2024-22 part 2 in haskell

2024/22-Monkey_Market/example2

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2024/22-Monkey_Market/second.hs

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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Bits
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import           Data.Ord             (comparing)
+import qualified Data.Set             as S
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 23
+
+type Input = [Int]
+
+type Parser = Parsec Void String
+
+parseInput' :: Parser Input
+parseInput' = some (read <$> some digitChar <* eol) <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle  -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+type Seq = (Int, Int, Int, Int)
+type PriceMap = M.Map Seq Int
+
+compute :: Input -> Int
+compute input = trace (show bestSequence) maxBananas
+  where
+    (bestSequence, maxBananas) = S.foldl (\(s, n) s' -> let n' = fromJust $ M.lookup s' bananas
+                                                        in if n >= n' then (s, n)
+                                                                      else (s', n')
+                                         ) ((0, 0, 0, 0), 0) allSeqs
+    bananas :: PriceMap
+    bananas = S.foldl (\acc s -> M.insert s (sum $ catMaybes $ map (M.lookup s) allPriceMaps) acc) M.empty allSeqs
+    allSeqs :: S.Set Seq
+    allSeqs = L.foldl' (\acc p -> S.union acc $ S.fromList $ M.keys p) S.empty allPriceMaps
+    allPriceMaps :: [PriceMap]
+    allPriceMaps = map (seqPrices M.empty) allPrices
+    seqPrices :: PriceMap -> [Int] -> PriceMap
+    seqPrices m s@(a:b:c:d:e:_) = seqPrices (let seq = (b-a, c-b, d-c, e-d) in case M.lookup seq m of
+                                                Just _  -> m
+                                                Nothing -> M.insert seq e m
+                                            ) (tail s)
+    seqPrices m _               = m
+    allPrices = map prices input
+    prices :: Int -> [Int]
+    prices s = fst $ L.foldl' next ([s `mod` 10], s) [1..2000]
+    next :: ([Int], Int) -> Int -> ([Int], Int)
+    next (acc, s) _ = let s' = prune $ mix (shift s 6) s
+                          s'' = prune $ mix (shift s' (-5)) s'
+                          s''' = prune $ mix (shift s'' 11) s''
+                      in (acc ++ [s''' `mod` 10], s''')
+    mix :: Int -> Int -> Int
+    mix = xor
+    prune :: Int -> Int
+    prune n = n `mod` 16777216
+
+main :: IO ()
+main = do
+  example <- parseInput "example2"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input