// double position[12][3] and double velocity[12][3] are global 
//  arrays of 3D vectors
// int edge[0] and int edge[1] are the indices in position[][3] and
//  velocity[][3] of the two nodes making up this edge, or link

// double friction is about 0.9999
// double natural is about 1 

void icosahedron::cord(const int *edge) {
    double s[3], rr;
    rr=0;
    for (int i=0; i<3; i++) {
        s[i]=position[edge[1]][i]-position[edge[0]][i];
        rr+=s[i]*s[i];
    }
    if (rr>0) {
        double strain=spring*(natural/sqrt(rr)-1);
        for (int i=0; i<3; i++) {
            rr=strain*s[i];
            velocity[edge[0]][i]-=rr;
            velocity[edge[1]][i]+=rr;
        }
    }
}
//-------------------------------------------------
void icosahedron::move() {
    double *pos,*vel;
    for (pos=*position, vel=*velocity; pos-*position<36; pos++, vel++)
        *pos+=*vel;
    for (vel=*velocity; vel-*velocity<36; vel+=3) 
        *vel*=friction;
}