(ql:quickload '(fiveam uiop))
(defconstant +num-encoding+
'((2 . #\2)
(1 . #\1)
(0 . #\0)
(-1 . #\-)
(-2 . #\=)))
(defun encode-char (c)
(cdr (assoc c +num-encoding+)))
(defun decode-char (c)
(car (rassoc c +num-encoding+)))
(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"))))
