From dde69103925d2ed8848cd2549187b46358cf3224 Mon Sep 17 00:00:00 2001
From: Jacopo De Simoi <wilderjds@protonmail.com>
Date: Sun, 7 Dec 2025 20:49:13 -0500
Subject: [PATCH] [p7] better whitespace and comments

---
 p7/p7.org | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/p7/p7.org b/p7/p7.org
index ee82220..604c3e6 100644
--- a/p7/p7.org
+++ b/p7/p7.org
@@ -17,15 +17,11 @@ Yay, another 2D problem
         height (length data-char))
 #+end_src
 
-This is for part 1; 
+This is for part 1; record the splitting points and count them at the end
 #+begin_src emacs-lisp
-
-
-    (defun position-valid-p (pos)
-      (and (and (>= (car pos) 0) (< (car pos) width))
-           (and (>= (cadr pos) 0) (< (cadr pos) height))))
-
-  (advent/char-at '(-1 0) data-char)
+  (defun position-valid-p (pos)
+    (and (>= (car pos) 0) (< (car pos) width)
+         (>= (cadr pos) 0) (< (cadr pos) height)))
 
   (defun tachyon-step (el)
     (if (not (position-valid-p el)) (list el)
@@ -35,27 +31,40 @@ This is for part 1;
   	  (list down)
   	(push down splits)
   	(--map (advent/neighbour el it) '((-1 0) (1 0)))))))
-  (setq tachyons (advent/coordinates-of ?S data-char))
-  (setq splits nil)
-  (length (--fix (-distinct (-mapcat 'tachyon-step it)) tachyons))
+
+
+  (setq tachyons (advent/coordinates-of ?S data-char)
+        splits nil)
+  (--fix (-distinct (-mapcat 'tachyon-step it)) tachyons)
   (length (-distinct splits))
 #+end_src
 
 #+RESULTS:
 : 1507
 
+This is for part 2. At each step record how many beams are at a given
+position.  Each beam has some multiplicity. ~compress~ merges beams at
+the same position in one beam with multiplicity equal to the sum of
+the multiplicities of each beam
 #+begin_src emacs-lisp
   (defun tachyon-step-freq (el)
-      (--map (cons it (cdr el)) (tachyon-step (car el))))
+      (--map (cons it (cdr el))
+  	     (tachyon-step (car el))))
 
   (setq tachyons (cons (car (advent/coordinates-of ?S data-char)) 1))
 
   (defun compress (li)
-    (--map (cons it (-sum (-map #'cdr (-filter (lambda (x) (equal it (car x))) li)))) (-distinct (-map #'car li))))
-
-  (-sum (-map #'cdr (--fix (compress (-mapcat 'tachyon-step-freq it)) (list tachyons))))
+    (--map (cons it (-sum (-map #'cdr
+  			      (-filter (lambda (x)
+  					 (equal it (car x)))
+  				       li))))
+  	 (-distinct (-map #'car li))))
 
+  (-sum (-map #'cdr
+  	    (--fix (compress (-mapcat 'tachyon-step-freq it))
+  		   (list tachyons))))
 #+end_src
 
 #+RESULTS:
 : 1537373473728
+