main :: IO ()
main = do
  raw <- readFile "day3.txt"
  let ls = map parse $ lines raw
      ansA = solveSlope 3 1 ls
      ansB = foldr (*) 1 $
        [ (solveSlope 1 1 ls)
        , (solveSlope 3 1 ls)
        , (solveSlope 5 1 ls)
        , (solveSlope 7 1 ls)
        , (solveSlope 1 2 ls)
        ]
   in do 
     putStrLn $ "day3a: " ++ (show ansA)
     putStrLn $ "day3b: " ++ (show ansB)

parse :: String -> [Bool]
parse = map (== '#')

solveLine :: [Bool] -> Int -> Bool
solveLine [] _ = error "empty list"
solveLine bs i = bs !! index
  where
    index = i `mod` (length bs)

solveSlope :: Int -> Int -> [[Bool]] -> Int
solveSlope r d bs =
  length $ filter (== True) $ solveSlope' r d bs 0 []

solveSlope' :: Int -> Int -> [[Bool]] -> Int -> [Bool] -> [Bool]
solveSlope' _ _ [] _ as = as
solveSlope' r d bs@(b:cs) i as =
  if ns == []
  then as
  else solveSlope' r d ns ni $ (solveLine (head ns) ni) : as
    where
      ni = (i + r) `mod` (length b)
      ns = if   (length bs) <= d
           then []
           else (drop d bs)