ummels-4clojure-solution150.clj

;; ummels's solution to Palindromic Numbers
;; https://4clojure.com/problem/150

(fn [n]
  (letfn [(decode [n] (if (< n 10) [n] (conj (decode (quot n 10)) (rem n 10))))
          (encode [x] (reduce #(+ (* % 10) %2) 0 x))
          (next-pal [n]
            (let [N (decode n)
                  l (count N)
                  d (quot l 2)
                  H (take d N)
                  H1 (decode (inc (encode H)))
                  Hr (reverse H)
                  h (encode Hr)
                  p (nth N d)
                  t (encode (take-last d N))]
              (cond
                (and (even? l) (>= h t)) (encode (concat H Hr))
                (and (odd? l) (>= h t)) (encode (concat H [p] Hr))
                (even? l) (encode (concat H1 (reverse H1)))
                (and (odd? l) (< p 9)) (encode (concat H [(inc p)] Hr))
                :else (encode (concat H1 [0] (reverse H1))))))]
  (iterate (comp next-pal inc) (next-pal n))))