chisqr.c

// By: Lisheng Zhou

#include <string.h>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>

/* HEADER */
#include "chisqr.h"

/* Chi-square distribution*/


static double igf( double S, double Z )
{
 if( Z < 0.0 )
 {
        return 0.0;
 }
 double Sc = ( 1.0 / S );
 Sc *= pow( Z, S );
 Sc *= exp( -Z );

 double Sum = 1.0;
 double Nom = 1.0;
 double Denom = 1.0;

 for( int I = 0; I < 200; I++ )
 {
        Nom *= Z;
        S++;
        Denom *= S;
        Sum += ( Nom / Denom );
 }

 return Sum * Sc;
}

double chisqr ( double Cv, int Dof )
{
 if ( Cv < 0 || Dof < 1 )
 {
  return 0.0;
 }
 double K = (( double ) Dof ) * 0.5;
 double X = Cv * 0.5;
 if( Dof == 2 )
 {
        return exp( -1.0 * X );
 }

 double PValue = igf( K, X );
 if( isnan(PValue) || isinf(PValue) || PValue <= 1e-8 )
 {
  return 1e-14;
 }

 PValue /= tgamma ( K );
 //PValue /= tgamma(K);

 return ( 1.0 - PValue );
}