Solución en Clojure al Desafío de la Gran Capicuá (Programando.org)

core.clj

(ns grancapicua.core)

(defn reverso [res n]
  (if (zero? n)
    res
    (recur (+ (* 10 res) (rem n 10)) (quot n 10))))

(defn capicua? [n]
  (= n (reverso 0 n)))

(defn digits [x]
  (cond
    (< x 10) 10
    (< 10 x 100) 100
    (< 100 x 1000) 1000
    :else 10000))

(defn iso-fmt [y m d]
  [(+ (* 1000 y) (* 100 m) d)])

(defn ddmmyyyy [y m d]
  [(+ (* 10000 d) (* 100 m) y)
   (+ (* (digits m) (digits y) d) (* (digits y) m) y)])

(defn mmddyyyy [y m d]
  [(+ (* 10000 m) (* 100 d) y)
   (+ (* (digits d) (digits y) m) (* (digits y) d) y)])

(defn capicua-date? [y m d]
  (->> (mapcat #(% y m d) [iso-fmt ddmmyyyy mmddyyyy])
       (some capicua?)))

(def days-month [nil 31 28 31 30 31 30 31 31 30 31 30 31])

(defn solve []
  (for [y (range 1 10000)
        m (range 1 13)
        d (range 1 (inc (days-month m)))
        :when (capicua-date? y m d)]
    (iso-fmt y m d)))

;; (capicua-date? 1 1 1) ;; => true
;; (capicua-date? 1111 11 11) ;; => true
;; (capicua-date? 2020 2 2) ;; => true

;; (time (count (solve)))
;; "Elapsed time: 35227.353732 msecs"
;; 50645