Little Common Lisp program that returns the winners (and each winner's earnings) of a pool. Uses shallow copies to avoid mutation.

world_cup_pool.lisp

;;; Based on this gist: http://gist.github.com/439264

(defstruct bet name amount guess rate)

(defun pool-amount (pool)
  (reduce #'+ (mapcar #'bet-amount pool)))

(defun pool-rates (pool)
  (let ((sum (pool-amount pool)))
    (mapcar (lambda (bet)
              (let ((x (copy-bet bet)))
                (setf (bet-rate x) (/ (bet-amount x) sum))
                x)) pool)))

(defun pool-winners (pool result)
  (let ((sum (pool-amount pool))
        (winners (loop for bet in pool when (equal (bet-guess bet) result) collect bet)))
    (mapcar (lambda (winner)
              (let ((x (copy-bet winner)))
                (setf (bet-amount x) (* (bet-rate x) sum))
                x)) (pool-rates winners))))

;;; Usage

(defvar *pool*
  (list                                   
    (make-bet :name "Daniel" :amount 2.0 :guess '(2 0)) ;;; Daniel bets $2 on 2x0
    (make-bet :name "John"   :amount 4.0 :guess '(0 0)) ;;; and so on...
    (make-bet :name "Maria"  :amount 1.0 :guess '(2 0))))

(pool-winners *pool* '(0 2)) ;;; NIL
(pool-winners *pool* '(0 0)) ;;; (#S(BET :NAME "John" ... :RATE 1.0))
(pool-winners *pool* '(2 0)) ;;; (#S(BET :NAME "Daniel" ... :RATE 0.6666667) #S(BET :NAME "Maria" ... :RATE 0.33333334))