advent-2025/p8/p8.org

Solution to p8

  (with-temp-buffer
    (insert-file-contents "input")
    (advent/replace-multiple-regex-buffer
     '(("," . " ")
       ("^" . "(")
       ("$" . ")")))
    (goto-char (point-min))
    (insert "(setq data '(")
    (goto-char (point-max))
    (insert "))")
    (eval-buffer))

First of all, sort the pair of boxes by means of their distance.

  (defun square (x)
    (* x x))

  (defun distance-squared (el)
   (let ((a (car el))
         (b (cdr el)))
     (+ (square (- (car a) (car b)))
        (square (- (cadr a) (cadr b)))
        (square (- (caddr a) (caddr b))))))

  (defun symmetric-pairs (list)
   (apply #'append (--map-indexed (-map (lambda (other) (cons it other))
                         (-drop (1+ it-index) list))
                   list)))

  (setq sorted-pairs
        (--sort (< (distance-squared it) (distance-squared other)) (symmetric-pairs data)))

This is for part 1

  (defun insert-pair (circuits el)
    (let* ((list (list (car el) (cdr el)))
           (ind-a (--find-index (-contains-p it (car el)) circuits))
           (ind-b (--find-index (-contains-p it (cdr el)) circuits)))
      (cond
       ((not (or ind-a ind-b)) (cons list circuits))
       ((and ind-a ind-b (not (eq ind-a ind-b)))
        (-remove-at ind-b (-replace-at ind-a
                                       (-union (nth ind-a circuits)
                                               (nth ind-b circuits))
                                       circuits)))
       (t  (-replace-at (or ind-a ind-b)
                        (-union list (nth (or ind-a ind-b) circuits))
                        circuits)))))

  (setq circuits
        (-reduce-from #'insert-pair nil (-take 1000 sorted-pairs)))

  (-product (-take 3 (-sort '> (-map #'length circuits))))

81536

This is for part 2. Will it explode?

  (let ((sorted-pairs sorted-pairs)
        (circuits nil)
        (last-junction nil))
    (while (or (> 1 (length circuits))
               (> (length data) (length (apply 'append circuits))))
      (setq last-junction (pop sorted-pairs)
            circuits (insert-pair circuits last-junction)))
    last-junction
    ;(* (caar last-junction) (cadr last-junction))
    )

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