1 files changed, 53 insertions, 0 deletions
diff --git a/2020/13-Shuttle_Search/first.hs b/2020/13-Shuttle_Search/first.hs
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+++ b/2020/13-Shuttle_Search/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List (elemIndex)
+import Data.Maybe (catMaybes, fromJust)
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 295
+
+type Input = (Int, [Maybe Int])
+
+type Parser = Parsec Void String
+
+parseShuttle :: Parser (Maybe Int)
+parseShuttle = do
+  num <- (Just . read <$> some digitChar) <|> (char 'x' *> return Nothing)
+  void . optional $ char ','
+  return $ num
+
+parseOps :: Parser Input
+parseOps = do
+  time <- some digitChar
+  void $ char '\n'
+  shuttles <- some parseShuttle
+  void $ char '\n'
+  void $ eof
+  return $ ((read time), shuttles)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseOps filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+compute :: Input -> Int
+compute (time, shuttles) = departure * busId
+  where
+    busId = allShuttles !! (fromJust $ elemIndex departure nextDepartures)
+    departure = minimum nextDepartures
+    nextDepartures = map (\a -> (-time) `mod` a) allShuttles
+    allShuttles = catMaybes shuttles
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input