[p7] cached-recursive approach

p7/p7.org

@@ -68,3 +68,30 @@ the multiplicities of each beam
 #+RESULTS:
 : 1537373473728
 
+now try part2 with a recursive approach. This should look better.
+Of course it will blow in my face if I did not cache the results
+(took 1.1s on Pixel 7)
+#+begin_src emacs-lisp
+  (setq max-lisp-eval-depth 10000000) ; burn baby burn
+  (setq cache nil)
+  (defun cache-result (fun pos)
+    (let ((c (assoc pos cache)))
+      (if c (cdr c)
+        (let ((res (funcall fun pos)))
+    	(push (cons pos res) cache)
+    	res ))))
+  (advice-add 'timelines-at :around 'cache-result)
+
+  (defun timelines-at (pos)
+    (if (not (position-valid-p pos)) 1
+      (if (not (eq ?^ (advent/char-at pos data-char)))
+          (timelines-at (advent/neighbour pos '(0 1)))
+        (-sum (--map (timelines-at (advent/neighbour pos it))
+                     '((1 0) (-1 0)))))))
+
+  (setq tachyons (car (advent/coordinates-of ?S data-char)))
+  (timelines-at tachyons)
+#+end_src
+
+#+RESULTS:
+: 1537373473728