2020-03 in haskell

2020/03-Toboggan_Trajectory/second.hs

@@ -0,0 +1,60 @@
+-- requires cabal install --lib megaparsec
+module Main (main) where
+
+import Control.Monad (void, when)
+import Data.List (foldl')
+import Data.Void
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 336
+
+type Line = [Bool]
+type Field = [Line]
+
+type Parser = Parsec Void String
+
+parseElt :: Parser Bool
+parseElt = do
+  c <- (char '#') <|> (char '.')
+  return $ c == '#'
+
+parseLine :: Parser Line
+parseLine = some parseElt <* char '\n'
+
+parseField :: Parser Field
+parseField = some parseLine <* eof
+
+parseInput :: String -> IO Field
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseField filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right field -> return field
+
+computeOne :: Field -> Int-> Int
+computeOne field step = snd $ foldl' tree (0, 0) field
+  where
+    tree :: (Int, Int) -> Line -> (Int, Int)
+    tree (index, trees) l = (next index, trees + if l !! index then 1 else 0)
+    next n = (n + step) `mod` fieldLen
+    fieldLen = length $ field !! 0
+
+steps = [1, 3, 5, 7]
+
+compute :: Field -> Int
+compute field = foldr (*) 1 (computeOne (skipEveryOtherLine field) 1 : map (computeOne field) steps)
+  where
+    skipEveryOtherLine :: Field -> Field
+    skipEveryOtherLine [] = []
+    skipEveryOtherLine (x:[]) = []
+    skipEveryOtherLine (x:y:xs) = x : skipEveryOtherLine xs
+
+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