aoc-2020-day3.hs

p1.hs

module Main where

import qualified Data.Map.Strict as M
import Data.List (unfoldr)
import Control.Monad (guard)

input = [
          "....#...............#.#..###.##",
          -- etc...
        ]

grid = M.fromList $ do
  (y, row) <- zip [0..] input
  (x, cell) <- zip [0..] row
  pure ((x, y), cell)

height = length input
width = length $ head input

spots = unfoldr (stepAdjust (3, 1)) (0, 0)
  where
    stepAdjust (dx, dy) (x, y) = do
      guard $ y < height
      cell <- M.lookup (x, y) grid
      pure (cell, ((x+dx) `mod` width, y+dy))

main = print $ length $ filter (== '#') spots

p2.hs

module Main where

import qualified Data.Map.Strict as M
import Data.List (unfoldr)
import Control.Monad (guard)

input = [
          "....#...............#.#..###.##",
          -- etc...
        ]

grid = M.fromList $ do
  (y, row) <- zip [0..] input
  (x, cell) <- zip [0..] row
  pure ((x, y), cell)

height = length input
width = length $ head input

spotsDelta delta = unfoldr (stepAdjust delta) (0, 0)
  where
    stepAdjust (dx, dy) (x, y) = do
      guard $ y < height
      cell <- M.lookup (x, y) grid
      pure (cell, ((x+dx) `mod` width, y+dy))

treesDelta = length . filter (== '#') . spotsDelta

-- I know, I know... `product` is not great, but it's over 5 elements!
main = print $ product $ map treesDelta [(1, 1), (3, 1), (5, 1), (7, 1), (1, 2)]