(defun read-maps (filespec)
  (with-open-file (stream filespec)
    (loop for line = (read-line stream nil)
          while line
          collect (loop collect line
                        while (setf line (read-line stream nil))
                        while (> (length line) 0)))))

(defun count-diff (list1 list2)
  (loop for string1 in list1
        for string2 in list2
        sum (loop for c1 across string1
                  for c2 across string2
                  sum (if (equal c1 c2) 0 1))))

(defun find-horizontal-mirror (map &optional (ndiff 0))
  (loop for prev = () then (cons (car rem) prev)
        for rem on map
        for i upto (length map)
        if (and prev rem (= ndiff (count-diff prev rem)))
          return i))

(defun rotate-map (map)
  (loop for i below (length (first map))
        collect (coerce (loop for line in map collect (aref line i)) '(string))))

(defun find-vertical-mirror (map &optional (ndiff 0))
  (find-horizontal-mirror (rotate-map map) ndiff))

(defun solve (filespec &optional (ndiff 0))
  (let ((maps (read-maps filespec)))
    (loop for map in maps
          as aux = (find-horizontal-mirror map ndiff)
          if aux
            sum (* 100 aux)
          else
            sum (find-vertical-mirror map ndiff))))

(defun solve1 (filespec) (solve filespec))

(defun solve2 (filespec) (solve filespec 1))

(print (solve1 "data/13/example.txt")) ;; 405
(print (solve1 "data/13/input.txt"))   ;; 30535
(print (solve2 "data/13/example.txt")) ;; 400
(print (solve2 "data/13/input.txt"))   ;; 30844