDistArray(dist) ;

			// note: above two lines can be replaced with 

			// BiasRandom br 	= 	new BiasRandom(dist) ;

			

			int [] history = new int [dist.length] ;

			int numThrows = 10000000 ;



			for (int i = 0; i < numThrows; i++)

			{

				int ev = br.selectEvent() ;

				history[ev] = history[ev] + 1 ;

			}

	

			int sum = 0 ;

			for (int i = 0; i < history.length; i++)

			{

				sum = sum + history[i] ;

				System.out.printf("Event %d percentage selected %f\n",i,(100.0 * history[i])/numThrows) ;

				

			}

			System.out.printf("Total %d\n",sum) ;

			

	}

	public static void main(String [] args)

	{

		test() ;

	}

}