src/mathb.cpp

/*****************************************************************************
* This file is provided under the Creative Commons Attribution 3.0 license.
*
* You are free to share, copy, distribute, transmit, or adapt this work
* PROVIDED THAT you attribute the work to the authors listed below.
* For more information, please see the following web page:
* http://creativecommons.org/licenses/by/3.0/
*
* This file is a component of the Sleipnir library for functional genomics,
* authored by:
* Curtis Huttenhower (chuttenh@princeton.edu)
* Mark Schroeder
* Maria D. Chikina
* Olga G. Troyanskaya (ogt@princeton.edu, primary contact)
*
* If you use this library, the included executable tools, or any related
* code in your work, please cite the following publication:
* Curtis Huttenhower, Mark Schroeder, Maria D. Chikina, and
* Olga G. Troyanskaya.
* "The Sleipnir library for computational functional genomics"
*****************************************************************************/
#include "stdafx.h"
#include "mathb.h"

namespace Sleipnir {

vector<double>  CMathImpl::s_vecdLogFact;

double CMath::LogFact( size_t iN ) {
    size_t  i;

    if( s_vecdLogFact.empty( ) ) {
        s_vecdLogFact.resize( c_iFactCutoff );
        s_vecdLogFact[ 0 ] = s_vecdLogFact[ 1 ] = 0;
        for( i = 2; i < s_vecdLogFact.size( ); ++i )
            s_vecdLogFact[ i ] = s_vecdLogFact[ i - 1 ] + log( (double)i ); }

    return ( ( iN >= c_iFactCutoff ) ? LogFactStirling( iN ) : s_vecdLogFact[ iN ] ); }

double CMathImpl::LogFactStirling( size_t iN ) {

    if( iN < 3 )
        return ( ( iN == 2 ) ? log( 2.0 ) : 0 );

    return ( log( 2.5066282746271 ) + ( ( iN + 0.5 ) * log( (double)iN ) ) - iN ); }

double CMathImpl::LogFactRec( size_t iN ) {

    if( iN < 3 )
        return ( ( iN == 2 ) ? log( 2.0 ) : 0 );

    return ( log( (double)iN ) + LogFactRec( iN - 1 ) ); }

}