advent-2024/p22/p22.org

Solution to p22

• Solution

Solution

  (require 'dash)
  (with-temp-buffer
      (insert-file-contents "input")
      (goto-char (point-min))
      (insert "(setq data '(")
      (goto-char (point-max))
      (insert "))")
      (eval-buffer))

Define the operations needed to create the pseudorandom sequence

  (defun mix (a b)
    (logxor a b))

  (defun prune (a)
    (logand a 16777215))

  (defun pseudo-next-1 (a)
    (prune (mix a (ash a 6))))

  (defun pseudo-next-2 (a)
    (prune (mix a (ash a -5))))

  (defun pseudo-next-3 (a)
    (prune (mix a (ash a 11))))

  (defun pseudo-next (a)
    (pseudo-next-3
     (pseudo-next-2
      (pseudo-next-1 a))))

This yields the solution to part 1

  (apply #'+ (--map (-last-item  (-iterate 'pseudo-next it 2001)) data))

19241711734

Create an alist with (quadruple . price)

  (defun price-sequence (seed)
    (--map (mod it 10) (-iterate 'pseudo-next seed 2001)))

  (defun take-quadruples (l)
    (-zip-lists l (cdr l) (cddr l) (cdddr l)))

  (defun create-quadruples (s)
    (-zip-pair (take-quadruples
           (-zip-with '- (cdr s) s))
          (cddddr s)))

create-quadruples

  (setq quadruples-data
        (--map
         (-quadruples (price-sequence it))
         data))

2256

  ; this seems super slow; it is not needed if we concatenate lists
  ; properly
  (defun normalize-alist (al)
    "Remove from the alist the secondary occurrences of each key"
    (--filter (eq it (assoc (car it) al)) al))

  (defun sum-stuff (b a)
    "A and B are two alists. "
    (append
     (--map (cons (car it)
                  (+ (cdr it)
                     (or (cdr (assoc (car it) b)) 0)))
            a)
     b))
  (native-compile 'sum-stuff); just in case

  (setq summed-quadruples
        (let ((index 0))
          (--reduce (progn
                      (message (format "Processing %d/2256" index))
                      (setq index (+ 1 index))
                      (sum-stuff acc it))
                    quadruples-data)))

  (--max-by (> (cdr it) (cdr other))
            summed-quadruples)

((-2 1 -1 3) . 23)