Add solution to day 6.

2024/haskell/app/day6/Day6.hs

@@ -0,0 +1,118 @@
+module Main (main) where
+
+import Control.Parallel.Strategies (parList, rdeepseq, using)
+import Data.List (nub)
+import Prelude hiding (Left, Right)
+
+-- | Part 1
+data Tile = Empty | Blockage | Guard deriving (Show, Eq)
+
+newtype Grid = Grid [[Tile]] deriving (Show)
+
+data Direction = Up | Down | Left | Right deriving (Show, Eq)
+
+data WalkResult = Forward (Int, Int) | BlockEncountered Direction | OutOfBounds
+
+tileFromString :: Char -> Tile
+tileFromString '.' = Empty
+tileFromString '#' = Blockage
+tileFromString '^' = Guard
+
+guardRow :: [Char] -> [Tile]
+guardRow = map tileFromString
+
+guardGrid :: String -> Grid
+guardGrid = Grid . map guardRow . lines
+
+findStart :: Grid -> (Int, Int)
+findStart grid = findStart' grid 0 0
+  where
+    findStart' :: Grid -> Int -> Int -> (Int, Int)
+    findStart' (Grid grid') row col
+      | col >= length (grid' !! row) = findStart' (Grid grid') (row + 1) 0
+      | (grid' !! row) !! col == Guard = (row, col)
+      | otherwise = findStart' (Grid grid') row (col + 1)
+
+move :: Direction -> (Int, Int) -> (Int, Int)
+move Up (row, col) = (row - 1, col)
+move Down (row, col) = (row + 1, col)
+move Left (row, col) = (row, col - 1)
+move Right (row, col) = (row, col + 1)
+
+turnRight :: Direction -> Direction
+turnRight Up = Right
+turnRight Right = Down
+turnRight Down = Left
+turnRight Left = Up
+
+outOfBounds :: Grid -> (Int, Int) -> Bool
+outOfBounds (Grid g) (row, col) = row < 0 || row >= length g || col < 0 || col >= length (g !! row)
+
+stepGridWith :: Grid -> Direction -> (Int, Int) -> WalkResult
+stepGridWith (Grid grid) direction pos
+  | outOfBounds (Grid grid) pos = OutOfBounds
+  | outOfBounds (Grid grid) (move direction pos) = Forward (move direction pos)
+  | otherwise =
+      let (r', c') = move direction pos
+          newDirection = turnRight direction
+       in case (grid !! r') !! c' of
+            Blockage -> BlockEncountered newDirection
+            _ -> Forward (r', c')
+
+walkGrid :: Grid -> Direction -> (Int, Int) -> [(Int, Int)]
+walkGrid grid direction pos =
+  case stepGridWith grid direction pos of
+    OutOfBounds -> []
+    Forward newPos -> pos : walkGrid grid direction newPos
+    BlockEncountered newDirection -> walkGrid grid newDirection pos
+
+part1 :: String -> IO ()
+part1 s =
+  let grid = guardGrid s
+      start = findStart grid
+   in print $ length $ nub $ walkGrid grid Up start
+
+-- | Part 2
+isLoop :: Direction -> (Int, Int) -> Grid -> Bool
+isLoop direction pos grid = isLoop' grid direction pos []
+  where
+    isLoop' :: Grid -> Direction -> (Int, Int) -> [((Int, Int), Direction)] -> Bool
+    isLoop' grid' direction' pos' visited
+      | (pos', direction') `elem` visited = True
+      | otherwise =
+          case stepGridWith grid' direction' pos' of
+            OutOfBounds -> False
+            Forward newPos -> isLoop' grid' direction' newPos ((pos', direction') : visited)
+            BlockEncountered newDirection -> isLoop' grid' newDirection pos' visited
+
+blockPos :: Grid -> (Int, Int) -> Grid
+blockPos (Grid g) (row, col) =
+  let (g', gs) = splitAt row g
+   in Grid $ g' ++ [blockPosInCol' (g !! row) col] ++ drop 1 gs
+  where
+    blockPosInCol' :: [Tile] -> Int -> [Tile]
+    blockPosInCol' row' col' =
+      let (r, rs) = splitAt col' row'
+       in r ++ [Blockage] ++ drop 1 rs
+
+countLoopers :: Grid -> (Int, Int) -> Int
+countLoopers (Grid grid) startPos =
+  let guardPath = nub $ walkGrid (Grid grid) Up startPos
+      emptyPath = filter (\(row, col) -> (grid !! row) !! col == Empty) guardPath
+      loops = map countLooper emptyPath `using` parList rdeepseq
+   in sum loops
+  where
+    countLooper :: (Int, Int) -> Int
+    countLooper (row, col)
+      | isLoop Up startPos (blockPos (Grid grid) (row, col)) = 1
+      | otherwise = 0
+
+part2 :: String -> IO ()
+part2 s =
+  let grid = guardGrid s
+      start = findStart grid
+   in print $ countLoopers grid start
+
+-- | Main
+main :: IO ()
+main = readFile "resources/day6.txt" >>= part2

2024/haskell/haskell.cabal

@@ -108,6 +108,21 @@ executable day5
     , haskell
   default-language: Haskell2010
 
+executable day6
+  main-is: Day6.hs
+  other-modules:
+      Paths_haskell
+  autogen-modules:
+      Paths_haskell
+  hs-source-dirs:
+      app/day6
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , haskell
+    , parallel
+  default-language: Haskell2010
+
 test-suite haskell-test
   type: exitcode-stdio-1.0
   main-is: Spec.hs

2024/haskell/package.yaml

@@ -87,6 +87,17 @@ executables:
     dependencies:
     - haskell
 
+  day6:
+    main:                Day6.hs
+    source-dirs:         app/day6
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - haskell
+    - parallel
+
 tests:
   haskell-test:
     main:                Spec.hs

2024/haskell/resources/day6.txt

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2024/haskell/resources/day6_example.txt

@@ -0,0 +1,10 @@
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...