(defparameter *depths-input* "./input.txt")

(defun read-depths ()
  "Read input file, returning a list of its depth readings."
  (let ((input (open *depths-input* :if-does-not-exist nil))
        (input-depths '()))
    (when input
      (loop for line = (read-line input nil)
            while line do
              (setq input-depths (cons (parse-integer line) input-depths)))
      (close input))

    (reverse input-depths)))

(defun traverse (depths)
  "Calculate number of descents into the deep."
  (defun iter-traverse (current-depth remaining-readings descents)
    (let ((next-depth (car remaining-readings)))
      (cond ((eq remaining-readings 'nil) descents)        
            ((< current-depth next-depth)
             (iter-traverse next-depth
                            (cdr remaining-readings)
                            (1+ descents)))
            ('true
             (iter-traverse next-depth
                            (cdr remaining-readings)
                            descents)))))          
          
  (iter-traverse (car depths) depths 0))

;; get results!!
(traverse (read-depths))     ;; 1215 descents


(defun traverse-wide-window (depths)
  "Calculate number of descents into the deep using a three-measurement window."
  (defun iter-traverse (previous-window remaining-readings descents)
    (if (< (length remaining-readings) 3)
        descents
        (let* ((a (car remaining-readings))
               (b (cadr remaining-readings))
               (c (caddr remaining-readings))
               ;; 199  A      
               ;; 200  A B    
               ;; 208  A B C      Start by comparing the first and second three-measurement windows.
               ;; 210    B C D    The measurements in the first window are marked A (199, 200, 208);
               ;; 200  E   C D    their sum is 199 + 200 + 208 = 607. The second window is marked 
               ;; 207  E F   D    B (200, 208, 210); its sum is 618. The sum of measurements in the
               ;; 240  E F G      second window is larger than the sum of the first, so this first 
               ;; 269    F G H    comparison increased.
               ;; 260      G H
               ;; 263        H
               (window (+ a b c))
               (remaining-depths (cdr remaining-readings)))
          (cond ((= previous-window 0)  ;; the first measurement -- not enough info to calculate depth
                 (iter-traverse window remaining-depths
                                descents))
                ((> window previous-window)
                 (iter-traverse window remaining-depths
                                (1+ descents)))
                ('true
                 (iter-traverse window remaining-depths
                                descents))))))
    
  (iter-traverse 0 depths 0))

;; get results!!
(traverse-wide-window (read-depths)) ;; 1150 descents