A function to determine if a move in Reversi is valid

reversi-move-valid.clj

(def directions
  {:N [-1 0] :NE [-1 1] :E [0 1] :SE [1 1] :S [1 0] :SW [1 -1] :W [0 -1] :NW [-1 -1]})

(defn move-valid? [board piece move]
  "Returns true if the specified move for the specified piece will cause
  a piece to be flipped in any direction."
  (let [r (move 0) c (move 1)]
    (cond
      (or (> r 7) (< r 0) (> c 7) (< c 0)) nil
      (not= nil ((board r) c)) nil
      :else (or
              (flippable? board piece move (directions :N ))
              (flippable? board piece move (directions :NE ))
              (flippable? board piece move (directions :E ))
              (flippable? board piece move (directions :SE ))
              (flippable? board piece move (directions :S ))
              (flippable? board piece move (directions :SW ))
              (flippable? board piece move (directions :W ))
              (flippable? board piece move (directions :NW ))))))

(defn flippable? [board piece move direction]
  "Examines each square on the board moving outward from the specified
  move in the specified direction, looking for an opponent-piece that
  could be flipped as a result of the move. If a flippable piece is found,
  returns true, else returns false."
  (loop [m (vec (map + move direction))] ; m is the current square
    (let [r (m 0)
          c (m 1)
          rd (Math/abs (- (move 0) r)) ; num rows between originating square and current square
          cd (Math/abs (- (move 1) c))] ; num cols between originating square and current square
      (cond
        (or (> r 7) (< r 0) (> c 7) (< c 0)) false ; 'm' is outside the board
        (= nil ((board r) c)) false ; 'm' is empty
        (= piece ((board r) c)) (or (> rd 1) (> cd 1)) ; return false if we're 1 move away from origin, else true
        :else (recur (vec (map + m direction))))))) ; start next iteration of loop with 'm' set to next square