Conway's game of life, transducers edition

(require '[net.cgrand.xforms :as x])

(defn step [cells]
  (into #{}
    (comp
      (x/for [[x y] % dx [-1 0 1] dy (if (zero? dx) [-1 1] [-1 0 1])] 
       [(+ dx x) (+ dy y)])
      (x/by-key identity x/count)
      (x/for [[cell n] %
              :when (or (= n 3) (and (= n 2) (cells cell)))]
        cell))
    cells))

=> (def board #{[1 0] [1 1] [1 2]}) ; blinker
=> (take 5 (iterate step board))
(#{[1 0] [1 1] [1 2]} #{[1 1] [2 1] [0 1]} #{[1 0] [1 1] [1 2]} #{[1 1] [2 1] [0 1]} #{[1 0] [1 1] [1 2]})

Alternative impl:

(defn step [cells]
  (x/into #{}
    (comp
      (x/for [[x y] % dx [-1 0 1] dy (if (zero? dx) [-1 1] [-1 0 1])] 
       [[(+ dx x) (+ dy y)] 1])
      (x/by-key (x/reduce +))
      (x/for [[cell n] %
              :when (or (= n 3) (and (= n 2) (cells cell)))]
        cell))
    cells))