package edu.northwestern.at.utils.corpuslinguistics.statistics;

/*	Please see the license information at the end of this file. */

import java.util.*;
import edu.northwestern.at.utils.math.*;
import edu.northwestern.at.utils.math.distributions.*;

/**	Computes Dunnett's log-likelihood for bigrams.
 */

public class BigramLogLikelihood
{
	/**	Compute one part of log likelihood value.
	 */

	protected static double logLike( double k , double n , double x )
	{
		return
			( k * ArithUtils.safeLog( x ) ) +
			( ( n - k ) * ArithUtils.safeLog( 1.0D - x ) );
	}

	/**	Compute log likelihood value for a bigram.
	 *
	 *	@param	c1			Count of first word in bigram.
	 *	@param	c2			Count of second word in bigram.
	 *	@param	c12			Count of bigram.
	 *	@param	n			Corpus size.
	 *
	 *	@return				The log-likelihood value.
	 */

	public static double calculateLogLikelihood
	(
		double c1 ,
		double c2 ,
		double c12 ,
		double n
	)
	{
		double p	= c2 / n;
		double p1	= c12 / c1;
		double p2	= ( c2 - c12 ) / ( n - c1 );

		double logLikelihood	=
			logLike( c12 , c1 , p ) +
			logLike( c2 - c12 , n - c1 , p ) -
			logLike( c12 , c1 , p1 ) -
			logLike( c2 - c12 , n - c1 , p2 );

		logLikelihood			= -2.0D * logLikelihood;

		return logLikelihood;
	}

	/**	Don't allow instantiation but do allow overrides.
	 */

	protected BigramLogLikelihood()
	{
	}
}

/*
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   Northwestern University
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