Atbash Cipher in Clojure

atbash.clj

(ns atbash.core
  (require [clojure.string :refer [join lower-case]]))

(def lookup
  (let [alphabet "abcdefghijklmnopqrstuvwxyz"]
    (zipmap alphabet (reverse alphabet))))

(defn encode
  [s]
  (->> (lower-case s)
       (map lookup)
       (filter identity)
       (partition 5 5 ())
       (map join)
       (join " ")))