1 files changed, 89 insertions, 0 deletions
diff --git a/2020/12-Rain_Risk/first.hs b/2020/12-Rain_Risk/first.hs
new file mode 100644
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+++ b/2020/12-Rain_Risk/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 (foldl')
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 25
+
+data Action = N | S | E | W | L | R | F
+data Heading = HN | HS | HE | HW
+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, HE) ops
+  where
+    step :: (Int, Int, Heading) -> Op -> (Int, Int, Heading)
+    step (x, y, h) (Op N i) = (x, y+i, h)
+    step (x, y, h) (Op S i) = (x, y-i, h)
+    step (x, y, h) (Op E i) = (x+i, y, h)
+    step (x, y, h) (Op W i) = (x-i, y, h)
+    step (x, y, HN) (Op F i) = (x, y+i, HN)
+    step (x, y, HS) (Op F i) = (x, y-i, HS)
+    step (x, y, HE) (Op F i) = (x+i, y, HE)
+    step (x, y, HW) (Op F i) = (x-i, y, HW)
+    step (x, y, h) (Op a i) = (x, y, turn h a i)
+    turn :: Heading -> Action -> Int -> Heading
+    turn h a i = turn' h a mi
+      where mi = (i `mod` 360) `div` 90
+    turn' HN L 1 = HW
+    turn' HN L 2 = HS
+    turn' HN L 3 = HE
+    turn' HS L 1 = HE
+    turn' HS L 2 = HN
+    turn' HS L 3 = HW
+    turn' HE L 1 = HN
+    turn' HE L 2 = HW
+    turn' HE L 3 = HS
+    turn' HW L 1 = HS
+    turn' HW L 2 = HE
+    turn' HW L 3 = HN
+    turn' HN R 1 = HE
+    turn' HN R 2 = HS
+    turn' HN R 3 = HW
+    turn' HS R 1 = HW
+    turn' HS R 2 = HN
+    turn' HS R 3 = HE
+    turn' HE R 1 = HS
+    turn' HE R 2 = HW
+    turn' HE R 3 = HN
+    turn' HW R 1 = HN
+    turn' HW R 2 = HE
+    turn' HW R 3 = HS
+    turn' h _ _ = h
+
+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