1 files changed, 50 insertions, 0 deletions
diff --git a/2024/07-Bridge_Repair/first.hs b/2024/07-Bridge_Repair/first.hs
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+++ b/2024/07-Bridge_Repair/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+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 = 3749
+
+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]) $ 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