Serializing Mazes!


Last week we improved our game so that we could solve additional random mazes after the first. This week, we'll step away from the randomness and look at how we can serialize our mazes. This will allow us to have a consistent and repeatable game. It will also enable us to save the game state later.

We'll be using the Megaparsec library as part of this article. If you aren't familiar with that (or parsing in Haskell more generally), check out our Parsing Series!

A Serialized Representation

The serialized representation of our maze doesn't need to be human readable. We aren't trying to create an ASCII art style representation. That said, it would be nice if it bore some semblance to the actual layout. There are a couple properties we'll aim for.

First, it would be good to have one character represent one cell in our maze. This dramatically simplifies any logic we'll use for serializing back and forth. Second, we should layout the cell characters in a way that matches the maze's appearance. So for instance, the top left cell should be the first character in the first row of our string. Then, each row should appear on a separate line. This will make it easier to avoid silly errors when coming up with test cases.

So how can we serialize a single cell? We could observe that for each cell, we have sixteen possible states. There are 4 sides, and each side is either a wall or it is open. This suggests a hexadecimal representation.

Let's think of the four directions as being 4 bits, where if there is a wall, the bit is set to 1, and if it is open, the bit is set to 0. We'll order the bits as up-right-down-left, as we have in a couple other areas of our code. So we have the following example configurations:

  1. An open cell with no walls around it is 0.
  2. A totally surrounded cell is 1111 = F.
  3. A cell with walls on its top and bottom would be 1010 = A.
  4. A cell with walls on its left and right would be 0101 = 5.

With that in mind, we can create a small 5x5 test maze with the following representation:


And this ought to look like so:


This serialization pattern lends itself to a couple helper functions we'll use later. The first, charToBoundsSet, will take a character and give us four booleans. These represent the presence of a wall in each direction. First, we convert the character to the hex integer. Then we use patterns about hex numbers and where the bits lie. For instance, the first bit is only set if the number is at least 8. The last bit is only set for odd numbers. This gives us the following:

charToBoundsSet :: Char -> (Bool, Bool, Bool, Bool)
charToBoundsSet c =
  ( num > 7,
  , num `mod` 8 > 3
  , num `mod` 4 > 1
  , num `mod` 2 > 0

Then, we also want to go backwards. We want to take a CellBoundaries item and convert it to the proper character. We'll look at each direction. If it's an AdjacentCell, it contributes nothing to the final Int value. But otherwise, it contributes the hex digit value for its place. We add these up and convert to a char with intToDigit:

cellToChar :: CellBoundaries -> Char
cellToChar bounds =
  let top = case upBoundary bounds of
        (AdjacentCell _) -> 0
        _ -> 8
  let right = case rightBoundary bounds of
        (AdjacentCell _) -> 0
        _ -> 4
  let down = case downBoundary bounds of
        (AdjacentCell _) -> 0
        _ -> 2
  let left = case leftBoundary bounds of
        (AdjacentCell _) -> 0
        _ -> 1
  in toUpper $ intToDigit (top + right + down + bottom)

We'll use both of these functions in the next couple parts.

Serializing a Maze

Let's move on now to determining how we can take a maze and represent it as Text. For this part, let's first apply a type synonym on our maze type:

type Maze = Map.Map Location CellBoundaries

dumpMaze :: Maze -> Text
dumpMaze = ...

First, let's imagine we have a single row worth of locations. We can convert that row to a string easily using our helper function from above:

dumpMaze = …
    rowToString :: [(Location, CellBoundaries)] -> String
    rowToString = map (cellToChar . snd)

Now we'd like to take our maze map and group it into the different rows. The groupBy function seems appropriate. It groups elements of a list based on some predicate. We'd like to take a predicate that checks if the rows of two elements match. Then we'll apply that against the toList representation of our map:

rowsMatch :: (Location, CellBoundaries) -> (Location, CellBoundaries) -> Bool
rowsMatch ((_, y1), _) ((_, y2), _) = y1 == y2

We have a problem though because groupBy only works when the elements are next to each other in the list. The Map.toList function will give us a column-major ordering. We can fix this by first creating a transposed version of our map:

dumpMaze maze = …
    transposedMap :: Maze
    transposedMap = Map.mapKeys (\(x, y) -> (y, x)) maze

Now we can go ahead and group our cells by row:

dumpMaze maze = …
    transposedMap = …

    cellsByRow :: [[(Location, CellBoundaries)]]
    cellsByRow = groupBy (\((r1, _), _) ((r2, _), _) -> r1 == r2) 
                   (Map.toList transposedMap)

And now we can complete our serialization function! We get the string for each row, and combine them with unlines and then pack into a Text.

dumpMaze maze = pack $ (unlines . reverse) (rowToString <$> cellsByRow)
    transposedMap = …

    cellsByRow = …

    rowToString = ...

