Day 6, Part 1

Day6/puzzle1.hs

@@ -0,0 +1,74 @@
+import Data.List (elemIndex)
+import Data.Maybe (isJust, fromMaybe)
+
+type Grid = [String]
+type Position = (Int, Int)
+data Direction = U | R | D | L deriving Eq
+
+getDirection :: Char -> Direction
+getDirection '^' = U
+getDirection '>' = R
+getDirection 'v' = D
+getDirection '<' = L
+
+getStartPositionition :: Char -> Grid -> Position
+getStartPositionition c grid = (x, y)
+                            where x = fromMaybe (-1) . elemIndex True . map (isJust . elemIndex c) $ grid
+                                  y = if x == -1
+                                           then -1
+                                           else fromMaybe (-1) . elemIndex c $ grid !! x
+
+getGridVal :: Position -> Grid -> Char
+getGridVal (x, y) grid = (grid !! x) !! y
+
+moveUp :: Position -> Position
+moveUp (x, y) = (x - 1, y)
+
+moveDown :: Position -> Position
+moveDown (x, y) = (x + 1, y)
+
+moveLeft :: Position -> Position
+moveLeft (x, y) = (x, y - 1)
+
+moveRight :: Position -> Position
+moveRight (x, y) = (x, y + 1)
+
+isInside :: Position -> Grid -> Bool
+isInside (x, y) grid = x >= 0 && y >= 0 && x < length grid && y < length (head grid)
+
+getNextPosition :: Position -> Direction -> Grid -> (Position, Direction)
+getNextPosition (x, y) U grid = let newPos = moveUp (x, y)
+                                in  if newPos `isInside` grid && getGridVal newPos grid == '#'
+                                       then (moveRight (x, y), R)
+                                    else (newPos, U)
+getNextPosition (x, y) R grid = let newPos = moveRight (x, y)
+                                in  if newPos `isInside` grid && getGridVal newPos grid == '#'
+                                       then (moveDown (x, y), D)
+                                    else (newPos, R)
+getNextPosition (x, y) D grid = let newPos = moveDown (x, y)
+                                in  if newPos `isInside` grid && getGridVal newPos grid == '#'
+                                       then (moveLeft (x, y), L)
+                                    else (newPos, D)
+getNextPosition (x, y) L grid = let newPos = moveLeft (x, y)
+                                in  if newPos `isInside` grid && getGridVal newPos grid == '#'
+                                       then (moveUp (x, y), U)
+                                    else (newPos, L)
+
+markVisited :: Position -> Grid -> Grid
+markVisited (x, y) grid = let row = grid !! x
+                              newRow = take y row ++ ['X'] ++ drop (y + 1) row
+                          in  take x grid ++ [newRow] ++ drop (x + 1) grid
+
+visitGrid :: Position -> Direction -> Grid -> Grid
+visitGrid (x, y) direction grid = let newGrid = markVisited (x, y) grid
+                                      (newPosition, newDirection) = getNextPosition (x, y) direction grid
+                                  in  if newPosition `isInside` newGrid
+                                         then visitGrid newPosition newDirection newGrid
+                                         else newGrid
+
+main = do
+    contents <- lines <$> readFile "day6.txt"
+    let (x, y) = (\w x y z -> head $ filter ((>= 0) . fst) [w, x, y, z]) <$> getStartPositionition 'v' <*> getStartPositionition '^'
+                                                                             <*> getStartPositionition '<' <*> getStartPositionition '>' $ contents
+        direction = getDirection $ (contents !! x) !! y
+    print . length . filter (== 'X') . concat $ visitGrid (x, y) direction contents