2024-19 in haskell

2024/19-Linen_Layout/second.hs

@@ -0,0 +1,48 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 16
+
+type Input = ([String], [String]) -- patterns, designs
+
+type Parser = Parsec Void String
+
+parseInput' :: Parser Input
+parseInput' = (,) <$> some (some letterChar <* optional (string ", ")) <* eol <* eol
+                  <*> some (some letterChar <* 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 Memo = M.Map String Int
+
+compute :: Input -> Int
+compute (stripes, designs) = snd $ L.foldl' compute' (M.empty, 0) designs
+  where
+    compute' :: (Memo, Int) -> String -> (Memo, Int)
+    compute' (memo, c) [] = (memo, c + 1)
+    compute' (memo, c) design = case M.lookup design memo of
+      Just c' -> (memo, c + c')
+      Nothing -> let (memo', c') = L.foldl' compute' (memo, 0) $ catMaybes $ map (\s -> L.stripPrefix s design) stripes
+                 in (M.insert design c' memo', c + c')
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input