2 files changed, 86 insertions(+), 0 deletions(-)

A day12.lisp
A input/day12.txt

@@ 0,0 1,65 @@
+(defpackage #:advent2021.day12
+  (:use #:cl #:alexandria #:cl-ppcre #:advent2021.util)
+  (:export #:solve-part-1 #:solve-part-2))
+
+(in-package #:advent2021.day12)
+
+
+
+(defparameter +input+
+  (loop :with map := (make-hash-table)
+        :for line :in (parse-lines #'identity)
+        :for (from to) := (mapcar (lambda (kw)
+                                    (intern kw :keyword))
+                                  (split "-" line))
+        :do (push to (gethash from map))
+            (push from (gethash to map))
+        :finally (return map)))
+
+(defun possible-paths (start &optional route)
+  (cond
+    ((eq start :|end|) 1)
+    ((member start route) 0)
+    (t
+     (loop :for next-node :in (gethash start +input+)
+           :sum (possible-paths next-node
+                                (if (upper-case-p
+                                      (char (string start) 0))
+                                    route
+                                    (cons start route)))))))
+
+(defun solve-part-1 ()
+  (possible-paths :|start|))
+
+(defun large? (node)
+  (upper-case-p (char (string node) 0)))
+
+(defun cave-visited-twice? (route)
+  (loop :with small-caves := ()
+        :for cave :in route
+        :do (when (not (large? cave))
+              (if (member cave small-caves)
+                  (return t)
+                  (push cave small-caves)))))
+
+(defun should-visit? (node route)
+  (when (eq node :|start|)
+      (return-from should-visit? nil))
+
+  (or (large? node)
+      (< (loop :for n :in route
+            :count (eq n node))
+         (if (cave-visited-twice? route)
+             1 2))))
+
+(defun possible-paths* (start &optional route)
+  (cond
+    ((eq start :|end|)
+     ;; (print (list :path route))
+     1)
+    (t (loop :for next-node :in (gethash start +input+)
+             :when (should-visit? next-node (cons start route))
+             :sum (possible-paths* next-node (cons start route))))))
+
+(defun solve-part-2 ()
+  (possible-paths* :|start|))

@@ 0,0 1,21 @@
+TR-start
+xx-JT
+xx-TR
+hc-dd
+ab-JT
+hc-end
+dd-JT
+ab-dd
+TR-ab
+vh-xx
+hc-JT
+TR-vh
+xx-start
+hc-ME
+vh-dd
+JT-bm
+end-ab
+dd-xx
+end-TR
+hc-TR
+start-vh