commit db0986abd6d2a985fff0dd0baa13c97a708a6b53
from: Lukas Henkel <lh@entf.net>
date: Fri Dec 06 07:04:56 2024 UTC

Slight optimization

Check only the spots in task 2 where the guard visited in task 1

commit - c7c064a8bf3e2b6aaaa8c6dc5e00d0553fc20728
commit + db0986abd6d2a985fff0dd0baa13c97a708a6b53
blob - cfbc66e5a32a3916228ab874d072a61e7e9b951d
blob + 2a21094e0eeba156116e0da9544abf12ddad2d62
--- src/day-6.lisp
+++ src/day-6.lisp
@@ -49,23 +49,22 @@
                                     next (next pos dir))
                            finally (return next)))
            (push pos visited)
-        finally (return visited)))
+        finally (return (remove-duplicates (cdr visited) :test #'equal))))
 
-(defun task-2 (map initial-pos)
+(defun task-2 (map initial-pos visited)
   (loop with task-2 = 0
-        for y from 0 below (input-map-height map)
-        do (loop for x from 0 below (input-map-width map)
-                 for point = (cons x y)
-                 for cell = (map-cell map point)
-                 when (char= cell #\.)
-                   do (setf (map-cell map point) #\#)
-                      (when (eq (walk-map map initial-pos) :loop)
-                        (incf task-2))
-                      (setf (map-cell map point) #\.))
+        for point in visited
+        for cell = (map-cell map point)
+        when (char= cell #\.)
+          do (setf (map-cell map point) #\#)
+             (when (eq (walk-map map initial-pos) :loop)
+               (incf task-2))
+             (setf (map-cell map point) #\.)
         finally (return task-2)))
 
 (defun day-6 (input)
   (let* ((map (make-map input))
-         (pos (find-guard map)))
-    (values (1- (length (remove-duplicates (walk-map map pos) :test #'equal)))
-            (task-2 map pos))))
+         (pos (find-guard map))
+         (visited (walk-map map pos)))
+    (values (length visited)
+            (task-2 map pos visited))))