Day 19

[advent-of-code-16.git] / advent19.hs

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+{-# LANGUAGE BangPatterns #-}
+
+import Prelude hiding (length, take, drop)
+import Data.Sequence
+
+-- input = 5 
+input = 3012210 
+
+main :: IO ()
+main = do 
+    part1 
+    part2
+
+part1 :: IO ()
+part1 = print $ 2 * (input - 2 ^ (toInteger (floor $ logBase 2 (fromIntegral input)))) + 1
+
+part2 :: IO ()
+part2 = print $ index (presentSteps initial) 0
+
+presentSteps :: Seq Int -> Seq Int
+presentSteps elves 
+    | isFinished elves = elves
+    | otherwise = presentSteps $ next elves
+
+initial :: Seq Int
+initial = fromList [1..input] 
+
+isFinished :: Seq Int -> Bool
+isFinished elves = length elves == 1
+
+next :: Seq Int -> Seq Int
+next elves = prefix >< (midfix |> suffix)
+    where 
+        target = length elves `quot` 2
+        prefix = drop 1 $ take target elves
+        midfix = drop (target+1) elves
+        suffix = index elves 0