1 files changed, 21 insertions, 0 deletions

diff --git a/execs/Day15.hs b/execs/Day15.hs
new file mode 100644
index 0000000..cd498fb
--- /dev/null
+++ b/execs/Day15.hs
@@ -0,0 +1,21 @@
+module Main where
+
+import Data.Maybe
+import Data.List.Split
+import qualified Data.Map as M
+
+run :: [Int] -> Int -> [Int] -> Int
+run ls start input = fst $ foldl fn (start, startMap) ls
+    where startMap = M.fromList $ zip input [1..]
+          fn (last, seen) i = (i - last', seen')
+              where last' = fromMaybe i (M.lookup last seen)
+                    seen' = M.insert last i seen
+
+main :: IO ()
+main = do
+    n <- map read . splitOn "," <$> readFile "input/15"
+
+    -- holy off-by-one errors
+    print $ run [8..2020 - 1] 0 n
+    print $ run [8..30000000 - 1] 0 n
+