beta.cljc

(ns meander.util.beta)


(defn cljs-env?
  "true if compiling ClojureScript or in a ClojureScript setting,
  false otherwise."
  [env]
  #?(:clj (some? (:ns env))
     :cljs true))


(defn parse-int
  "Parse the string s as an integer."
  [s]
  #?(:clj (Integer/parseInt s)
     :cljs (js/parseInt s)))


(defn swap
  "Swap the elements at positions `i` and `j` in `v`."
  {:private true}
  [v i j]
  (-> v
      (assoc i (nth v j))
      (assoc j (nth v i))))


;; SEE: https://en.wikipedia.org/wiki/Heap%27s_algorithm
(defn permutations
  "Return a sequence of all the ways to arrange coll."
  [coll]
  (let [v (vec coll)
        n (count v)]
    (loop [n n
           a [v]]
      (if (zero? n)
        a
        (let [n* (dec n)
              a* (mapcat
                  (fn step [v]
                    (map
                     (fn [i]
                       (swap v i n*))
                     (range n)))
                  a)]
          (recur n* a*))))))


(defn k-combinations
  "All the ways to choose k items from coll."
  [coll k]
  (if (= k 1)
    (sequence (map vector) coll)
    (let [coll (vec coll)
          n (count coll)]
      (sequence
       (comp
        (reduce comp
                (repeat (dec k)
                        (mapcat
                         (fn [v]
                           (let [i (peek v)]
                             (map conj (repeat v) (range i)))))))
        (mapcat permutations)
        (map
         (fn [ptrs]
           (mapv nth (repeat coll) ptrs))))
       (map vector (range n))))))


(defn vsplit-at
  "Like `clojure.core/split-at` but for vectors."
  ([n v]
   {:pre [(vector? v)]}
   (let [i (min n (count v))]
     [(subvec v 0 i) (subvec v i)])))


(defn vec-partitions [n v]
  {:private true}
  "

(let [coll [:a :b]
      n 3]
  (vec-partitions n coll))
;; => ([[] [] [:a :b]]
;;     [[] [:a] [:b]]
;;     [[:a] [] [:b]]
;;     [[] [:a :b] []]
;;     [[:a] [:b] []]
;;     [[:a :b] [] []])
"
  {:pre [(nat-int? n)]}
  (case n
    0 (list [])
    1 (list [v])
    2 (sequence
       (map
        (fn [i]
          [(subvec v 0 i) (subvec v i)]))
       (range (inc (count v))))
    ;; else
    (sequence
     (comp (map-indexed
            (fn [i _]
              [(subvec v 0 i) (subvec v i)]))
           (mapcat
            (fn [[a b]]
              (sequence
               (map conj)
               (vec-partitions (dec n) a)
               (repeat b)))))
     (range (inc (count v))))))

(defn coll-partitions
  {:private true}
  ([n coll]
   {:pre [(nat-int? n)]}
   (case n
     0 (list [])
     1 (list [coll])
     2 (sequence
        (map-indexed
         (fn [i _]
           (split-at i coll)))
        (cons 1 coll))
     ;; else
     (sequence
      (comp
       (map-indexed
        (fn [i _]
          (split-at i coll)))
       (mapcat
        (fn [[a b]]
          (sequence
           (map conj)
           (coll-partitions (dec n) a)
           (repeat b)))))
      ;; Adding one more element to the coll ensures we split at 0
      ;; *and* at (count coll) without counting the collection.
      (cons (first coll) coll)))))


(defn str-partitions [n str]
  "
Examples:

(let [str \"ab\"
      n 0]
  (str-partitions n str))
;; => ([])

(let [str \"ab\"
      n 1]
  (partitions n coll))
;; => ([\"ab\"])

(let [str \"ab\"
      n 2]
  (partitions n coll))
;; => ([[] [\"ab\"]
;;     [[\"a\"] [\"b\"]]
;;     [[\"ab\"] []])

(let [str \"ab\" 
      n 3]
  (partitions n coll))
;; => ([[] [] [\"ab\"]]
;;     [[] [\"a\"] [\"b\"]]
;;     [[\"a\"] [] [\"b\"]]
;;     [[] [\"ab\"] []]
;;     [[\"a\"] [\"b\"] []]
;;     [[\"ab\"] [] []])
"
  {:pre [(nat-int? n)]}
  (case n
    0 (list [])
    1 (list [str])
    2 (sequence
       (map
        (fn [i]
          [(subs str 0 i) (subs str i)]))
       (range (inc (.length str))))
    ;; else
    (sequence
     (comp
      (map
       (fn [i]
         [(subs str 0 i) (subs str i)]))
      (mapcat
       (fn [[a b]]
         (sequence
          (map conj)
          (str-partitions (dec n) a)
          (repeat b)))))
     (range (inc (.length str))))))


(defn partitions [n coll]
  "
Examples:

(def coll [:a :b])

(partitions 0 coll))
;; => ([])

(partitions 1 coll)
;; => ([[:a :b]])

(partitions 2 coll)
;; => '([[] [:a :b]]
;;      [[:a] [:b]]
;;      [[:a :b] []])

(partitions 3 coll)
;; => '([[] [] [:a :b]]
;;      [[] [:a] [:b]]
;;      [[:a] [] [:b]]
;;      [[] [:a :b] []]
;;      [[:a] [:b] []]
;;      [[:a :b] [] []])
"
  (cond
    (vector? coll)
    (vec-partitions n coll)

    (coll? coll)
    (coll-partitions n coll)

    (string? coll)
    (str-partitions n coll)

    :else
    (throw (ex-info "coll must be a string? or coll?" {:type (type coll)}))))