[hs] Solve 2020_03

hs/src/Aoc/Y2020/D03.hs

@@ -0,0 +1,43 @@
+module Aoc.Y2020.D03
+  ( day
+  ) where
+
+import           Data.List
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser [[Bool]]
+parser = manyLines $ many $ do
+  c <- lineChar
+  pure $ c == '#'
+
+slope :: Int -> Int -> [Maybe Int]
+slope dx dy = intercalate (replicate (dy - 1) Nothing) [[Just x] | x <- [0,dx..]]
+
+onSlope :: [[Bool]] -> [Maybe Int] -> Int
+onSlope trees s = length $ filter id $ [row !! x | (row, Just x) <- zip trees s]
+
+solver :: [[Bool]] -> IO ()
+solver trees = do
+  let infTrees = map cycle trees
+
+  putStrLn ">> Part 1"
+  let treesHit = length $ filter id $ zipWith (!!) infTrees [0,3..]
+  putStrLn $ "Trees hit for slope 3-1: " ++ show treesHit
+
+  putStrLn ">> Part 2"
+  let oneOne = onSlope infTrees $ slope 1 1
+      threeOne = onSlope infTrees $ slope 3 1
+      fiveOne = onSlope infTrees $ slope 5 1
+      sevenOne = onSlope infTrees $ slope 7 1
+      oneTwo = onSlope infTrees $ slope 1 2
+  putStrLn $ "right 1, down 1: " ++ show oneOne
+  putStrLn $ "right 3, down 1: " ++ show threeOne
+  putStrLn $ "right 5, down 1: " ++ show fiveOne
+  putStrLn $ "right 7, down 1: " ++ show sevenOne
+  putStrLn $ "right 1, down 2: " ++ show oneTwo
+  putStrLn $ "Product: " ++ show (oneOne * threeOne * fiveOne * sevenOne * oneTwo)
+
+day :: Day
+day = dayParse "2020_03" parser solver