source: sasmodels/sasmodels/models/lib/Si.c @ cf404cb

double Si(double x);
// integral of sin(x)/x: approximated to w/i 1%
double Si(double x)
{
        int i;
        int nmax=6;
        double out;
        long 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;
}

int factorial(int f);
int factorial(int f)
{
    if ( f == 0 ) 
        return 1;
    return(f * factorial(f - 1));
}