(ql:quickload '(fiveam uiop))

(defun encode-char (c)
  (ecase c
    (2 #\2)
    (1 #\1)
    (0 #\0)
    (-1 #\-)
    (-2 #\=)))

(defun decode-char (c)
  (ecase c
    (#\2 2)
    (#\1 1)
    (#\0 0)
    (#\- -1)
    (#\= -2)))

(defun parse-snafu (num)
  (loop for a across (reverse num)
        for pow = 1 then (* pow 5)
        sum (* pow (decode-char a))))

(defun write-snafu (num &optional coeffs)
  (if (zerop num)
      (concatenate 'string coeffs)
      (multiple-value-bind (div rest) (floor num 5)
        (if (> rest 2)
            (write-snafu (1+ div) (cons (encode-char (- rest 5)) coeffs))
            (write-snafu     div  (cons (encode-char    rest)    coeffs))))))

(defun solver (filename)
  (write-snafu (apply #'+ (mapcar #'parse-snafu (uiop:read-file-lines filename)))))

(fiveam:test solutions
  (fiveam:is (string= "2=-1=0" (solver "eg-in")))
  (fiveam:is (string= "2-212-2---=00-1--102" (solver "input"))))