al_fixatan2 - Allegro 5 API

       This is a fixed point version of the libc atan2() routine.  It computes the arc tangent of y / x, but the
       signs  of both arguments are used to determine the quadrant of the result, and x is permitted to be zero.
       This function is useful to convert Cartesian coordinates to polar coordinates.

       Example:

              al_fixed result;

              /* Sets `result' to binary angle 64. */
              result = al_fixatan2(al_itofix(1), 0);

              /* Sets `result' to binary angle -109. */
              result = al_fixatan2(al_itofix(-1), al_itofix(-2));

              /* Fails the assert. */
              result = al_fixatan2(0, 0);
              assert(!al_get_errno());

       al_fixatan2 - Allegro 5 API

       Returns the arc tangent of y / x in fixed point binary format angle, from -128 to 128.  If both x  and  y
       are zero, returns zero and sets Allegro’s errno to EDOM.

Allegro reference manual                                                                    al_fixatan2(3alleg5)

              #include <allegro5/allegro.h>

              al_fixed al_fixatan2(al_fixed y, al_fixed x)