2020-12 in haskell

2020/12-Rain_Risk/second.hs

@@ -0,0 +1,66 @@
+-- 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 = 286
+
+data Action = N | S | E | W | L | R | F
+data Op = Op Action Int
+
+type Parser = Parsec Void String
+
+parseOp :: Parser Op
+parseOp = do
+  action <- (char 'N' *> return N)
+        <|> (char 'S' *> return S)
+        <|> (char 'E' *> return E)
+        <|> (char 'W' *> return W)
+        <|> (char 'L' *> return L)
+        <|> (char 'R' *> return R)
+        <|> (char 'F' *> return F)
+  num <- some digitChar
+  void $ char '\n'
+  return $ Op action (read num)
+
+parseOps :: Parser [Op]
+parseOps = some parseOp <* eof
+
+parseInput :: String -> IO [Op]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseOps filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+compute :: [Op]-> Int
+compute ops = (\((x, y), _) -> abs(x)+abs(y)) $ foldl' step ((0, 0), (10, 1)) ops
+  where
+    step :: ((Int, Int), (Int, Int)) -> Op -> ((Int, Int), (Int, Int))
+    step (p, (wx, wy)) (Op N i) = (p, (wx, wy+i))
+    step (p, (wx, wy)) (Op S i) = (p, (wx, wy-i))
+    step (p, (wx, wy)) (Op E i) = (p, (wx+i, wy))
+    step (p, (wx, wy)) (Op W i) = (p, (wx-i, wy))
+    step ((x, y), (wx, wy)) (Op F i) = ((x+i*wx, y+i*wy), (wx, wy))
+    step (p, w) (Op a i) = (p, turn w a i)
+    turn :: (Int, Int) -> Action -> Int -> (Int, Int)
+    turn w a i = turn' w a mi
+      where mi = (i `mod` 360) `div` 90
+    turn' (wx, wy) L 1 = (-wy, wx)
+    turn' (wx, wy) _ 2 = (-wx, -wy)
+    turn' (wx, wy) L 3 = (wy, -wx)
+    turn' (wx, wy) R 1 = (wy, -wx)
+    turn' (wx, wy) R 3 = (-wy, wx)
+    turn' w _ _ = w
+
+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