day09

1 files changed, 41 insertions, 0 deletions

diff --git a/day09.lisp b/day09.lisp
new file mode 100644
index 0000000..4637471
--- /dev/null
+++ b/day09.lisp
@@ -0,0 +1,41 @@
+(defstruct problem reports)
+
+(defun parse-integers (string &key (start 0))
+  (loop for i = start then (cadr o)
+        as o = (multiple-value-list (parse-integer string :start i :junk-allowed t))
+        as n = (car o)
+        while n
+        collect n))
+
+(defun parse-reports (stream)
+  (loop for line = (read-line stream nil)
+        while line
+        collect (parse-integers line)))
+
+(defun parse-problem (filespec)
+  (with-open-file (stream filespec)
+    (make-problem :reports (parse-reports stream))))
+
+(defun report-next-level (report)
+  (loop for i in (cdr report)
+        for j in report
+        collect (- i j)))
+
+(defun report-next-value (report)
+  (if (every (lambda (v) (= v 0)) report)
+      0
+      (+ (car (last report)) (report-next-value (report-next-level report)))))
+
+(defun solve1 (filespec)
+  (let ((problem (parse-problem filespec)))
+    (reduce #'+ (mapcar #'report-next-value (problem-reports problem)))))
+
+(defun solve2 (filespec)
+  (let ((problem (parse-problem filespec)))
+    (reduce #'+ (mapcar #'report-next-value
+                        (mapcar #'reverse (problem-reports problem))))))
+
+(print (solve1 "data/09/example.txt"))  ;; 114
+(print (solve1 "data/09/input.txt"))    ;; 1916822650
+(print (solve2 "data/09/example.txt"))  ;; 2
+(print (solve2 "data/09/input.txt"))    ;; 966