acTan function
AcTan() ArcTangent of sin(x) / cos(x). The advantage of this function over arctan() is that it returns the correct quadrant of the angle.
Implementation
double acTan(double sinx, double cosx) {
if (cosx == 0.0) {
return (sinx > 0.0) ? (PI / 2.0) : (3.0 * PI / 2.0);
} else {
return (cosx > 0.0)
? (math.atan(sinx / cosx))
: (PI + math.atan(sinx / cosx));
}
}