Wednesday, November 21, 2012

A Formal Language for Data Definitions

Lately, I've started to think again about the irritating problem that there is no formal language for describing binary data layouts (at least not that I know of). So when people attempt to describe a file format or a network protocol they have to resort to vague and nondescript things like:

Each section in the file starts with a header with the format:

4 bytes   header identifier
2 bytes   header length
0--20 bytes  extra data in header

The extra data is described below.

As anyone who has tried to decipher such descriptions can testify, they are not always clear-cut, which leads to a lot of unnecessary work when trying to coax data out of a document.

It is even worse when I create my own data formats (for our engine's runtime data). I would like to document those format in a clear and unambiguous way, so that others can understand them. But since I have no standardized way of doing that, I too have to resort to ad-hoc methods.

This whole thing reminds me of the state of mathematics before formal algebraic notation was introduced. When you had to write things like: the sum of the square of these two numbers equals the square of the previous number. Formal notation can bring a lot of benefits (just look at what it has done for mathematics, music, and chess).

For data layouts, a formal definition language would allow us to write a tool that could open any binary file (that we had a data definition) for and display its content in a human readable way:

height = 128
width = 128
comment = "A funny cat animation"
frames = [
 {display_time = 0.1 image_data = [100 120 25 ...]}

The tool could even allow us to edit the readable data and save it back out as a binary file.

A formal language would also allow debuggers to display more useful information. By writing data definition files, we could make the debugger understand all our types and display them nicely. And it would be a lot cleaner than the hackery that is autoexp.dat.

Just to toss something out there, here's an idea of what a data definition might look like:

typdedef uint32_t StringHash;

struct Light
 StringHash name;
 Vector3  color;
 float  falloff_start;
 float   falloff_end;

struct Level
 uint32_t version;
 uint32_t num_lights;
 uoffset32_t light_data_offset;


 Light lights[num_lights];

This is a C-inspired approach, with some additions. Array lengths can be parametrized on earlier data in the file and a labels can be used to generate offsets to different sections in the file..

I'm still tossing around ideas in my head about what the best way would be to make a language like this a reality. Some of the things I'm thinking about are:

Use Case

I don't think it would do much good to just define a langauge. I want to couple it with something that makes it immediately useful. First, for my own motivation. Second, to provide a "reality check" to make sure that the choices I make for the language are the right ones. And third, as a reference implementation for anyone else who might want to make use of the language.

My current idea is to write a binary-to-JSON converter. I.e., a program that given a data definition file can automatically convert back and forth between a binary and a JSON-representation of that same data.


The syntax in the example is very "C like". The advantage of that is that it will automatically understand C structs if you just paste them into the data definition file, which reduces the work required to set up a file.

The disadvantage is that it can be confusing with a language that is very similar to C, but not exactly C. It is easy to make mistakes. Also, C++ (we probably want some kind of template support) is quite tricky to parse. If we want to add our own enhancements on top of that, we might just make a horrible mess.

So maybe it would be better to go for something completely different. Something Lisp-like perhaps. (Because: Yay, Lisp! But also: Ugh, Lisp.)

I'm still not 100 % decided, but I'm leaning towards a restricted variant of C. Something that retains the basic syntatic elements, but is easier to parse.


Should this system be able to describe any possible binary format out there?

Completeness would be nice of course. It is kind of annoying to have gone through all the trouble of defining language and creating the tools and still not be able to handle all forms of binary data.

On the other hand, there are a lot of different formats out there and some of them have a complexity that is borderline insane. The only way to be able to describe everything is to have a data definition language that is Turing complete and procedural (in other words, a detailed list of the instructions required to pack and unpack the data).

But if we go down that route, we haven't really raised the abstraction level. In that case, why even bothering with creating a new language. The format description could just be a list of the C instructions needed to unpack the data. That doesn't feel like a step forward.

Perhaps some middle ground could be found. Maybe we could make language that was simple and readable for "normal" data, but still had the power to express more esoteric constructs. One approach would be to regard the "declarative statements" as syntactic sugar in a procedural language. With this approach, the declaration:

struct LightCollection
 unsigned num_lights;
 LightData lights[num_lights];

Would just be syntactic sugar for:

function unpack_light_collection(stream)
 local res = {}
 res.num_lights = unpack_unsigned(stream)
 res.lights = []
 for i=1,res.num_lights do
  res.lights[i] = unpack_light_data(stream)

This would allow the declarative syntax to be used in most places, but we could drop out to full-featured Turing complete code whenever needed.

Thursday, November 1, 2012

Bitsquid Foundation Library

Today I want to talk a bit about the Bitsquid Foundation Library that we recently released on Bitbucket (under the permissive MIT license).

It's a minimalistic "foundation" library with things like memory management and collection classes. The idea is to have something that can be used as a reasonable starting-off point for other open source projects.

The library makes some interesting design choices that touches on topics that I have already talked about in this blog and that I think are worth elaborating on a bit further. It also serves an example on how these techniques can be used in "real world" code.

Separation of data and code

The foundation library implements the idea of separating data definitions and function implementation, that I talked about in this article.

Data is stored in structs with public members (prefixed with an underscore to indicate that you should not mess with them unless you know what you are doing) that are found in *_types.h files. Functions that operate on the data are written outside of the struct, in separate *.h files (and organized into namespaces).

For example, the data definition for the dynamic Array<T> class is found in collection_types.h:

template<typename T> struct Array
    Array(Allocator &a);
    Array(const Array &other);
    Array &operator=(const Array &other);
    T &operator[](uint32_t i);
    const T &operator[](uint32_t i) const;

    Allocator *_allocator;
    uint32_t _size;
    uint32_t _capacity;
    T *_data;

The struct contains only the data used by the array and the operators which C++ forces us to implement as member functions.

