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(in-package :aoc/2024)
(defparameter *input/d2*
(asdf:system-relative-pathname :advent "2024/d02.input"))
(defun split-by-one-space (string)
"Splits the string at spaces and parses the values as ints"
(loop for i = 0 then (1+ j)
as j = (position #\Space string :start i)
collect (parse-integer (subseq string i j))
while j))
(defun readlists (filename)
"Reads the given file into a list of reports containing levels"
(let ((reports '()))
(with-open-file (stream filename :direction :input)
(loop for line = (read-line stream nil nil)
while line
do (push (split-by-one-space line) reports)))
(nreverse reports)))
(defun diff-safe-p (a b)
"Checks if the given two numbers are within of 3 of each other"
(<= (abs (- a b)) 3))
(defun ordered-p (lst)
"Checks if all conditions apply"
(and (or (apply #'< lst) (apply #'> lst))
(every #'diff-safe-p lst (rest lst))))
(print (count-if #'ordered-p (readlists *input/d2*)))
; 287
(defun remove-nth (n lst)
"Remove the nth value from the given list"
(remove-if (constantly t) lst :start n :count 1))
(defun fixable (lst)
"Check if the given list if fixable when removing a value"
(loop for i from 0 below (length lst)
thereis (ordered-p (remove-nth i lst))))
(defun part-2 (lst)
"The conditions for the second part"
(or (ordered-p lst) (fixable lst)))
(print (count-if #'part-2 (readlists *input/d2*)))
; 354
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