Largest Palindrome of the mul of 2 n-digits

; Determines if a sequence is palindromic
(defn is-palindrome [_seq]
  (let [len (count _seq)
        mid (/ len 2)
        a (subvec _seq 0 mid)
        b (subvec _seq (+ mid (mod len 2)))]
    (= a (reverse b))))

; Builds a vector from an integer
; > 34 -> [3 4]
(defn int-to-vec [_num]
  (vec (map #(- (int %) 48) (str _num))))

; Repeats an integer i, n times
;   returns the concatenation as an integer
; > (repeat-int 9 3) -> 999
(defn repeat-int [i n]
  (Integer/parseInt (apply str (repeat n i))))

; Calculates the multiplier for some level
(defn num-for-level [highest level]
  (- highest (* 2 (inc level))))

; Generates palindromes for a specific depth level
(defn vals-for-level [highest level]
  (let [extra (num-for-level highest (dec level))
        mul (num-for-level highest level)
        dat (map #(num-for-level highest (dec %))
              (range 0 (inc level)))
        partdat (map #(* % mul) dat)
        fulldat (conj partdat (* extra extra))
        filtered (filter #(is-palindrome (int-to-vec %)) fulldat)]
    filtered))

; Calculates the largest palindrome
;   which is also a product of two
;   n-digit values
; > (largest-palindrome 2) -> 9009
; > (largest-palindrome 3) -> 906609
; > (largest-palindrome 4) -> 99000099
; > (largest-palindrome 5) -> 9966006699
(defn largest-palindrome [digit_count]
  (if (< digit_count 1)
    nil
    (let [maxval (repeat-int 9 digit_count)]
      (loop [level 0]
        (let [palindromes (vals-for-level maxval level)]
          (if (empty? palindromes)
            (if (< level maxval)
              (recur (inc level))
              nil)
            (let [value (apply max palindromes)]
              value)))))))