source: sasview/sansmodels/src/sans/models/c_extensions/libmultifunc/libfunc.c @ 8c8cb05

#include <math.h>
#include "libmultifunc/libfunc.h"
#include <stdio.h>

//used in Si func
int factorial(int i)
{
        int k, j;
        if (i<2){
                return 1;
        }
        k=1;
        for(j=1;j<i;j++)
        {
                k=k*(j+1);
        }
        return k;
}

// Used in pearl nec model
//integral of sin(x)/x up to namx term nmax>10 looks fine.
double Si(double x, int nmax )
{
        int i;
        double out;
        long double power;
        if (x > 5.0){
                double pi = 4.0*atan(1.0);
                return pi/2.0;
        }
        out = 0.0;
        for (i=0; i<nmax; i+=1)
        {
                if (i==0){
                        out += x;
                        continue;
                }
                power = pow(x,(2 * i + 1));
                out += (double)pow(-1, i) * power / ((2.0 * (double)i + 1.0) * (double)factorial(2 * i + 1));
                //printf ("Si=%g %g %d\n", x, out, i);
        }
        return out;
}

double sin_x(double x)
{
        if (x==0){
                return 1.0;
        }
        return sin(x)/x;
}