Day 3 in Haskell

03/Main.hs

@@ -0,0 +1,53 @@
+import Data.Array
+import Text.Printf (printf)
+
+ioAssert True _ = return ()
+ioAssert False msg = fail msg
+
+testInput =
+  [ "..##......."
+  , "#...#...#.."
+  , ".#....#..#."
+  , "..#.#...#.#"
+  , ".#...##..#."
+  , "..#.##....."
+  , ".#.#.#....#"
+  , ".#........#"
+  , "#.##...#..."
+  , "#...##....#"
+  , ".#..#...#.#"
+  ]
+
+data Field = Open | Tree deriving (Eq)
+
+parseField :: Char -> Field
+parseField '.' = Open
+parseField '#' = Tree
+
+parseInput :: [String] -> Array (Int, Int) Field
+parseInput rows =
+  listArray ((0, 0), (maxY - 1 , maxX - 1)) $ concat $ map (map parseField) rows
+  where maxX = (length $ head rows) - 1
+        maxY = (length rows) - 1
+
+solve :: Array (Int, Int) Field -> (Int, Int) -> Int
+solve input = length . filter (==Tree) . path input
+
+part1 = flip solve (3, 1)
+part2 input = product $ map (solve input) [(1, 1), (3, 1), (5, 1), (7, 1), (1, 2)]
+
+path arr (dX, dY) =
+  let ((_, _), (height, width)) = bounds arr
+  in [arr ! (y, x) | (i, y) <- zip [0,dX..] [0,dY..height],
+                     let x = mod i (width + 1)]
+
+main = do
+  input <- parseInput . lines <$> readFile "input"
+  runTests
+  putStr $ printf "Part 1: %d\n" $ part1 input
+  putStr $ printf "Part 2: %d\n" $ part2 input
+
+runTests = do
+  ioAssert (7 == (part1 $ parseInput testInput)) "part1"
+  ioAssert (336 == (part2 $ parseInput testInput)) "part2"
+  putStrLn "Tests ok"