Ethiopian multiplication: Difference between revisions

=={{header|Common Lisp}}==

Common Lisp already has <code>evenp</code>, but all three of <code>halve</code>, <code>double</code>, and <code>even-p</code> are locally defined within <code>ethiopian-multiple</code>. (Note that the termination condition is <code>(zerop l)</code> because we terminate 'after' the iteration wherein the left column contains 1, and <code>(halve 1)</code> is 0.)

<lang lisp>(defun ethiopian-multiply (l r)

(flet ((halve (n) (floor n 2))

(double (n) (* n 2))

(even-p (n) (zerop (mod n 2))))

(do ((product 0 (if (even-p l) product (+ product r)))

(l l (halve l))

(r r (double r)))

((zerop l) product))))</lang>