Nope, not Arcus tangent. But it's based on that function.

http://www.cplusplus.com/ref/cmath/atan2.html

Language doesn't matter, as long as I can read the code.

EDIT: Nevermind, I found out how atan2 is defined. If you are interested:

function atan2 v,u

if u=0 and v=0 then: not defined, but you might use 0 as answer

if u=0 then

if v > 0 then pi/2 else -pi/2

if u > 0 then atan(v/u) --the normal atan function

if u < 0 then

if v >= 0 then atan(v/u) + pi else atan(v/u) - pi