STK_Funct_poisson_raw.cpp

Go to the documentation of this file.

/*--------------------------------------------------------------------*/
/*     Copyright (C) 2004-2007  Serge Iovleff

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as
    published by the Free Software Foundation; either version 2 of the
    License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this program; if not, write to the
    Free Software Foundation, Inc.,
    59 Temple Place,
    Suite 330,
    Boston, MA 02111-1307
    USA

    Contact : Serge.Iovleff@stkpp.org
*/

/*
 * Project:  Analysis
 * Purpose:  implementation of the poisson function
 * Author:   Serge Iovleff, serge.iovleff@stkpp.org
 **/

#include <cmath>

#include "../include/STK_Const_Math.h"

#include "../include/STK_Funct_gamma.h"

#include "../include/STK_Funct_util.h"

#include "../include/STK_Funct_raw.h"

namespace STK
{
  
namespace Funct
{
Real poisson_pdf_raw(Real const& x, Real const& lambda)
{
  // check trivial values
  if (x<0.) return( 0. );
  // if lambda is 0, we have P(X=0) = 1
  if (lambda==0.) return( (x==0.) ? 1. : 0. );
  // special value
  if (x==0.) return( exp(-lambda) );
  // stirling approximation and deviance
  return( exp(-gammaLnStirlingError(x)-dev0(x, lambda))
        / (Const::_SQRT2PI_*sqrt(x))
        );
}

Real poisson_pdf_raw(Integer const& x, Real const& lambda)
{
  // check trivial values
  if (x<0) return( 0. );
  // if lambda is 0, we have P(X=0) = 1
  if (lambda==0) return( (x==0) ? 1. : .0 );
  // special value
  if (x==0) return( exp(-lambda) );
  // stirling approximation and deviance
  return( exp(-gammaLnStirlingError(x)-dev0(x, lambda))
        / (Const::_SQRT2PI_*sqrt(x))
        );
}


} // namespace Funct

} // namespace STK