1 files changed, 37 insertions, 0 deletions
diff --git a/AoC2022/25/solver.lisp b/AoC2022/25/solver.lisp
new file mode 100644
index 0000000..630cd03
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+++ b/AoC2022/25/solver.lisp
@@ -0,0 +1,37 @@
+(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"))))