Functions to help determine who the winner is

reversi-winner.clj

(defn count-pieces [board piece]
  "Returns the number of squares occupied by the specified piece"
  (count (filter #(= % piece) (flatten board))))

(defn winner [board]
  "Returns the winner, or nil if there's a tie"
  (let [w (count-pieces board :w ) b (count-pieces board :b )]
    (cond
      (> w b) :w (> b w) :b :else nil)))