advent-2025/p4/p4.org

Solution to p4

Yay, a maze. Cool. No, not cool.

  (with-temp-buffer
    (insert-file-contents "input")
    (advent/replace-multiple-regex-buffer
     '(("^\\(.*\\)$" . "\"\\1\" ")))
    (goto-char (point-min))
    (insert "(setq data '(")
    (goto-char (point-max))
    (insert "))")
    (eval-buffer))

  (setq data-char (-map #'advent/split-string-into-char-list data)
        width (length (car data-char))
        height (length data-char))

This is for part 1; easy

  (defun position-valid-p (pos)
    (and (and (>= (car pos) 0) (< (car pos) width))
         (and (>= (cadr pos) 0) (< (cadr pos) height))))

  (defun roll-at-p (pos data-char)
    (when (position-valid-p pos)
      (eq ?@ (advent/char-at pos data-char))))

  (setq adjacent '((-1 -1)
                   (-1 0)
                   (-1 1)
                   (0 1)
                   (0 -1)
                   (1 -1)
                   (1 0)
                   (1 1)))

  (defun rolls-adjacent (pos data-char)
    (length (-non-nil (--map (roll-at-p it data-char) (--map (advent/neighbour pos it) adjacent)))))

  (length (--filter (< it 4)
                    (--map (rolls-adjacent it data-char) (advent/coordinates-of ?@ data-char))))

1464

Now for part two, we create a new data-char at each removal step and count how many rolls have been removed

  (setq initial-roll-number (length (advent/coordinates-of ?@ data-char))
        current-roll-number (1+ initial-roll-number))

  (while (> current-roll-number
            (setq current-roll-number (length (advent/coordinates-of ?@ data-char))))
    (setq data-char
          (--2map-indexed (if (and (= ?@ it)
                                   (<= 4 (rolls-adjacent it-multi-index data-char)))
                              ?@ ?.)
                          data-char)))

  (- initial-roll-number current-roll-number)

8409

  (defun -2map (fun li)
    (--map (-map fun it) li))

  (defmacro --2map (form list)
    `(-2map (lambda (it) ,form) ,list))

  (defun -2map-indexed (fun li)
    (-map-indexed (lambda (i row)
                    (-map-indexed (lambda (j it)
                                    (funcall fun i j it))
                                  row))
          li))

  (defmacro --2map-indexed (form li)
    `(-2map-indexed (lambda (it-index-1 it-index-2 it) ,form) ,li))