commit 5cd36c441567ae4b5c7bd4bc399e39eea75e1206
from: Lukas Henkel <lh@entf.net>
date: Wed Dec 18 06:34:45 2024 UTC

Reverse task 2

This way is much faster

commit - ddf4527960184f991ca2d6abda4882b6c351312e
commit + 5cd36c441567ae4b5c7bd4bc399e39eea75e1206
blob - b2cf768455c8a52a6ceb50af7f6d1c4dd085205f
blob + 8463eca572917772cc0d753516a2ff486c87d515
--- src/day-18.lisp
+++ src/day-18.lisp
@@ -24,9 +24,14 @@
         until (null line)
         collect (parse-coordinate line)))
 
+(declaim (inline fall-byte unfall-byte))
+
 (defun fall-byte (map position)
   (setf (map-cell map position) #\#))
 
+(defun unfall-byte (map position)
+  (setf (map-cell map position) #\.))
+
 (defun draw-map (map visited)
   (loop for y from 0 below *height*
         do (loop for x from 0 below *width*
@@ -57,8 +62,10 @@
 
 (defun task-2 (map coordinates)
   (loop for byte in coordinates
-        do (fall-byte map byte)
-        unless (bfs map)
+        do (fall-byte map byte))
+  (loop for byte in (nreverse coordinates)
+        do (unfall-byte map byte)
+        when (bfs map)
           do (return byte)))
 
 (defun day-18 (input)