This is the implementation of the discrete arctangent used in Star Versus. It takes as input an X-delta, Y-delta, and quadrant number and returns a direction between 0 and 23. Documentation on parameter passing accompanies the function, of note, no registers are preserved. Assembly source uses ca65 syntax.
Using nes_unit_testing, the lisp based unit test verifies a couple of inputs to the arctan function. Run the tests using:
./run_tests.sh
Assumes SBCL.
For more information on the motivation, design, and usage of this function, see this blog post.