(defn- iter-perm [v]
  (let [len (count v),
	j (loop [i (- len 2)]
	     (cond (= i -1) nil
		   (< (v i) (v (inc i))) i
		   :else (recur (dec i))))]
    (when j
      (let [vj (v j),
	    l (loop [i (dec len)]
		(if (< vj (v i)) i (recur (dec i))))]
	(loop [v (assoc v j (v l) l vj), k (inc j), l (dec len)]
	  (if (< k l)
	    (recur (assoc v k (v l) l (v k)) (inc k) (dec l))
	    v))))))


(defn- vec-lex-permutations [v]
  (when v (cons v (lazy-seq (vec-lex-permutations (iter-perm v))))))

(defn lex-permutations
  "Fast lexicographic permutation generator for a sequence of numbers"
  [c]
  (lazy-seq
   (let [vec-sorted (vec (sort c))]
     (if (zero? (count vec-sorted))
       (list [])
       (vec-lex-permutations vec-sorted)))))

(defn permutations
  "All the permutations of items, lexicographic by index"
  [items]
  (let [v (vec items)]
    (map #(map v %) (lex-permutations (range (count v))))))

(println (permutations [1 2 3]))