2 files changed, 31 insertions, 0 deletions
diff --git a/2020/15.hs b/2020/15.hs
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+++ b/2020/15.hs
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+module Main where
+
+import Data.Foldable (toList)
+import qualified Data.IntMap.Strict as IM
+import Data.IntMap.Strict (IntMap)
+import Data.List (foldl')
+
+import Input
+import Util
+
+
+type State = IntMap Int
+
+next :: Int -> Int -> State -> Int
+next idx num mp = maybe 0 (idx -) (IM.lookup num mp)
+
+vaneck :: [Int] -> [Int]
+vaneck values = init values ++ expand (length values) (last values) (seed (init values))
+  where
+    expand idx num state =
+        let num' = next idx num state
+        in num : expand (idx + 1) num' (IM.insert num idx state)
+    seed = foldl' go mempty . zip [1..]
+      where go mp (idx, num) = IM.insert num idx mp
+
+main :: IO ()
+main = do
+    input <- map read . toList . splitOn (== ',') . head <$> getInput 15 :: IO [Int]
+    print (vaneck input !! 2019)
+    print (vaneck input !! 29999999)
diff --git a/2020/15.in b/2020/15.in
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--- /dev/null
+++ b/2020/15.in
@@ -0,0 +1 @@
+1,20,11,6,12,0