AoC 2021, day 9

day9.lisp

(defun read-lines (path)
  (with-open-file (stream path :direction :input :if-does-not-exist :error)
    (flet ((line->vector (line)
             (let ((vec (make-array 0 :adjustable t :fill-pointer 0)))
               (loop :for ch :across line
                     :do (vector-push-extend (- (char-code ch) 48) vec))
               vec)))
      (let ((res (make-array 0 :adjustable t :fill-pointer 0)))
        (Loop :for line := (read-line stream nil nil)
              :while line
              :do (vector-push-extend (line->vector line) res))
        res))))


(defun arref (vector i j)
  (aref (aref vector i) j))

(defun check-neighbours (array i j)
  (and (if (> i 0)
           (< (arref array i j) (arref array (- i 1) j))
           t)
       (if (< i (- (length array) 1))
           (< (arref array i j) (arref array (+ i 1) j))
           t)
       (if (> j 0)
           (< (arref array i j) (arref array i (- j 1)))
           t)
       (if (< j (- (length (aref array 0)) 1))
           (< (arref array i j) (arref array i (+ j 1)))
           t)))

(defun day9.1 (path)
  (let ((array (read-lines path)))
    (loop :for i :from 0 :below (length array)
          :sum (loop :for j :from 0 :below (length (aref array 0))
                     :when (check-neighbours array i j)
                       :sum (+ 1 (arref array i j))))))