(exponent-of-in n m)returns the highest power of n that divides evenly into m.
(exponent-of-in 2 3888) ⇒ 4 (exponent-of-in 3 3888) ⇒ 5The selector is now straightforward:
(define size-of-pile (lambda (game-state pile-number) (if (= pile-number 1) (exponent-of-in 2 game-state) (exponent-of-in 3 game-state))))