两个向量的夹角(顺时针)

double getRotateAngle(double x1, double y1, double x2, double y2)
{
    const double epsilon = 1.0e-6;
    const double nyPI = acos(-1.0);
    double dist, dot, degree, angle;

 // normalize
    dist = sqrt( x1 * x1 + y1 * y1 );
    x1 /= dist;
    y1 /= dist;
    dist = sqrt( x2 * x2 + y2 * y2 );
    x2 /= dist;
    y2 /= dist;
// dot product
    dot = x1 * x2 + y1 * y2;
    if ( fabs(dot-1.0) <= epsilon )
    angle = 0.0;
    else if ( fabs(dot+1.0) <= epsilon )
    angle = nyPI;
    else {
        double cross;

        angle = acos(dot);
  //cross product
        cross = x1 * y2 - x2 * y1;
  // vector p2 is clockwise from vector p1
  // with respect to the origin (0.0)
        if (cross < 0 ) {
            angle = 2 * nyPI - angle;
        }
    }
    degree = angle *  180.0 / nyPI;
    return degree;
}