BCMath.cxx

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/*
 * Copyright (C) 2008, Daniel Kollar and Kevin Kroeninger.
 * All rights reserved.
 *
 * For the licensing terms see doc/COPYING.
 */

// ---------------------------------------------------------

#include "BAT/BCMath.h"

#include <math.h>

#include <TMath.h>

// ---------------------------------------------------------

double BCMath::LogGaus(double x, double mean, double sigma, bool norm)
{
   // if we have a delta function, return fixed value
   if(sigma==0.)
      return 0;

   // if sigma is negative use absolute value
   if(sigma<0.)
      sigma *= -1.;

   double arg = (x-mean)/sigma;
   double result = -.5 * arg * arg;

   // check if we should add the normalization constant
   if(!norm)
      return result;

   // subtract the log of the denominator of the normalization constant
   // and return
   return result - log( sqrt( 2.* M_PI ) * sigma );
}

// ---------------------------------------------------------

double BCMath::LogPoisson(double x, double par)
{
   if (par > 899)
      return BCMath::LogGaus(x,par,sqrt(par),true);

   if (x<0)
      return 0;

   if (x == 0.)
      return  -par;

   return x*log(par)-par-BCMath::ApproxLogFact(x);
}

// ---------------------------------------------------------

double BCMath::ApproxBinomial(int n, int k, double p)
{
   return exp( BCMath::LogApproxBinomial(n, k, p) );
}

// ---------------------------------------------------------

double BCMath::LogApproxBinomial(int n, int k, double p)
{
   // check p 
   if (p == 0)
      return -1e99; 

   else if (p == 1)
      return 0; 

   // switch parameters if n < k
   if(n<k)
   {
      int a=n;
      n=k;
      k=a;
   }

   return BCMath::LogBinomFactor(n,k) + (double)k*log(p) + (double)(n-k)*log(1.-p);
}

// ---------------------------------------------------------

double BCMath::LogBinomFactor(int n, int k)
{
   // switch parameters if n < k
   if(n<k)
   {
      int a=n;
      n=k;
      k=a;
   }

   if(n==k || k==0) return 0.;
   if(k==1 || k==n-1) return log((double)n);

   // if no approximation needed
   if(n<BCMATH_NFACT_ALIMIT || (n-k)<5) return BCMath::LogNoverK(n,k);

   // calculate final log(n over k) using approximations if necessary
   return BCMath::ApproxLogFact((double)n) - BCMath::ApproxLogFact((double)k) - BCMath::ApproxLogFact((double)(n-k));
}

// ---------------------------------------------------------

double BCMath::ApproxLogFact(double x)
{
   if(x>BCMATH_NFACT_ALIMIT)
      return x*log(x) - x + log(x*(1.+4.*x*(1.+2.*x)))/6. + log(M_PI)/2.;

   else
      return BCMath::LogFact((int)x);
}

// ---------------------------------------------------------
double BCMath::LogFact(int n)
{
   double ln = 0.;

   for(int i=1;i<=n;i++)
      ln += log((double)i);

   return ln;
}

// ---------------------------------------------------------

double BCMath::LogNoverK(int n, int k)
{
   // switch parameters if n < k
   if(n<k)
   {
      int a = n;
      n = k;
      k = a;
   }

   if(n==k || k==0) return 0.;
   if(k==1 || k==n-1) return log((double)n);

   int lmax = BCMath::Max(k,n-k);
   int lmin = BCMath::Min(k,n-k);

   double ln = 0.;

   for(int i=n;i>lmax;i--)
      ln += log((double)i);
   ln -= BCMath::LogFact(lmin);

   return ln;
}

// ---------------------------------------------------------

int BCMath::Nint(double x)
{
   // round to integer
   int i;

   if (x >= 0)
   {
      i = (int)(x + .5);

      if (x + .5 == (double)i && i & 1)
         i--;
   }

   else
   {
      i = int(x - 0.5);

      if (x - 0.5 == double(i) && i & 1)
         i++;
   }

   return i;
}

// ---------------------------------------------------------

double BCMath::rms(int n, const double *a)
{
   if (n <= 0 || !a)
      return 0;

   double sum = 0., sum2 = 0.;

   for (int i = 0; i < n; i++)
   {
      sum  += a[i];
      sum2 += a[i] * a[i];
   }

   double n1 = 1./(double)n;
   double mean = sum * n1;

   return sqrt(fabs(sum2 * n1 - mean * mean));
}

// ---------------------------------------------------------

double BCMath::LogBreitWignerNonRel(double x, double mean, double Gamma, bool norm)
{
   double bw = log(Gamma) - log( (x - mean) * (x - mean) + Gamma * Gamma / 4.0);

   if (norm)
      bw -= log(2. * M_PI);

   return bw;
}

// ---------------------------------------------------------

double BCMath::LogBreitWignerRel(double x, double mean, double Gamma)
{
   return -log( (x*x - mean*mean) * (x*x - mean*mean) + mean*mean*Gamma*Gamma );
}

// ---------------------------------------------------------

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