Day 3

aoc20.lisp

@@ -73,3 +73,45 @@
   (let ((data (read-2 file)))
     (time (progn (print (%solve-2 data #'%solve-2-1))
 		 (print (%solve-2 data #'%solve-2-2))))))
+
+(defun read-3 (file)
+  (with-open-file (stream file)
+    (let ((temp-map (loop for line = (read-line stream nil)
+			  while line
+			  collect line)))
+      (make-array (list (length temp-map) (length (first temp-map)))
+		  :initial-contents temp-map
+		  :element-type 'character))))
+
+(defun step-slope (position direction map)
+  (let* ((temp-position (map 'list #'+ direction position))
+	 (overflow (map 'list #'> temp-position (array-dimensions map))))
+    (cond ((first overflow) :bottom)
+	  ((second overflow)
+	   (progn (decf (second temp-position) (second (array-dimensions map)))
+		  temp-position))
+	  (t temp-position))))
+
+(defun solve-3-1 (direction map)
+  (let ((tree-count 0))
+    (labels ((rec (position)
+	       (let ((zero-pos (map 'list #'1- position))
+		     (new-pos (step-slope position direction map)))
+		 (if (char= (aref map (first zero-pos) (second zero-pos))
+			    #\#)
+		     (incf tree-count))
+		 (unless (eq new-pos :bottom)
+		   (rec new-pos)))))
+      (rec '(1 1))
+      tree-count)))
+
+;; (height width)
+
+(defun solve-3-2 (slopes map)
+  (reduce #'* (loop for slope in slopes
+		    collecting (solve-3-1 slope map))))
+
+(defun solve-3 (map)
+  (time (let ((data (read-3 map)))
+	  (print (solve-3-1 '(1 3) data))
+	  (print (solve-3-2 '((1 1) (1 3) (1 5) (1 7) (2 1)) data)))))