Merge two ascii (art) files like image layers, treating spaces as transparent.

merge_ascii.clj

(ns merge-ascii
  (:require [clojure.java.io :refer [reader writer]]))

(defn choose-char [a b]
  (cond (nil? a) b
        (nil? b) a
        (not= b \space) b
        :else a))

(defn merge [a b out]
  (with-open [read-a (reader a)
              read-b (reader b)
              w (writer out)]
    (let [lines-a (line-seq read-a)
          lines-b (line-seq read-b)]
      (loop [[al & als] lines-a
             [bl & bls] lines-b]
        (when (or al bl)
          (loop [[ac & acs] al
                 [bc & bcs] bl]
            (when (or ac bc)
              (.write w (str (choose-char ac bc)))
              (recur acs bcs)))
          (.newLine w)
          (recur als bls))))))