source: sasview/sansmodels/src/c_extensions/libmultifunc/libfunc.c @ 98fdccd

// by jcho
#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
// Sine integral function: approximated within 1%!!!
// integral of sin(x)/x up to namx term nmax=6 looks the best.
double Si(double x)
{
        int i;
        int nmax=6;
        double out;
        long double power;
        double pi = 4.0*atan(1.0);
        if (x >= pi*6.2/4.0){

                double out_sin = 0.0;
                double out_cos = 0.0;
                out = pi/2.0;
                for (i=0; i<nmax-2; i+=1){
                        out_cos += pow(-1.0, i) * (double)factorial(2*i) / pow(x, 2*i+1);
                        out_sin += pow(-1.0, i) * (double)factorial(2*i+1) / pow(x, 2*i+2);
                }
                out -= cos(x) * out_cos;
                out -= sin(x) * out_sin;
                return out;
        }
        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 sinc(double x)
{
        if (x==0.0){
                return 1.0;
        }
        return sin(x)/x;
}