aoc2021-day1

aoc2021-day1.lisp

;;;; +----------------------------------------------------------------+
;;;; | Advent of Code 2021                                            |
;;;; +----------------------------------------------------------------+

(defpackage #:snippets/aoc2021/day1
  (:use #:cl)
  (:export
   #:day1))

(in-package #:snippets/aoc2021/day1)

(defun count-increasing-sums (input k)
  (do ((c 0)
       (n (- (length input) k))
       (s1 (reduce #'+ input :end k))
       (i 0 (1+ i)))
      ((= i n) c)
    (let ((s2 (+ (- s1 (aref input i)) (aref input (+ i k)))))
      (when (> s2 s1)
        (incf c))
      (setf s1 s2))))

(defun day1 (input)
  (let ((vector (coerce input 'vector)))
    (list (count-increasing-sums vector 1)
          (count-increasing-sums vector 3))))