Day 14, part 1

14.hs

@@ -0,0 +1,31 @@
+import Data.List.Split (chunksOf)
+import Data.Char (ord)
+import Data.Bits (xor)
+import Text.Printf (printf)
+
+type Length = Int
+type State = ([Int], Int, Int)
+(%) = mod
+
+twist :: State -> Length -> State
+twist (ring, position, skip) len =
+    let rotated = slice position 256 $ cycle ring
+        twisted = (reverse $ take len rotated) ++ drop len rotated
+        newRing = slice (256 - position) 256 $ cycle twisted
+    in  (newRing, (position + len + skip) % 256, skip + 1)
+    where slice start amount = take amount . drop start
+    
+hash :: [Length] -> State -> State
+hash lengths state = foldl twist state lengths
+
+sparseHash :: [Int] -> String
+sparseHash lengths =
+    let (hashed, _, _) = iterate (hash lengths) ([0..255], 0, 0) !! 64
+    in  concat . map (printf "%08b" . foldr xor 0) . chunksOf 16 $ hashed
+
+main :: IO ()
+main = do
+    let hashes = map (sparseHash . (++ [17, 31, 73, 47, 23]) . map ord . ("ffayrhll-" ++) . show) [0..127]
+        used   = length . filter (== '1') . concat $ hashes
+    print $ used
+    mapM_ print hashes