AoC 2024 Day 2

day2.lisp

(defun safep (levels)
  (loop for (a b) on levels by #'cdr
        with sign = (signum (- (cadr levels) (car levels)))
        do
           (if (null b)
               (return t)
               (let ((delta (- b a)))
                 (unless (and (= sign (signum delta))
                              (<= 1 (abs delta) 3))
                   (return nil))))))

(defun drop-elt (n list)
  (append (subseq list 0 n) (subseq list (1+ n))))

(defun dampened-safe-p (levels)
  (if (safep levels)
      t
      (loop for i from 0 below (length levels)
            when (safep (drop-elt i levels))
              return t)))

(defvar *data*)
(progn
  (setf *data*
        (mapcar (lambda (string) (mapcar #'parse-integer (split-sequence #\space string)))
                (uiop:read-file-lines "day2.txt")))
  (length *data*))

(count-if #'safep *data*)
(count-if #'dampened-safe-p *data*)