As a last trick, note we reverse the order of the rows. This way, we get that the top row appears first, rather than the row corresponding to y = 0.

Parsing a Maze

Now that we can dump our maze into a string, we also want to be able to go backwards. We should be able to take a properly formatted string and turn it into our Maze type. We'll do this using the Megaparsec library, as we discussed in part 4 of our series on parsing in Haskell. So we'll create a function in the Parsec monad that will take the dimensions of the maze as an input:

import qualified Text.Megaparsec as M

mazeParser :: (Int, Int) -> M.Parsec Void Text Maze
mazeParser (numRows, numColumns) = ...

We want to parse the input into a format that will match each character up with its location in the (x,y) coordinate space of the grid. This means parsing one row at a time, and passing in a counter argument. To make the counter match with the desired row, we'll use a descending list comprehension like so:

mazeParser (numRows, numColumns = do
  rows <- forM [(numRows - 1), (numRows - 2)..0] $ \i -> do

For each row, we'll parse the individual characters using M.hexDigit and match them up with a column index:

mazeParser (numRows, numColumns = do
  rows <- forM [0..(numRows - 1)] $ \i -> do
    (columns :: [(Int, Char)]) <-
      forM [0..(numColumns - 1)] $ \j -> do
        c <- M.hexDigitChar
        return (j, c)

We conclude the parsing of a row by reading the newline character. Then we make the indices match the coordinates in discrete (x,y) space. Remember, the "column" should be the first item in our location.

mazeParser (numRows, numColumns = do
  (rows :: [[(Location, Char)]]) <-
    forM [0..(numRows - 1)] $ \i -> do
      columns <- forM [0..(numColumns - 1)] $ \j -> do
        c <- M.hexDigitChar
        return (j, c)
      return $ map (\(col, char) -> ((col, i), char)) columns

Now we'll need a function to convert one of these Location, Char pairs into CellBoundaries. For the most part, we just want to apply our charToBoundsSet function and get the boolean values. Remember these tell us if walls are present or not:

mazeParser (numRows, numColumns = do
  rows <- …
    cellSpecToBounds :: (Location, Char) -> (Location, CellBoundaries)
    cellSpecToBounds (loc@(x, y), c) =
      let (topIsWall, rightIsWall, bottomIsWall, leftIsWall) = 
            charToBoundsSet c

Now it's a matter of applying a case by case basis in each direction. We just need a little logic to determine, in the True case, if it should be a Wall or a WorldBoundary. Here's the implementation:

cellSpecToBounds :: (Location, Char) -> (Location, CellBoundaries)
cellSpecToBounds (loc@(x, y), c) =
  let (topIsWall, rightIsWall, bottomIsWall, leftIsWall) = 
         charToBoundsSet c
      topCell = if topIsWall
        then if y + 1 == numRows
          then WorldBoundary
          else Wall
        else (AdjacentCell (x, y + 1))
      rightCell = if rightIsWall
        then if x + 1 == numColumns
          then WorldBoundary
          else Wall
        else (AdjacentCell (x + 1, y))
      bottomCell = if bottomIsWall
        then if y == 0
          then WorldBoundary
          else Wall
        else (AdjacentCell (x, y - 1))
      leftCell = if leftIsWall
        then if x == 0
          then WorldBoundary
          else Wall
        else (AdjacentCell (x - 1, y))
  in  (loc, CellBoundaries topCell rightCell bottomCell leftCell)

And now we can complete our parsing function by applying this helper over all our rows!

mazeParser (numRows, numColumns = do
  (rows :: [[(Location, Char)]]) <-
    forM [0..(numRows - 1)] $ \i -> do
      columns <- forM [0..(numColumns - 1)] $ \j -> do
        c <- M.hexDigitChar
        return (j, c)
      return $ map (\(col, char) -> ((col, i), char)) columns
  return $ Map.fromList (cellSpecToBounds <$> (concat rows))
    cellSpecToBounds = ...


This wraps up our latest part on serializing maze definitions. The next couple parts will still be more code-focused. We'll look at ways to improve our data structures and an alternate way of generating random mazes. But after those, we'll get back to adding some new game features, such as wandering enemies and combat!

