It is not that difficult to those of us who know math. Obvious analogy to the function below exists in 'perl'.
double vectorAngle( double x, double y )
{
double r2 = x*x+y*y;
if( r2 == 0 ) return -10; // error case
int quadrant = x > 0 ? (y >= 0 : 0 : 3) : (y > 0 ? 1 : 2);
return pi/2 * quadrant + asin( abs(y)/sqrt( r2 ) );
}
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