# Pastebin pBTpHp1C
module Day3 where

import           Data.Coerce     (coerce)
import           Data.Foldable   (fold)
import           Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map
import           Data.Monoid     (Product (..), Sum (..))

import           Advent.AoC
import           Advent.TwoD

type World = Map Point (Sum Int)

getInput :: FilePath -> IO World
getInput fn = parseGrid (\case '#' -> 1; _ -> 0) <$> readFile fn

-- Count the trees along a given slope.
trees :: World -> (Int, Int) -> Sum Int
trees m (xoff, yoff) = fold [ at i | i <- [1 .. (maxy `div` yoff)] ]
  where
    ((maxx, maxy),_) = Map.findMax m
    at i = m Map.! (i*xoff `mod` (maxx+1), i*yoff)

part1 :: World -> Int
part1 w = getSum $ trees w (3,1)

part2 :: World -> Int
part2 w = getProduct . foldMap (coerce . trees w) $ [(1,1), (3,1), (5, 1), (7, 1), (1, 2)]