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module Day19 where
import Data.Maybe (maybe)
import Data.List (find, findIndex, unfoldr)
import Data.Char (isLetter)
day19text :: IO [[Char]] -- We could use a better data structure, but the code only gets run once
day19text = lines <$> readFile "C:\\Users\\me\\advent\\day19.txt"
isWire :: Char -> Bool
isWire = (/= ' ')
data Point = Point { x :: Int, y :: Int }
safeIndex :: Int -> [a] -> Maybe a
safeIndex _ [] = Nothing
safeIndex y _ | y < 0 = Nothing
safeIndex 0 (x:_) = Just x
safeIndex i (_:xs) = safeIndex (i-1) xs
val :: [[Char]] -> Point -> Maybe Char
val maze Point { x = x, y = y } = safeIndex x =<< safeIndex y maze
data Direction = No | So | Ea | We
move :: Direction -> Point -> Point
move d Point { x = x, y = y } = Point {x = fx d, y = fy d}
where fx Ea = x + 1
fx We = x - 1
fx _ = x
fy No = y - 1
fy So = y + 1
fy _ = y
nextDirection :: [[Char]] -> Direction -> Point -> Maybe Direction
nextDirection maze d p = find v (d:others d)
where others No = [Ea, We]
others So = [Ea, We]
others _ = [No, So]
v dir = maybe False isWire (val maze $ move dir p)
data State19 = State19 { position :: Point, direction :: Direction}
createState :: Point -> Direction -> State19
createState p d = State19 { position = p, direction = d}
startState :: [Char] -> Maybe State19
startState chars = (toState . toPoint) <$> findIndex isWire chars
where toPoint x = Point { x = x, y = 0 }
toState p = createState p So
nextState :: [[Char]] -> State19 -> Maybe (Char, State19)
nextState maze State19 { position = p, direction = d } = (,) <$> v2 <*> s2
where d2 = nextDirection maze d p
p2 = flip move p <$> d2
s2 = createState <$> p2 <*> d2
v2 = val maze =<< p2
path :: [[Char]] -> Maybe [Char]
path maze = unfoldr (nextState maze) <$> startState (head maze)
(<$$>) = fmap . fmap
day19a = filter isLetter <$$> path <$> day19text
day19b = ((1+) . length) <$$> path <$> day19text
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