advent-2025/p1/p1.org

Solution to p1

First load the data as a list of positive or negative numbers

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

Then add the rotations starting from the initial offset and count the number of 0's mod 100; that is what we need for part 1

      (length (--filter (= 0 it)
    		    (--map (mod it 100)
  			   (-running-sum (cons 50 data)))))

962

For part 2, This works:

• we count the number of full turns that have been done in each rotation. Since there are corner cases we need to average over two offsets of ½

  (defun advent/consecutive-pairs (list)
    (-zip-pair list (cdr list)))

  (defun average (li)
    (/ (-sum li) (length li)))

  (let ((number-lists (--map (-running-sum (cons it data))
  			   (list 50.5 49.5))))
    (average (--map 
  	    (-sum (--map (abs (- (car it) (cdr it)))
  			 (advent/consecutive-pairs
  			  (--map (floor it 100) it))))
  	    number-lists)))

5782