EvtdFunctionSingle.cpp

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#include "EvtGenBase/EvtdFunctionSingle.hh"

#include "EvtGenBase/EvtPatches.hh"

#include <assert.h>
#include <iostream>
#include <math.h>
#include <stdlib.h>

EvtdFunctionSingle::EvtdFunctionSingle()
{
    _j = 0;
    _m1 = 0;
    _m2 = 0;
    _coef = 0;
    _kmin = 0;
    _kmax = 0;
}

EvtdFunctionSingle::~EvtdFunctionSingle()
{
    if ( _coef != 0 )
        delete[] _coef;
}

void EvtdFunctionSingle::init( int j, int m1, int m2 )
{
    assert( abs( m2 ) >= abs( m1 ) );
    assert( m2 >= 0 );

    _j = j;
    _m1 = m1;
    _m2 = m2;

    _kmin = _m2 - _m1;
    _kmax = _j - _m1;

    assert( _kmin <= _kmax );

    _coef = new double[( _kmax - _kmin ) / 2 + 1];

    int k;

    for ( k = _kmin; k <= _kmax; k += 2 ) {
        int sign = 1;
        if ( ( k - _m2 + _m1 ) % 4 != 0 )
            sign = -sign;
        double tmp = fact( ( _j + _m2 ) / 2 ) * fact( ( _j - _m2 ) / 2 ) *
                     fact( ( _j + _m1 ) / 2 ) * fact( ( _j - _m1 ) / 2 );
        _coef[( k - _kmin ) / 2] =
            sign * sqrt( tmp ) /
            ( fact( ( _j + _m2 - k ) / 2 ) * fact( k / 2 ) *
              fact( ( _j - _m1 - k ) / 2 ) * fact( ( k - _m2 + _m1 ) / 2 ) );
    }
}

double EvtdFunctionSingle::d( int j, int m1, int m2, double theta )
{
    assert( j == _j );
    _unused( j );
    assert( m1 == _m1 );
    assert( m2 == _m2 );

    double c2 = cos( 0.5 * theta );
    double s2 = sin( 0.5 * theta );

    double d = 0.0;

    int k;
    for ( k = _kmin; k <= _kmax; k += 2 ) {
        d += _coef[( k - _kmin ) / 2] *
             pow( c2, ( 2 * _j - 2 * k + m2 - m1 ) / 2 ) *
             pow( s2, ( 2 * k - m2 + m1 ) / 2 );
    }

    return d;
}

int EvtdFunctionSingle::fact( int n )
{
    assert( n >= 0 );

    int f = 1;

    int k;
    for ( k = 2; k <= n; k++ )
        f *= k;

    return f;
}