2020-05 in haskell

2020/05-Binary_Boarding/first.hs

@@ -0,0 +1,55 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Monad (void, when)
+import Data.List (foldl')
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 820
+
+data BoardingPass = BoardingPass { row :: Int
+                                 , column :: Int
+                                 } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseColumn :: Parser Int
+parseColumn = do
+  colStr <- some (char 'L' <|> char 'R')
+  return $ foldl' (\acc c -> acc * 2 + if c == 'R' then 1 else 0) 0 colStr
+
+parseRow :: Parser Int
+parseRow = do
+  rowStr <- some (char 'B' <|> char 'F')
+  return $ foldl' (\acc c -> acc * 2 + if c == 'B' then 1 else 0) 0 rowStr
+
+parseBP :: Parser BoardingPass
+parseBP = do
+  row <- parseRow
+  column <- parseColumn
+  void (char '\n')
+  return $ BoardingPass { row=row, column=column }
+
+parseBPs :: Parser [BoardingPass]
+parseBPs = some (parseBP) <* eof
+
+parseInput :: String -> IO [BoardingPass]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseBPs filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right bps -> return bps
+
+compute :: [BoardingPass]-> Int
+compute = foldl' (\max BoardingPass{row=r, column=c} -> let n = r * 8 + c in if max >= n then max else n) 0
+
+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