permutations.clj

(ns permutations
  (:require [midje.sweet :refer :all]))

(defn permute
  ([in]
   (set (permute [] in)))
  ([concrete possible-next]
   (mapcat
     (fn [n]
       (let [new-next     (disj possible-next n)
             new-concrete (conj concrete n)]
         (if (empty? new-next)
           [new-concrete]
           (permute new-concrete new-next))))
     possible-next)))

(fact (permute #{1})
      => #{[1]})

(fact (permute #{1 2})
      => #{[1 2]
           [2 1]})

(fact (permute #{1 2 3})
      => #{[1 2 3]
           [1 3 2]
           [2 1 3]
           [2 3 1]
           [3 2 1]
           [3 1 2]})