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Co-routines

Co-routines is a way to allow multiple execution paths in parallel, but without the hazards of pre-emptive scheduling in multithreading where one thread can be suspended at any moment so another can resume. Because co-routines always voluntarily suspend themselves in favor of the next co-routine, there is no need to worry about atomic instructions and critical sections.

Co-routines are not natively built-into the AngelScript library, but it can easily be implemented from the application side. In fact, the Context manager add-on already provides a default implementation for this.

To implement your own version of co-routines you will need a couple of pieces:

- The co-routine itself, which is just an instance of the asIScriptContext object. Each co-routine will have its own context object that holds the callstack of the co-routine.
- A function that will permit the script to create, or spawn, new co-routines. This function will need to be able to refer to starting function of the new co-routine. This reference can be done by name, or perhaps more elegantly with a function pointer. Once invoked this function will instanciate the new context, and prepare it with the starting function.
- A function that will permit a co-routine to yield control to the next co-routine. This function will simply suspend the current context so the next co-routine can be resumed.
- A simple control algorithm for the co-routines. This can just be a loop that will iterate over an array of co-routines, i.e. contexts, until all of them have finished executing. When a new co-routine is created it is simply appended to the array to be picked up when the current co-routine yields the control.

A simple implementation of the function that spawns a new co-routine may look like this:

void CreateCoRoutine(string &func)
{
asIScriptContext *ctx = asGetActiveContext();
if( ctx )
{
asIScriptEngine *engine = ctx->GetEngine();
string mod = ctx->GetFunction()->GetModuleName();
// We need to find the function that will be created as the co-routine
string decl = "void " + func + "()";
asIScriptFunction *funcPtr = engine->GetModule(mod.c_str())->GetFunctionByDecl(decl.c_str());
if( funcPtr == 0 )
{
// No function could be found, raise an exception
ctx->SetException(("Function '" + decl + "' doesn't exist").c_str());
return;
}
// Create a new context for the co-routine
asIScriptContext *coctx = engine->CreateContext();
coctx->Prepare(funcPtr);
// Add the new co-routine context to the array of co-routines
coroutines.push_back(coctx);
}
}

The yield function is even simpler:

void Yield()
{
asIScriptContext *ctx = asGetActiveContext();
if( ctx )
{
// Suspend the context so the next co-routine can be resumed
ctx->Suspend();
}
}

A basic control algorithm might look like this:

std::vector<asIScriptContext *> coroutines;
void Execute()
{
int n = 0;
while( coroutines.size() > 0 )
{
// Resume the co-routine
int r = coroutines[n]->Execute();
if( r == asEXECUTION_SUSPENDED )
{
// Resume the next co-routine
if( ++n == coroutines.size() )
n = 0;
}
else
{
// The co-routine finished so let's remove it
coroutines[n]->Release();
coroutines.erase(n);
}
}
}

See Also
  Context manager, Co-routines, Concurrent scripts