adventofcode-2021/2/navigator.hs

import           Control.Exception (assert)

main :: IO ()
main = do
  instructions <- getContents
  putStrLn (solution1 instructions)
  putStrLn (solution2 instructions)

solution1 :: String -> String
solution1 input = show (uncurry (*) (navigate (parseInstructions input)))

solution2 :: String -> String
solution2 input = show (uncurry (*) (navigate2 (parseInstructions input)))

parseInstructions :: String -> [(Int, Int)]
parseInstructions instructions =
  map parseLine (lines instructions)
  where
    parseLine line = case break (== ' ') line of
      ("forward", distance) -> (read distance, 0)
      ("down", distance)    -> (0, read distance)
      ("up", distance)      -> (0, (-read distance))
      _                     -> error "Invalid line"

navigate :: [(Int, Int)] -> (Int, Int)
navigate instructions =
  foldl1 sumTuples instructions
  where
    sumTuples :: (Int, Int) -> (Int, Int) -> (Int, Int)
    sumTuples (a, b) (c, d) = (a + c, b + d)

navigate2 :: [(Int, Int)] -> (Int, Int)
navigate2 instructions =
  takePosition (foldl navigate (0, 0, 0) instructions)
  where
    navigate :: (Int, Int, Int) -> (Int, Int) -> (Int, Int, Int)
    navigate (horz, depth, aim) (speed, dir) = (horz + speed, depth + aim * speed, aim + dir)
    takePosition :: (Int, Int, Int) -> (Int, Int)
    takePosition (x,y,_) = (x,y)

-- Tests

testInput1 :: String
testInput1 = unlines [
  "forward 5",
  "down 5",
  "forward 8",
  "up 3",
  "down 8",
  "forward 2"
  ]

test1 :: String
test1 = assert ((solution1 testInput1) == "150") "success"

test2 :: String
test2 = assert ((solution2 testInput1) == "900") "success"