day09

3 files changed, 244 insertions, 0 deletions

diff --git a/data/09/example.txt b/data/09/example.txt
new file mode 100644
index 0000000..539a763
--- /dev/null
+++ b/data/09/example.txt
@@ -0,0 +1,3 @@
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diff --git a/data/09/input.txt b/data/09/input.txt
new file mode 100644
index 0000000..8351871
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+++ b/data/09/input.txt
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+28 50 81 121 170 228 295 371 456 550 653 765 886 1016 1155 1303 1460 1626 1801 1985 2178
+5 26 66 131 233 400 698 1273 2421 4694 9050 17055 31145 54956 93730 154805 248197 387282 589586 877691 1280265
+3 9 30 78 165 303 504 780 1143 1605 2178 2874 3705 4683 5820 7128 8619 10305 12198 14310 16653
+18 34 65 123 224 388 654 1139 2180 4618 10326 23168 50732 107452 220199 438224 850758 1618146 3027071 5588849 10214624
+6 23 51 87 131 187 264 395 711 1647 4440 12244 32508 81875 195993 448650 988108 2104215 4348929 8746789 17152577
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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