Add 2023/08

src/2023/day08.lisp

@@ -0,0 +1,35 @@
+(defpackage :aoc/2023/08 #.cl-user::*aoc-use*)
+(in-package :aoc/2023/08)
+
+(defun parse-input (&optional (strings (uiop:read-file-lines #P"src/2023/day08.txt")))
+  (destructuring-bind (instructions ignore . nodes) strings
+    (flet ((node (s)
+             (mapcar #'as-keyword (cl-ppcre:all-matches-as-strings "\\w+" s))))
+      (cons (coerce instructions 'list)
+            (mapcar #'node nodes)))))
+
+; (loop with (inst . nodes) = (parse-input)
+;       for step from 0
+;       for node = (assoc :aaa nodes) then (assoc next nodes)
+;       for in in (ncycle inst) for next = (if (eq in #\L) (second node) (third node))
+;       when (eq (car node) :zzz) return step)
+; => 21409
+
+(defun node-name-ends-with-char? (ch node)
+  (char= (char (reverse (mkstr (first node))) 0) ch))
+(node-name-ends-with-char? #\A '(:aaz))
+
+(defun find-part2 (start &optional (strings (uiop:read-file-lines #P"src/2023/day08.txt")))
+  (loop with (inst . nodes) = (parse-input)
+        for step from 0
+        for node = (assoc start nodes) then (assoc next nodes)
+        for in in (ncycle inst) for next = (if (eq in #\L) (second node) (third node))
+        when (node-name-ends-with-char? #\Z node) return step
+        never (not node)))
+
+; (apply
+;   #'lcm
+;   (looping
+;     (dolist (start (remove-if-not [node-name-ends-with-char? #\A _] (cdr (parse-input))))
+;       (collect! (find-part2 (car start))))))
+; => 21165830176709