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1/submarine.lisp

@@ -0,0 +1,68 @@
+(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