advent-2025/p2/p2.org

Solution to p2

First load the data as a list of cons cells (start . end)

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

This is for part 1; only two repetitions

  (defun create-range (rng)
    (-iterate #'1+ (car rng) (1+ (- (cdr rng) (car rng)))))

  (defun invalid-id-p (num)
    (let* ((num-digits (1+ (floor (log num 10))))
           (mult (expt 10 (floor (/ num-digits 2)))))
      (= (mod num mult) (/ num mult))))

  (-sum
   (--mapcat (-filter #'invalid-id-p it)
  	   (-map #'create-range data)))

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This is for part 2: very inefficient, but it works

  (defun factors (num)
    (let ((candidates (-iterate #'1+ 1 num)))
      (--filter (= num (* it (floor (/ num it)))) candidates)))

  (defun chop-num (num len)
    (let ((mult (expt 10 len)))
      (--unfold (when (> it 0) (cons (mod it mult) (floor (/ it mult)))) num)))

  (defun invalid-id-p (num)
    (let* ((num-digits (1+ (floor(log num 10))))
           (factors (-drop-last 1 (factors num-digits))))
      (--any (= 1 (length it))
             (-map #'-distinct (--map (chop-num num it) factors)))))

    (-sum
     (--mapcat (-filter #'invalid-id-p it)
  	     (-map #'create-range data)))

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