The implementation of these functions, as well as the declaration and definition of all other functions that operate on the arrays are found in the array.h file. It contains things like:

namespace array
    template<typename T>
    inline uint32_t size(const Array<T> &a)
    {return a._size;}
    template<typename T>
    inline bool any(const Array<T> &a)
    {return a._size != 0;}
    template<typename T>
    inline bool empty(const Array<T> &a)
    {return a._size == 0;}

template <typename T>
inline Array<T>::Array(Allocator &allocator) :
    _allocator(&allocator), _size(0), _capacity(0), _data(0)

This way of arranging data and code fills two purposes.

First, it improves compile times by reducing header inclusion. Header files that want to make use of arrays only need to include collection_types.h, which just contains a few struct definitions. They don't have to drag in array.h, with all its inline code.

Headers including other headers indiscriminately because they need their types is what leads to exploding compile times. By only including the minimal thing we need (the type definitions), compile times are minimized.

Second, and more importantly, this design allows the collection types to be freely extended. Is there anything you miss in the array interface? Perhaps you would like shift() and unshift() methods? Or binary_search()?

No problem. If you want them you can just add them, and you don't even need to modify array.h. Just create your own file array_extensions.h or whatever, and add some new functions to the array namespace, that manipulate the data in the Array<T> interface. The functions you create will be just as good as the functions I have created.

Note that this isn't true for traditional class designs, where you have first-class citizens (methods) and second-class citizens (external functions).

The foundation library has some interesting examples of this. For example, the string_stream functions don't operate on any special StringStream class, they just directly use an Array<char>. Also, the hash and multi_hash interfaces both work on the same underlying Hash<T> struct.

I believe that this design leads to simpler, more orthogonal code that is easier to extend and reuse.

Memory management

The library implements the allocator system mentioned in this article. There is an abstract Allocator interface, and implementations of that interface can provide different allocation strategies (e.g. ArenaAllocator, HeapAllocator, SlotAllocator, etc).

Since I want to keep the library platform independent, I haven't implemented a PageAllocator. Instead, the MallocAllocator is used as the lowest allocator level. If you want to, you can easily add a PageAllocator for your target platform.

For the same reason, I haven't added any critical section locking to the allocators, so they aren't thread safe. (I'm thinking about adding an interface for that though, so that you can plug in a critical section implementation if needed.)

The system for temporary allocations is kind of interesting and deserves a bit further explanation.

Most games have a need for temporary memory. For example, you might need some temporary memory to hold the result of a computation until it is done, or to allow a function to return an array of results.

Allocating such memory using the ordinary allocation system (i.e., malloc) puts a lot of unnecessary stress on the allocators. It can also create fragmentation, when long lived allocations that need to stay resident in memory are mixed with short lived temporary allocations.

The foundation library has two allocators for dealing with such temporary allocations, the ScratchAllocator and the TempAllocator.

The ScratchAllocator services allocation requests using a fixed size ring buffer. An allocate pointer advances through the buffer as memory is allocated, and a corresponding free pointer advances as memory is freed. Memory can thus be allocated and deallocated with simple pointer arithmetic. No calls need to be made to the underlying memory management system.

If the scratch buffer is exhausted (the allocate pointer wraps around and catches up with the free pointer), the ScratchAllocator will revert to using the ordinary MallocAllocator to service requests. So it won't crash or run out of memory. But it will run slower, so try to avoid this by making sure that your scratch buffer is large enough.

If you forget to free something allocated with the ScratchAllocator, or if you accidentally mix in a long-lived allocation among the short-lived ones, that allocation will block the free pointer from advancing, which will eventually exhaust your scratch buffer, so keep an eye out for such situations.

TempAllocator<BYTES> is a scoped allocator that automatically frees all its allocated memory when it is destroyed (meaning you don't have to explicitly call deallocate(), you can just let the allocator fall out of scope). This means you can use it everywhere where you need a little extra memory in a function scope:

void test()
     TempAllocator1024 ta;
     Array<char> message(ta);

The BYTES argument to TempAllocator<BYTES> specifies how much stack space the allocator should reserve. The TempAllocator contains char buffer[BYTES] that gets allocated on the stack together with the TempAllocator.

Allocation requests are first serviced from the stack buffer, then (if the stack buffer is exhausted) from the ScratchAllocator.

This means that TempAllocator gives you an allocator that can be used by all collection classes and will use the fastest allocation method possible (local stack memory, followed by scratch buffer memory, followed by malloc() if all else fails).

Minimalistic collection types

The collection classes in the library are distinctly anti-STL. Some of the important differences:

  • They use the allocation system described above (taking an Allocator as argument to the constructor). They can thus be used sensibly with different allocators (unlike STL types).

  • The use the data/function separation also described above, which means that the headers are cheap to include, and that you can extend them with your own functionality.

  • They use a minimalistic design. They assume that the stored data consists of plain-old-data objects (PODs). Constructors and destructors are not called for the stored objects and they are moved with raw memmove() operations rather than with copy constructors.

This simplifies the code and improves the performance (calling constructors and destructors is not free). It also saves us the headache of dealing with storing objects that must be constructed with Allocators.

Personally I like this minimalistic approach. If I want to keep non-POD data in a collection, I prefer to store it as pointers anyway, so I have control over when and how the data is constructed and destroyed. I don't like those things happening "behind my back". You may disagree of course, but in that case you are probably happy to use STL (or boost).

Another example of choosing minimalism is the Hash<T> class. The hash uses a fixed key type which is a uint64_t. If you want to use a key that doesn't fit into 64 bits, you have to hash it yourself before using it to access the data.

And more?

I'm planning to add some basic math code to the library, but haven't gotten around to it yet.

Is there anything else you'd like to see in a library like this?