我可以在clojure中做出确定性的随机播放吗？

Question

我想制作一些套装,每次运行程序都是一样的:

这是一种方法:

(def colours ["red" "blue" "green" "yellow" "cyan" "magenta" "black" "white"])

(defn colour-shuffle [n] 
  (let [cs (nth (clojure.math.combinatorics/permutations colours) n)]
    [(first cs) (drop 1 cs)]))

; use (rand-int 40320) to make up numbers, then hard code:
(def colour-shuffle-39038 (colour-shuffle 39038))
(def colour-shuffle-28193 (colour-shuffle 28193))
(def colour-shuffle-5667  (colour-shuffle 5667))
(def colour-shuffle-8194  (colour-shuffle 8194))
(def colour-shuffle-13895 (colour-shuffle 13895))
(def colour-shuffle-2345  (colour-shuffle 2345))

colour-shuffle-39038 ; ["white" ("magenta" "blue" "green" "cyan" "yellow" "red" "black")]

但是评估需要一段时间,而且看起来很浪费而且相当不优雅.

有没有一种方法可以直接生成shuffle 39038,而不会生成和消耗所有序列？

(我已经意识到我可以对它们进行硬编码,或者用宏来重新编译时间.这也似乎有点垃圾.)

Answer 1

clojure.core/shuffle使用java.util.Collection/shuffle,它采用可选的随机数生成器. clojure.core/shuffle不使用此参数,但您可以使用它来创建一个带有附加种子值参数的shuffle变体,并使用该种子值创建一个随机数生成器以传递给 java.util.Collection/shuffle:

(defn deterministic-shuffle
  [^java.util.Collection coll seed]
  (let [al (java.util.ArrayList. coll)
        rng (java.util.Random. seed)]
    (java.util.Collections/shuffle al rng)
    (clojure.lang.RT/vector (.toArray al))))

Answer 2

听起来你想对排列进行编号：

(def factorial (reductions * 1 (drop 1 (range))))

(defn factoradic [n] {:pre [(>= n 0)]}
   (loop [a (list 0) n n p 2]
      (if (zero? n) a (recur (conj a (mod n p)) (quot n p) (inc p)))))

(defn nth-permutation [s n] {:pre [(< n (nth factorial (count s)))]}
  (let [d (factoradic n)
        choices (concat (repeat (- (count s) (count d)) 0) d)]
    ((reduce 
        (fn [m i] 
          (let [[left [item & right]] (split-at i (m :rem))]
            (assoc m :rem (concat left right) 
                     :acc (conj (m :acc) item))))
      {:rem s :acc []} choices) :acc)))

我们来尝试一下：

(def colours ["red" "blue" "green" "yellow" "cyan" "magenta" "black" "white"])

(nth-permutation colours 39038)
=> ["white" "magenta" "blue" "green" "cyan" "yellow" "red" "black"]

...如问题中所示，但不会生成任何其他排列。

好吧，但是我们能把它们全部拿到吗？

(def x (map (partial nth-permutation colours) (range (nth factorial (count colours)))))

(count x)
=> 40320
(count (distinct x))
=> 40320
(nth factorial (count colours))
=> 40320

请注意，排列是按（按索引字典顺序）顺序生成的：

user=> (pprint (take 24 x))
(["red" "blue" "green" "yellow" "cyan" "magenta" "black" "white"]
 ["red" "blue" "green" "yellow" "cyan" "magenta" "white" "black"]
 ["red" "blue" "green" "yellow" "cyan" "black" "magenta" "white"]
 ["red" "blue" "green" "yellow" "cyan" "black" "white" "magenta"]
 ["red" "blue" "green" "yellow" "cyan" "white" "magenta" "black"]
 ["red" "blue" "green" "yellow" "cyan" "white" "black" "magenta"]
 ["red" "blue" "green" "yellow" "magenta" "cyan" "black" "white"]
 ["red" "blue" "green" "yellow" "magenta" "cyan" "white" "black"]
 ["red" "blue" "green" "yellow" "magenta" "black" "cyan" "white"]
 ["red" "blue" "green" "yellow" "magenta" "black" "white" "cyan"]
 ["red" "blue" "green" "yellow" "magenta" "white" "cyan" "black"]
 ["red" "blue" "green" "yellow" "magenta" "white" "black" "cyan"]
 ["red" "blue" "green" "yellow" "black" "cyan" "magenta" "white"]
 ["red" "blue" "green" "yellow" "black" "cyan" "white" "magenta"]
 ["red" "blue" "green" "yellow" "black" "magenta" "cyan" "white"]
 ["red" "blue" "green" "yellow" "black" "magenta" "white" "cyan"]
 ["red" "blue" "green" "yellow" "black" "white" "cyan" "magenta"]
 ["red" "blue" "green" "yellow" "black" "white" "magenta" "cyan"]
 ["red" "blue" "green" "yellow" "white" "cyan" "magenta" "black"]
 ["red" "blue" "green" "yellow" "white" "cyan" "black" "magenta"]
 ["red" "blue" "green" "yellow" "white" "magenta" "cyan" "black"]
 ["red" "blue" "green" "yellow" "white" "magenta" "black" "cyan"]
 ["red" "blue" "green" "yellow" "white" "black" "cyan" "magenta"]
 ["red" "blue" "green" "yellow" "white" "black" "magenta" "cyan"])