78 lines
2.0 KiB
Haskell
78 lines
2.0 KiB
Haskell
module Main where
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import qualified Data.Set as S
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type Grid = [[Char]]
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type Dimensions = (Int, Int)
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type Coord = (Int, Int)
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data Dir = North | East | South | West deriving (Ord, Eq, Show)
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type State = (Coord, Dir)
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next :: Dir -> Coord -> Coord
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next North (x, y) = (x, y - 1)
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next East (x, y) = (x + 1, y)
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next South (x, y) = (x, y + 1)
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next West (x, y) = (x - 1, y)
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rotate :: Dir -> Dir
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rotate North = East
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rotate East = South
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rotate South = West
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rotate West = North
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getPath :: S.Set Coord -> Dimensions -> Coord -> S.Set Coord
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getPath obs (m, n) start = go S.empty start North
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where
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oob :: Coord -> Bool
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oob (x, y) = x < 0 || y < 0 || x >= m || y >= n
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go :: S.Set Coord -> Coord -> Dir -> S.Set Coord
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go path c d
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| oob n = S.insert c path
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| n `S.member` obs = go path c (rotate d)
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| otherwise = go (S.insert c path) n d
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where
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n = next d c
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isLoop :: S.Set Coord -> Dimensions -> Coord -> Bool
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isLoop obs (m, n) start = go S.empty start North
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where
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oob :: Coord -> Bool
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oob (x, y) = x < 0 || y < 0 || x >= m || y >= n
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go :: S.Set State -> Coord -> Dir -> Bool
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go states c d
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| oob n = False
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| (c, d) `S.member` states = True
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| n `S.member` obs = go states c (rotate d)
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| otherwise = go (S.insert (c, d) states) n d
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where
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n = next d c
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main :: IO ()
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main =
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do
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grid <- lines <$> readFile "./inputs/day6.in"
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let m = length grid
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n = length $ head grid
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let grid' = [((x, y), ch) | (y, row) <- zip [0 ..] grid, (x, ch) <- zip [0 ..] row]
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obstacles = S.fromList [coord | (coord, ch) <- grid', ch == '#']
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start = head [coord | (coord, ch) <- grid', ch == '^']
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path = getPath obstacles (m, n) start
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let part1 = S.size path
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part2 =
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S.size $
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S.filter
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(\p -> isLoop (S.insert p obstacles) (m, n) start)
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(S.delete start path)
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putStr "Part 1: " >> print part1
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putStr "Part 2: " >> print part2
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