;; https://adventofcode.com/2021/day/1

;; *day1-input* is a text file of integer values delimited by a newline.

(defun d1p1 ()
  "Find the number of times the value of a line increases from the previous line."
  (with-open-file (stream *day1-input* :if-does-not-exist :error)
    (loop for line = (read-line stream NIL NIL)
          while line
          for prev = NIL then value
          for value = (parse-integer line)
          counting line into line-count
          counting (and prev (< prev value)) into count
          finally (return (values count line-count)))))

;; Answer: 1162.

(defun d1p2 ()
  "Find the number of times the sum of three lines increases from the sum of the previous sliding window of three lines."
  (with-open-file (stream *day1-input* :if-does-not-exist :error)
    (let ((values (make-array 3 :element-type 'integer :initial-contents '(0 0 0)))
          (index 0))
      (labels ((sum () (loop for value across values summing value))
               (next (line)
                 (setf (aref values (mod index 3)) (parse-integer line))
                 (incf index)
                 (sum)))
        ;; First three.
        (next (read-line stream))
        (next (read-line stream))
        (next (read-line stream))
        (loop for line = (read-line stream NIL NIL)
              while line
              for prev = (sum) then current
              for current = (next line)
              counting (< prev current)))))))

;; Answer: 1190