steinhaus.clj

;; 順列生成アルゴリズム
;; 『群論の味わい』p.69より

(ns steinhaus
  (:use [clojure.contrib.seq :only (rec-cat)]))

(defn- insert-at [item n coll]
  (concat (take n coll) (vector item) (drop n coll)))

(defn stein-rec [max]
  (if (= max 1)
    '((1))
    (reduce into
            (for [perm (stein-rec (dec max))]
              (map #(insert-at max % perm)
                   (range max))))))

(defn stein-loop [max]
  (lazy-seq
   (loop [perms '((1)), n 2]
     (if (> n max)
       perms
       (recur (reduce into
                      (for [p perms] (map #(insert-at n % p) (range n))))
              (inc n))))))