AoC 2021, day 6

day6.lisp

(defun read-cycle-num (path)   ;;reading input
  (flet ((convert (char)
           (- (char-code char) 48)))
    (with-open-file (stream path :direction :input :if-does-not-exist :error)
      (let ((res (make-array 0 :adjustable t :fill-pointer 0 :element-type 'fixnum)))
        (loop :for cycle := (read-char stream nil nil)
              :while cycle
              :do (vector-push-extend (convert cycle) res)
                  (read-char stream nil nil))
        res))))


(defun growth (n a0 a1 a2 a3 a4 a5 a6 a7 a8)  ;;progression, it's x_n = x_(n-7) + x_(n-9)
  (if (< n 9)
      (elt (vector a0 a1 a2 a3 a4 a5 a6 a7 a8) n)
      (loop :repeat (- n 8) :do (shiftf a0 a1 a2 a3 a4 a5 a6 a7 a8 (+ a0 a2))
            :finally (return a8))))


(defun calc-first-9-days (array)   ;;need first 9 elements
  (loop :repeat 9
        :collect (length array)
        :do (loop :for i :from 0 :below (length array)
                  :if (= 0 (aref array i))
                    :do (vector-push-extend 8 array)
                        (setf (aref array i) 6)
                  :else :do (decf (aref array i)))))


(defun day6.1 (path)
  (let ((initial (read-cycle-num path)))
    (apply #'growth 80 (calc-first-9-days initial))))


(defun day6.2 (path)
  (let ((initial (read-cycle-num path)))
    (apply #'growth 256 (calc-first-9-days initial))))