2024-07 in haskell

2024/07-Bridge_Repair/second.hs

@@ -0,0 +1,51 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
+module Main (main) where
+
+import           Control.Applicative.Permutations
+import           Control.Monad                    (void, when)
+import           Data.Void                        (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 11387
+
+data Equation = Equation Int [Int] deriving Show
+type Input = [Equation]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* optional hspace
+
+parseEquation :: Parser Equation
+parseEquation = Equation <$> parseNumber <* string ": "
+                         <*> some parseNumber
+
+parseInput' :: Parser Input
+parseInput' = some (parseEquation <* 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'
+
+compute :: Input -> Int
+compute = sum . map total . filter valid
+  where
+    total (Equation t _) = t
+    valid (Equation total numbers) = elem total $ combinations numbers
+    combinations [x] = [x]
+    combinations l = let x = last l
+                         xs = init l
+                     in concatMap (\n -> [x+n, x*n, read (show n ++ show x)]) $ combinations xs
+
+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