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import Text.Parsec.String (Parser)
import Text.Parsec
import System.Environment
import Control.Monad
import Debug.Trace
data Tile = EmptyT | Digit Int | Gear deriving (Show,Eq)
tile :: Parser Tile
tile = char '.' *> pure EmptyT <|> Digit <$> (read.pure <$> digit) <|> char '*' *> pure Gear <|> noneOf "\n" *> pure EmptyT
board = sepBy (many1 tile) newline
gearPositions l = [(x,y) | (y,r) <- zip [0..] l, (x,Gear) <- zip [0..] r]
digitPositions l = [(n,x,y) | (y,r) <- zip [0..] l, (x,Digit n) <- zip [0..] r]
groupBy :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy p = map (uncurry (:)) . groupByNonEmpty p
groupByNonEmpty :: (a -> a -> Bool) -> [a] -> [(a,[a])]
groupByNonEmpty p =
foldr
(\x0 yt ->
let (xr,yr) =
case yt of
(x1,xs):ys ->
if p x0 x1
then (x1:xs,ys)
else ([],yt)
[] -> ([],yt)
in (x0,xr):yr)
[]
fromDigits :: [Int] -> Int
fromDigits = foldl ((+) . (10 *)) 0
neighbours = foldMap (\(x,y) -> map (\(dx,dy) -> (x+dx,y+dy)) neigh) where
neigh = liftM2 (,) [1,0,-1] [1,0,-1]
findGearParts gears nums = do
(x,y) <- gears
let nb = neighbours [(x,y)]
let adj = filter (any (\(_,x',y') -> (x',y') `elem` nb)) nums
guard $ length adj == 2
let (a:b:[]) = adj
return ((fromDigits.map (\(n,_,_) -> n)) a,(fromDigits.map (\(n,_,_) -> n)) b)
solution :: [[Tile]] -> Int
solution board = sum $ uncurry (zipWith (*)) $ unzip ratios where
gears = gearPositions board
digs = groupBy (\(_,x1,_) (_,x2,_) -> x2 == succ x1) $ digitPositions board
ratios = findGearParts gears digs
main = liftM2 (>>=) readFile (((print . solution . either (error.show) id) .) . parse board) =<< head <$> getArgs
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