My solution to https://4clojure.com/problem/138

ummels-4clojure-solution138.clj

;; ummels's solution to Squares Squared
;; https://4clojure.com/problem/138

(fn [start end]
  (letfn
    [(squares [start end]
       (take-while #(<= % end) (iterate #(* % %) start)))
     (arrange [x]
       (let [n (count x)
             dim (int (Math/ceil (Math/sqrt n)))
             dim1 (dec (* 2 dim))
             y (concat x (repeat (- (* dim dim) n) \*))
             rot (fn [[i j]] (if (= j 0) [(* i -1) i] [i 0]))
             legal? (fn [[i j]] (and (>= i 0) (>= j 0) (< i dim1) (< j dim)))]
         (loop [p (if (odd? dim) [(dec dim) (dec dim)] [(dec dim) 0])
                d (if (odd? dim) [-1 0] [1 0])
                in (reverse y)
                res (vec (repeat dim1 (vec (repeat dim nil))))]
           (cond
             (empty? in) res
             (and (legal? (map + p d)) (nil? (get-in res (map + p d))))
               (recur (map + p d) d (rest in) (assoc-in res p (first in)))
             :else
               (recur (map + p (rot d)) (rot d) (rest in) (assoc-in res p (first in)))))))
     (pretty [x]
       (for [r x]
         (let [s (take-while (complement nil?) (drop-while nil? r))
               m (- (count r) (count s))]
           (apply str
             (concat (repeat m \space)
                     (rest (interleave (repeat \space) s))
                     (repeat m \space))))))]
    (pretty (arrange (mapcat str (squares start end))))))