To learn more about serialization, you should read our series on parsing. You can also download our Production Checklist for more ideas!

Declaring Victory! (And Starting Again!)


In last week's article, we used a neat little algorithm to generate random mazes for our game. This was cool, but nothing happens yet when we "finish" the maze! We'll change that this week. We'll allow the game to continue re-generating new mazes when we're finished! You can find all the code for this part on the part-2 branch on the Github repository for this project!

If you're a beginner to Haskell, hopefully this series is helping you learn simple ways to do cool things! If you're a little overwhelmed, try reading our Liftoff Series first!


Our objectives for this part are pretty simple. We want to make it so that when we reach the "end" location, we get a "victory" message and can restart the game by pressing a key. We'll get a new maze when we do this. There are a few components to this:

  1. Reaching the end should change a component of our World.
  2. When that component changes, we should display a message instead of the maze.
  3. Pressing "Enter" with the game in this state should start the game again with a new maze.

Sounds pretty simple! Let's get going!

Game Result

We'll start by adding a new type to represent the current "result" of our game. We'll add this piece of state to our World. As an extra little piece, we'll add a random generator to our state. We'll need this when we re-make the maze:

data GameResult = GameInProgress | GameWon
  deriving (Show, Eq)

data World = World
  { playerLocation :: Location
  , startLocation :: Location
  , endLocation :: Location
  , worldBoundaries :: Maze
  , worldResult :: GameResult
  , worldRandomGenerator :: StdGen

Our generation step needs a couple small tweaks. The function itself should now return its final generator as an extra result:

generateRandomMaze :: StdGen -> (Int, Int) -> (Maze, StdGen)
generateRandomMaze gen (numRows, numColumns) =
  (currentBoundaries finalState, randomGen finalState)
    finalState = execState dfsSearch initialState

Then in our main function, we incorporate the new generator and game result into our World:

main = do
  gen <- getStdGen
  let (maze, gen') = generateRandomMaze gen (25, 25)
    (World (0, 0) (0, 0) (24, 24) maze GameInProgress gen')

Now let's fix our updating function so that it changes the game result if we hit the final location! We'll add a guard here to check for this condition and update accordingly:

updateFunc :: Float -> World -> World
updateFunc _ w
  | playerLocation w == endLocation w = w { worldResult = GameWon }
  | otherwise = w

We could do this in the eventHandler but it seems more idiomatic to let the update function handle it. If we use the event handler, we'll never see our token enter the final square. The game will jump straight to the victory screen. That would be a little odd. Here there's at least a tiny gap.

Displaying Victory!

Now our game will update properly. But we have to respond to this change by changing what the display looks like! This is a quick fix. We'll add a similar guard to our drawingFunc:

drawingFunc :: (Float, Float) -> Float -> World -> Picture
drawingFunc (xOffset, yOffset) cellSize world
  | worldResult world == GameWon =
      Translate (-275) 0 $ Scale 0.12 0.25
        (Text "Congratulations! You've won!\
              \Press enter to restart with a new maze!")
  | otherwise = ...

Note that Text here is the Gloss Picture constructor, not Data.Text. We also scale and translate it a bit to make the text appear on the screen. This is all we need to get the victory screen to appear on completion!


Restarting the Game

The last step is that we have to follow through on our process to restart the game if they hit enter! This involves changing our inputHandler to give us a brand new World. As with our other functions, we'll add a guard to handle the GameWon case:

inputHandler :: Event -> World -> World
inputHandler event w
  | worldResult w == GameWon = …
  | otherwise = case event of

We'll want to make a new case section that accounts for the user pressing the "Enter" key. All this section needs to do is call generateRandomMaze and re-initialize the world!

inputHandler event w
  | worldResult w == GameWon = case event of
      (EventKey (SpecialKey KeyEnter) Down _ _) ->
        let (newMaze, gen') = generateRandomMaze 
              (worldRandomGenerator w) (25, 25)
        in  World (0, 0) (0, 0) (24, 24) newMaze GameInProgress gen'
      _ -> w

And with that, we're done! We can restart the game and navigate random mazes to our heart's content!


The ability to restart the game is great! But if we want to make our game re-playable instead of random, we'll need some way of storing mazes. In the next part, we'll look at some code for dumping a maze to an output format. We'll also need a way to re-load from this stored representation. This will ultimately allow us to make a true game with saving and loading state.

In preparation for that, you can read our series on Parsing. You'll especially want to acquaint yourself with the Megaparsec library. We go over this in Part 4 of the series!