Pacman Dissected

“If Pac-Man had affected us as kids, we’d all be running around in dark rooms, munching pills and listening to repetitive electronic music”

Markus Brigstocke

In my previous post, I wrote about learning TypeScript by writing a game.

The game I chose was Pacman (play it here).

I never intended to make the source code available, but a few people asked for it, so I’ve tidied it up a bit and put it on GitHub (it’s far from tidy though, so go easy – plus it’s my first attempt at TypeScript!)

I’ve described the major bits of the code below.  I’ve described:

  • the startup – how scripts and assets are loaded
  • the game-loop – what bits of code are called 60 times per second
  • the game flow – how the code flows from one screen to another
  • the graphics – spritesheets, sprites, and Canvas and how they fit together
  • the maze – how things interact with the maze
  • ghosts – most of the logic in the game is associated with the ghosts
  • timing and difficulty – getting the game to play like the real arcade game

If there’s anything I’ve missed, please let me know.

I hope you find this useful.  Please be aware that this is not a shining example of TypeScript or the best patterns to use in TypeScript.  The style leans heavily towards C# as that’s my day-to-day language.  It barely scrapes the surface of TypeScript features and I’m sure there are many things in it that could be made more elegant (readable) by using other TypeScript features.  I’d love to get feedback on the code as I’d like to evolve it over time.  So please free to provide feedback, pull-requests, etc. etc.


Build and Run

The following should download and run the game (assuming you’ve got git and npm installed):

Game Startup

index.html loads the JavaScript scripts for howler (sound), hammer (touch), the loading screen, the control panel, and require.js.

When the page loads, it loads all of the sound files and then all of js files.  require.js fires an event (load), when a script is loaded.  We subscribe to this event and tell the loading screen that a script is loaded (loadState.scriptLoaded(moduleName)).

The main game is held within a div named gameDiv.  Within that div is a canvas:

The next bit then instantiates the Engine:

Engine (in Engine.ts) is a small type which handles:

  • running the game-loop
  • handling ‘credits’ (when 1 or 2 player buttons are pressed)
  • showing/hiding the control panel

Game Loop

The game-loop runs 60 times per second via a call to window.requestAnimationFrame

The game-loop updates and draws everything (60 times per second).  It calls MainWindow.Update (MainWindow.ts) with the time elapsed since the last call.  The time elapsed is important as it allows timers to be run accurately.  MainWindow is the, er, main window.  It handles:

  • updating the current act (see below)
  • drawing the current act
  • updating and drawing the score and status panels
  • handling game events, such as ‘pacManEaten’, ‘ghostEaten’ etc.

Game Flow

Everything in the game is an Act:

Here are the different Acts:

The welcome screen (or the ‘attract screen’ as they call it in arcade circles) is called the AttractAct.  You can see it being set as the main Act in MainWindow.ts:

When the update method returns Finished, the game-loop starts to run the Act returned by nextAct.

Graphics

The graphics are drawn onto an HTML Canvas.  A sprite-sheet is loaded in index.html:

It looks like this:

It contains all of the graphics in one image.  The sprites then reference a particular rectangle of this image and are drawn on the canvas. All sprites derive from Sprite:

Each sprite has the following facets:

  • position specifies where in the ‘game world’ the sprite currently is
  • spriteSheetPos specifies the point in the sprite-sheet where the image for this sprite begins
  • origin specifies the offset from the top left of the sprite that acts as the origin.  The origin is used to calculate the top left position of the sprite and can be used for rotation (rotating something whos origin is top left will have a different effect that animating something whos origin is center)
  • size specifies the pixel extent of the sprite.  Everything drawn in this game is zoomed in by 3 times, so the size here is the pixel size, and not the output ‘screen size’

The Maze

The maze (as shown above) is drawn to the canvas every frame.  The ‘pills’ (normal pills and ‘power pills’)  are removed from maze (well, a copy of each as there’s one for each player) when the pill is eaten.

The maze is broken down into ’tiles’ that are 8×8 pixels in size.  Sprite positions are converted to the associate ’tile’.  The game then refers to a lookup that says what’s in the current tile.  The lookup looks like this:

  • o represents a cell containing a pill
  • * represents a cell containing a power-pill
  • + represents a cell containing nothing
  • [space] represents a wall

A tile is represented by the Tile class.  Some of the main methods on here are:

  • isInCenter – is the sprite’s position near the center of the tile?
  • nextTile – it is common to get the next tile, based on the direction that an actor is headed
  • nextTileWrapper – the next tile, but taking into account ‘wrapping’ (the two tunnels at either side of the maze)

Ghosts

Ghosts move around the maze and either chase pacman or run away from him.  Here’s the various states of a ghost:

The state of a ghost can differ from the ‘movement mode’ of a ghost:

I mentioned that the ‘ghost state’ and ‘movement mode’ can differ; an example is that a ghost can be ‘blue’ (Frightened) while still being in the ghost house

There are a number of types responsible for moving ghosts.  They all implement GhostMover:

The general logic of a ghost comprises of ‘head to the home corner for X seconds, chase pacman for X seconds’.  The time spent in each phase varies throughout the level.  Each level specifies different patterns.

