day15

1 files changed, 20 insertions, 0 deletions

diff --git a/day15.clj b/day15.clj
new file mode 100644
index 0000000..569c5fb
--- /dev/null
+++ b/day15.clj
@@ -0,0 +1,20 @@
+(def data [5 2 8 16 18 0 1])
+
+(defn doit [n]
+  (letfn [(rec [lasta lastb last i]
+            (if (<= i n)
+              (let [new-last (if (and (lasta last) (lastb last))
+                               (- (lastb last) (lasta last))
+                               0)]
+                (recur (assoc lasta new-last (lastb new-last))
+                       (assoc lastb new-last i)
+                       new-last
+                       (inc i)))
+              last))]
+    (rec {}
+         (into {} (map-indexed #(vector %2 %1) data))
+         (last data)
+         (count data))))
+
+(doit 2019)  ; => 517
+(doit 30000000)  ; => 3303657 slow