Saturday, May 5, 2012

Embracing Dynamism

Are you stuck in static thinking? Do you see your program as a fixed collection of classes and functions with unchanging behavior.

While that view is mostly true for old school languages such as C++ and Java, the game is different for dynamic languages: Lua, JavaScript, Python, etc. That can be easy to forget if you spend most of your time in the static world, so in this article I'm going to show some of the tricks you can apply when everything is fluid and malleable.

At Bitsquid our dynamic language of choice is Lua. Lua has the advantage of being fast, fully dynamic, small, simple and having a traditional (i.e. non-LISP-y) syntax. We use Lua for most gameplay code and it interfaces with the engine through an API with exposed C functions, such as World.render() or Unit.set_position().

I will use Lua in all the examples below, but the techniques can be used in most dynamic languages.

1. Read-eval-print-loop

Dynamic languages can compile and execute code at runtime. In Lua, it is as simple as:


This can be used to implement a command console where you can type Lua code and directly execute it in the running game. This can be an invaluable debugging and tuning tool. For example if you need to debug some problem with the bazooka:

World.spawn_unit("bazooka", Unit.position(player))

Or tune the player's run speed:

Unit.set_data(player, "run_speed", 4.3)

2. Reload code

The console can be used for more than giving commands, you can also use it to redefine functions. If the gameplay code defines a scoring rule for kills:

function Player.register_kill(self, enemy)
 self.score = self.score + 10

you can use the console to redefine the function and change the rules:

function Player.register_kill(self, enemy)
 if enemy.type == "boss" then
  self.score = self.score + 100
  self.score = self.score + 10

Executing this code will replace the existing Player.register_kill function with the new one. All code that previously called the old function will now call the new one and the new scoring rules will apply immediately.

If you take some care with how you use the global namespace you can write your Lua code so that all of it is reloadable using this technique. Then the gameplay programmer can just edit the Lua files on disk and press a key to reload them in-game. The game will continue to run with the new gameplay code, without any need for a reboot. Pretty cool.

You can even get this to work for script errors. If there is an error in the Lua code, don't crash the game, just freeze it and allow the gameplay programmer to fix the error, reload the code and continue running.

3. Override system functions

The functions in the engine API don't have any special privileges, they can be redefined just as other Lua functions. This can be used to add custom functionality or for debugging purposes.

Say, for example, that you have some units that are mysteriously popping up all over the level. You know they are being spawned somewhere in the gameplay code, but you can't find where. One solution would be to override the World.spawn_unit function and print a stack trace whenever the offending unit is spawned:

old_spawn_unit = World.spawn_unit
function World.spawn_unit(type, position)
 if type == "tribble" then
  print "Tribble spawned by:"
 old_spawn_unit(type, position)

Now, whenever a tribble is spawned by the script, a call stack will be printed and we can easily find who is doing the spawning.

Note that before we replace World.spawn_unit, we save the original function in the variable old_spawn_unit. This enables us to call old_spawn_unit() to do the actual spawning.

This technique could also be used to find all (potentially expensive) raycasts being done by the script.

4. Handle deprecated functions

Sometimes we need to deprecate functions in the engine API. It can be annoying to the people using the engine of course, but backwards compatibilty is the mother of stagnation. If you never throw away old code, you will eventually have a huge ugly code mess on your hands.

Luckily, since the script can create functions in the engine namespace, the script can provide the backwards compatibility when needed.

For example, we used to have a function PhysicsWorld.clear_kinematic(world, actor). That naming was inconsistent with some of our other functions so we changed it to Actor.set_kinematic(actor, false).

One way of dealing with this change would be to go through all the code in the project, find all uses of PhysicsWorld.clear_kinematic and change them to use Actor.set_kinematic instead. But another way would be to just implement PhysicsWorld.clear_kinematic in the script:

function PhysicsWorld.clear_kinematic(world, actor)
 Actor.set_kinematic(actor, false)

Now the rest of the code can go on using PhysicsWorld.clear_kinematic without even caring that the function has been removed from the engine API. You could even use a combination of the two strategies -- implementing the deprecated function in Lua for a quick fix, and then looking into removing the uses of it.

5. Dynamically inserting profiling

Top-down profiling with explicit profiler scopes is a good way of finding out where a game is spending most of its time. However, to be useful, explicit profiler scopes need to be inserted in all the "right" places (all potentially expensive functions).

In C we need to guess where these right places are before compiling the program. In Lua, we can just insert the profiler scopes dynamically. We can even create a function that adds profiling to any function we want:

function profile(class_name, method_name)
 local f = _G[class_name][method_name]
 _G[class_name][method_name] = function (...)
  Profiler.start(class_name .. "." .. method_name)

When we call this function as profile('Player', 'update') it will first save the existing Player.update function and then replace it with a function that calls Profiler.start("Player.update") before calling the original function and Profiler.stop() before returning.

Using this techinque, we can dynamically add profiling to any function we want during our optimization session.

6. Tab completion

If you implement an interactive Lua console, it is nice to support tab completion, so the user doesn't have to remember all function names. But how do you build the list of callable functions to use with tab completion?

Using Lua of course! Just find all tables (i.e., classes) in the global namespace and all functions stored in those tables:

t = {}

for class_name,class in pairs(_G) do
 if type(class) == 'table' then
  for function_name,function in pairs(class) do
   if type(function) == 'function' then
    t[#t+1] = class_name .. '.' .. function_name

After running this, t will contain the full list of function names.

7. Looping through all objects

By recursing through _G you can enumerate all reachable objects in the Lua runtime.

function enumerate(f)
 local seen = {}
 local recurse = function(t)
  if type(t) ~= 'table' then return end
  if seen[t] == true then return end
  seen[t] = true
  for k,v in pairs(t) do

Calling enumerate(f) will call f(o) on all objects o in the runtime. (Assuming they are reachable from _G. Potentially, there could also be objects only reachable through Lua references held in C.)

Such an enumeration could be used for many things. For example, you could use it to print the health of every object in the game.

function print_health(o)
 if then print( end

The technique could also be used for memory optimizations. You could loop through all Lua objects and find the memory used by each object type. Then you could focus your optimization efforts on the resource hogs.


  1. Which Lua binding library did you use? Or, did you roll your own?

    1. We write our Lua wrappers by hand, i.e.:

      int lua_unit_set_position(lua_State *L)

      It is just a couple of lines for each wrapped function, so it is not that much work. And I've found that taking a few seconds to think about the bindings produces better results. A tighter, simpler API that is more Lua-like and not directly tied to the C++ functions (which means we can refactor the C code without changing the Lua API).

      I used to do automatic Lua bindings in the past, but I'm much more happy with this approach.

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