Timing and Difficulty

Getting the difficulty to match that of the arcade game was tricky.  There are many variables used throughout each level.  These variables are described in the type LevelProps:

At a glance, there are different speeds for:

  • Pacman – depending on whether he’s eating pills or he’s in an empty cell (a very minor difference, but it makes all the difference if you’ve got a ghost 2 pixels away!)
  • ghosts – depending on whether they’re on cells that contain pills or empty cells, and also whether they’re ‘frightened’ (blue)
  • the duration the ghosts are blue for (in later levels, this is one frame (1/60th of a second!))
  • Cruise Elroy‘ speed – Blinky is the only ghost that has different speeds throughout the level
  • tunnel speed – ghosts travel slower through tunnels

There’s a rather large array created in LevelStats that contains all of these variables for the first 21 levels.

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Learning TypeScript by Writing a Game

Want to learn a new programming language?  Tired of the usual Line-of-Business tutorials? Then write a game! I’ve always said that the best way to learn a new language is to write a game with it.  You’ll come across many problems when writing a game that you just won’t experience by writing an app that selects product items from a customer’s order.

That’s what I did recently when I wanted to learn TypeScript.  I decided to write a web-based version of PacMan.  This post describes a bit about my goals.  There’s nothing technical here; I may, at some point, describe the internals of the game and share the code.

**Update August 2017 – source code published and internals described**

My goals were to learn TypeScript and end up with a game that looked and played similar the original.

Here’s some comparison screenshots:

Arcade (Mame)

TypeScript

Arcade
TypeScript

Whilst learning the language, I didn’t want the overhead of also learning a game framework, so I chose to use the HTML canvas directly.  This worked out well as the canvas is very simple to use.

The feedback cycle with the canvas is really quick:  do the code changes and refresh the browser to see the results.

I used a combination of Visual Studio Code & node, and Visual Studio (2017) with IIS.  I found debugging with Visual Studio 2017 easier than with Visual Studio Code (and of course, I also had ReSharper which works quite nicely with TypeScript although it’s not quite as fast and robust as it is with C#).

I found it quite easy to pick up TypeScript, probably because of its similarities with other C based languages.  Like most languages, it’ll probably take years of use to master all of its nuances (my familiarity with the nuances of C# came from using it for years, and also from reading the fantastic More Effective C#).

In fact, I found that it was very quick and easy to get up and running in TypeScript.   This was both good and bad:  good because I could concentrate on the game more and worry less about the language, and bad because I could concentrate on the game more and worry less about the language!

There’s a fair bit to PacMan and I wanted mine to be as close to the original as possible.  This included things such as:

  • Everything has a different speed and those speeds vary depending on level (and where you are in the level). PacMan travels at a different speed to the ghosts (initially, a very small difference), but later levels the difference increases.  PacMan’s speed changes when he’s eating pills, which is barely noticeable unless a ghost is 5 pixels from your rear!
  • Cornering – Related to the different speeds of things is the technique of ‘cornering’: PacMan can gain a very slight speed increase over the ghosts by selecting the direction a couple of pixels before the turn:

This is very subtle but is critical in later levels; to evade faster ghosts, head to as many corners as possible until the ghost pattern changes (see below) – each corner gives PacMan a couple of pixels advantage of the ghost

  • Ghost patterns: ghosts are either chasing PacMan or returning to their ‘home corner’.  Every level has a different duration to the patterns.  I used this page for all the timings in the game and this page to understand where the ghosts should go.
  • BUGS! There are a few bugs in the original PacMan.  As far as I know, I replicated all but one bug (the kill screen bug) (but I’m sure I introduced a few of my own to make up for it!).  Bugs deliberately programmed in include the ghost house bug (where it’s possible to keep 3 ghosts from entering the maze for the whole level), and the bug where ghosts can (very rarely) change direction just before they eat PacMan.
  • Ghost eyes: ghost’s eyes look at where they’re going next. If they’re heading for a T-junction, they’ll either look left or right just before they get there.  If you’re very quick, you can use this to your advantage.  This is very subtle.  I’ve played PacMan for years and only recently discovered this.
  • Cutscenes: These are intermission screens that appear after certain levels.  They tell a ‘story’ that reveals the true identity of the ghosts!

I found that the game itself was only half the work.  I wanted it to run on as many platforms and devices as possible (PCs, phones, tablets).  A fair amount of work, after the game was finished, went into things like:

  • The loading screen to show the progress of script and asset loading
  • Audio issues – sound played on the PC but didn’t play on mobile devices. I ended up using the super Howler library
  • Control Panel – needed a control panel where users could ‘insert coins’ and select the number of players by clicking/touching (during development it was all keyboard based)
  • Touch/swipe – needing to handle touch/swipe correctly for tablets and phones
  • Sound – not playing on mobile devices until touched

Overall, I think it was a good idea to write a game to learn TypeScript.  Aside from the extra tasks involved after finishing writing the game, it was a quick way to learn the language with the added bonus of being able to visualise progress.  Click here to play.

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