Problem Statement

Finding the angle between two bearings is often confusing.[1]

Find the angle which is the result of the subtraction b2 - b1, where b1 and b2 are the bearings. Input bearings are expressed in the range   -180   to   +180   degrees. The  result  is also expressed in the range   -180   to   +180   degrees.

Compute the angle for the following pairs:

Allow the input bearings to be any (finite) value.

Let's start with the solution:

(defun angle-difference (b1 b2)
	   (let ((diff (mod (- b2 b1) 360)))
	     (if (< diff -180)
		 (incf diff 360)
		 (if (> diff 180)
		     (decf diff 360)
		     diff))))