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module Day3 where
import System.Environment (getArgs)
data Square = Empty | Tree
deriving (Eq, Ord, Show)
type Map = [[Square]]
-- Converts lines to a map
toMap :: [String] -> Map
toMap = map toRow
where toRow = map toSquare
toSquare '.' = Empty
toSquare '#' = Tree
toSquare _ = error "Invalid map character"
-- Get the size of a map
-- Note this returns (height, width)
getSize :: Map -> (Int, Int)
getSize m = (length m, length (head m))
-- Get the square at the given (y, x) coord
-- This wraps the x coord, but not the y
get :: Map -> Int -> Int -> Square
get m y x = m!!y!!(x `mod` w)
where (_, w) = getSize m
-- Get the squares encountered along a slop, starting at (y, x)
-- and descending along (f, r)
getSquaresAlongSlope :: Map -> Int -> Int -> Int -> Int -> [Square]
getSquaresAlongSlope m y x f r | y >= h = []
| otherwise = get m y x : getSquaresAlongSlope m (y + f) (x + r) f r
where (h, _) = getSize m
-- Count the number of trees along a given slope, starting at (0, 0)
checkSlope :: Map -> Int -> Int -> Int
checkSlope m f r = length $ filter (== Tree) $ getSquaresAlongSlope m 0 0 f r
-- Usage: ./3a inputs/day3
main :: IO ()
main = do
args <- getArgs;
content <- readFile $ head args;
let l = lines content;
let m = toMap l;
print $ checkSlope m 1 3
return ()
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