Day 10

10.hs

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+import Data.List.Split (splitOn)
+import Data.Sequence (Seq, update, fromList)
+import qualified Data.Sequence as S (lookup)
+import Data.Char (ord)
+import Data.Bits (xor)
+import Data.Foldable (toList)
+import Numeric (showHex)
+
+type Ring = Seq Int
+type Position = Int
+type Skip = Int
+type Length = Int
+type State = (Ring, Position, Skip)
+
+(%) = mod
+
+unsafeLookup :: Int -> Seq Int -> Int
+unsafeLookup index seq =
+    case S.lookup index seq of
+        Just i -> i
+
+twist :: State -> Length -> State
+twist (ring, position, skip) len =
+    let end       = position + len - 1
+        positions = map (% 256) [position..end]
+        posRev    = zip positions $ reverse positions
+        newRing   = foldl (\currRing (pos, rev) -> update pos (unsafeLookup rev ring) currRing) ring posRev
+    in  (newRing, (end + skip + 1) % 256, skip + 1)
+
+hash :: [Length] -> State -> State
+hash lengths state = foldl twist state lengths
+
+condense :: [Int] -> [Int]
+condense [] = []
+condense ns =
+    let block = foldr xor 0 $ take 16 ns
+    in  block : (condense   $ drop 16 ns)
+
+showHexPrepended :: Int -> (String -> String)
+showHexPrepended n
+    | n < 15    = showHex 0 . showHex n
+    | otherwise = showHex n
+
+main :: IO ()
+main = do
+    input <- readFile "10.txt"
+    let lengths    = map read $ splitOn "," input
+        newLengths = map ord input ++ [17, 31, 73, 47, 23]
+        (hashed,     _, _) =          hash lengths     (fromList [0..255], 0, 0)
+        (sparseHash, _, _) = iterate (hash newLengths) (fromList [0..255], 0, 0) !! 64
+        condensedHash = condense $ toList sparseHash
+        hexString     = foldr (\digit str -> showHexPrepended digit $ str) "" condensedHash
+    print $ (unsafeLookup 0 hashed) * (unsafeLookup 1 hashed)
+    print $ hexString

10.txt

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+18,1,0,161,255,137,254,252,14,95,165,33,181,168,2,188