Friday, December 20, 2013

SqueakJS: A Lively Squeak VM

I'm proud to announce SqueakJS, a new Squeak VM that runs on Javascript:


It was inspired by Dan's JSqueak/Potato VM for Java, and similarly only runs the old Squeak 2.2 mini.image for now. But I developed it inside the Lively Kernel, which allowed me to make a nice UI to look inside the VM (in addition to all the Lively tools):


It represents regular Squeak objects as Javascript objects with direct object references. SmallIntegers are represented as Javascript numbers, there is no need for tagging. Instance variables and indexable fields are held in a single array named "pointers". Word and byte binary objects store their data in arrays named "bytes" or "words". CompiledMethod instances have both "pointers" and "bytes". Float instances are not stored as two words as in Squeak, but have a single "float" property that stores the actual number (and the words are generated on-the-fly when needed).

For garbage collection, I came up with a hybrid scheme: the bulk of the work is delegated to the Javascript garbage collector. Only in relatively rare circumstances is a "manual" garbage collection needed. This hybrid GC is a semi-space GC with an old space and a new space. Old space is a linked list of objects, but newly allocated objects are not added to the list, yet. Therefore, unreferenced new objects will be automatically garbage-collected by Javascript. This is like Squeak's incremental GC, which only looks at objects in new space. The full GC is a regular mark-and-sweep: it's marking all reachable objects (old and new), then unmarked old objects get removed (a very cheap operation in a linked list), and new objects (identified by their missing link) are added to the old-space list. One nice feature of this scheme is that its implementation does not need weak references, which Javascript currently does not support.

This scheme also trivially supports object enumeration (Squeak's nextObject/nextInstance primitives): If the object is old, the next object is just the next link in the list. Otherwise, if there are new objects (newSpaceCount > 0) a GC is performed, which creates the next object link. But if newSpaceCount is 0, then this was the last object, and we're done.

The UI for now copies the Squeak display bitmap pixel-by-pixel to a typed array and shows it on the HTML 2D canvas using putImageData(). Clipboard copying injects a synthetic CMD-C keyboard event into the VM, then runs the interpreter until it has executed the clipboard primitive in response, then answers that string. This is because the web browser only allows clipboard access inside the copy/paste event handlers. You can drag an image file from your disk into the browser window to load it.

Besides running it on your desktop, you can install it as offline web app on an iPad:


On the iPad there is neither right-click nor command keys, but the menu is available on the inside of the flop-out scrollbars. It needs a fairly recent browser, too - it works in iOS 7, but apparently not in older ones. On Android it works in Chrome 31, but not quite as well (for example, the onscreen-keyboard does not come up on an Galaxy Note tablet).

Go to the project page to try it yourself. The sources are on GitHub, and contributions are very welcome.

Have a great Christmas!

Monday, February 25, 2013

Smalltalk Bindings for Minecraft Pi

The Raspberry Pi is a cute little computer. Quite cheap at $35, you plug in USB keyboard+mouse and a TV as monitor. And it is surprisingly capable, even for running 3D games.

One particularly interesting game is Minecraft: Pi Edition. As in other Minecraft versions, the main goal is to create a world. But unlike other versions, you can not only use the tools provided by the game, you can make your own tools! That's because it comes with a programming interface.

The Minecaft world is made of little cubes, and you normally place or remove these blocks by hand, one after another. This is fun, but for larger structures also quite cumbersome. For example, this rainbow here might take a long time to construct manually:


But I did not make the rainbow by hand. I programmed it, using the Smalltalk programming language. It's just these dozen lines of code in the Squeak programming environment:


Squeak is already installed on the Raspberry Pi, because Scratch was made in Squeak. Of course you need a little more to make this dozen lines of code work. Mojang (the developers of Minecraft) have provided "bindings" for the Python and Java programming languages, but not for Smalltalk. So I had to make these bindings first.

Here are the Bindings

Now you can use the bindings too, because I am publishing my code:
http://ss3.gemstone.com/ss/minecraft.html
Squeak can either run on the Raspberry Pi itself (a VM is already installed) or on another computer in your network.

There are two packages, 'Minecraft-Pi-Base' and 'Minecraft-Pi-Demo', load them in this order. At the time of writing, the demo package has only the rainbow method in it. The code is not heavily commented, but from the examples it should be fairly obvious how to use it. 

The bindings are still somewhat basic, but cover all the functions of the current Minecraft-Pi 0.1.1 release. There is certainly room for improvement. E.g. it would be nice to add symbolic block names, so you could write "wool" instead of "35". And the hit testing (when you right-click on a block with your sword) works, but could be made more convenient to use, perhaps by introducing an event class like in the other bindings.

I made the repository open, so anyone can easily contribute. I'm curious what others will come up with.   Like, control Minecraft from Etoys or Scratch? How about a Croquet bridge? Build a little game? In any case, have fun! :)