Day 10

aoc20.lisp

@@ -408,3 +408,45 @@
 	       (p1 (solve-9-1 step-size data)))
 	  (print  p1)
 	  (print (solve-9-2 data p1)))))
+
+;;; Day 10
+
+(defun read-10 (file)
+  (read-9 file))
+
+(defun solve-10-1 (data)
+  (let ((data (sort data #'<))
+	(steps (list 1 0 1)))
+    (reduce (lambda (x y)
+	      (let ((difference (- y x)))
+		(incf (nth (1- difference) steps))
+		y))
+	    data)
+    (* (first steps) (third steps))))
+
+(let ((ht (make-hash-table)))
+  (defun recurse-adapters (data)
+    (if (gethash (car data) ht)
+	(gethash (car data) ht)
+	(setf (gethash (car data) ht)
+	      (%recurse-adapters data)))))
+
+(defun %recurse-adapters (data)
+  (if (null (cdr data))
+      1
+      (let ((ceiling (+ 3 (first data))))
+	(reduce #'+ (map 'list #'recurse-adapters
+			 (remove-if-not
+			  #'consp
+			  (list (and (cdr data) (>= ceiling (cadr data)) (cdr data))
+				(and (cddr data) (>= ceiling (caddr data)) (cddr data))
+				(and (cdddr data) (>= ceiling (cadddr data)) (cdddr data)))))))))
+
+(defun solve-10-2 (data)
+  (let* ((data (cons 0 (coerce (sort data #'<) 'list))))
+    (recurse-adapters data)))
+
+(defun solve-10 (file)
+  (let ((data (read-10 file)))
+    (time (progn (print (solve-10-1 data))
+		 (print (solve-10-2 data))))))