commit 4af8595fb520002054c5e07b0661b6135c69420b
from: Lukas Henkel <lh@entf.net>
date: Fri Dec 08 05:39:55 2023 UTC

Day 8 task 2

commit - 0208257989a583a9dee119a5269316a42eb39b71
commit + 4af8595fb520002054c5e07b0661b6135c69420b
blob - 975f5f85ccebdfe07e0127c43fd49d2d4e1fa93f
blob + 9f0a8cff3c62f7a8d93f2213b9baa570b239843e
--- src/day-8.lisp
+++ src/day-8.lisp
@@ -22,16 +22,28 @@
         do (setf (gethash key map) left-right)
         finally (return map)))
 
+(defun walk-to-destination (instructions map starting-point test-fun)
+  (loop with current = starting-point
+        for steps from 0
+        for direction = (aref instructions (mod steps (length instructions)))
+        for (left right) = (gethash current map)
+        do (setf current
+                 (ecase direction
+                   (#\L left)
+                   (#\R right)))
+        when (funcall test-fun current)
+          do (return (1+ steps))))
+
+(defun destination-p (field)
+  (char= (last-elt field) #\Z))
+
 (defun day-8 (input)
   (let ((instructions (read-line input))
         (map (read-map input)))
-    (loop with current = "AAA"
-          for steps from 0
-          for direction = (aref instructions (mod steps (length instructions)))
-          for (left right) = (gethash current map)
-          do (setf current
-                   (ecase direction
-                     (#\L left)
-                     (#\R right)))
-          when (equal current "ZZZ")
-            do (return (1+ steps)))))
+    (values (if (gethash "AAA" map)
+                (walk-to-destination instructions map "AAA" (curry #'equal "ZZZ"))
+                0)
+            (apply #'lcm
+                   (loop for field string being the hash-key of map
+                         when (char= (last-elt field) #\A)
+                           collect (walk-to-destination instructions map field #'destination-p))))))
blob - 6525fd470b0a7ac48ebaf4524046fc026916d302
blob + c148247ceec030b4af7522e7d61056d012794527
--- t/day-8.lisp
+++ t/day-8.lisp
@@ -21,4 +21,18 @@ ZZZ = (ZZZ, ZZZ)")
 AAA = (BBB, BBB)
 BBB = (AAA, ZZZ)
 ZZZ = (ZZZ, ZZZ)")
-    (assert= 6 task-1)))
+    (assert= 6 task-1))
+
+  (multiple-value-bind (task-1 task-2)
+      (aoc:run-day 8 "LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)")
+    (declare (ignore task-1))
+    (assert= 6 task-2)))