Hello all,

I'm considering a rewrite of some old C/C++ simulation code in D.  This code is a "stock market" game where lots of different agents with different strategies compete and try to outdo one another at trading.

What I'm puzzling over is how to effectively store the collection of agents. These will be arbitrary in number (and agents might in principle be created or destroyed during the game), will all have the same interface, but will have different implementations -- some may use look-up tables, some may be controlled by neural networks, etc. etc.

When I originally wrote this in C, I just used an array of pointers to agents. In C++ I did something similar, defining an agent class of which all implementations were subclasses.

I'm just wondering what might be the best and most idiomatic way to do this in D.  My likely agent design will be a "policy class" style,

    struct Agent(alias X, alias Y, ...)
    {
        mixin X;
        mixin Y;
        // etc...
    }

... with template mixins to implement the individual components that vary by agent type.  The public interface will be identical for all agents.

What I can't work out is how to store a collection of these agents in an array or similarly efficient container.  Can anyone advise?

Thanks and best wishes,

    -- Joe

On 06/20/2012 02:01 PM, Joseph Rushton Wakeling wrote:

> What I'm puzzling over is how to effectively store the collection of
> agents. These will be arbitrary in number (and agents might in principle
> be created or destroyed during the game), will all have the same
> interface, but will have different implementations -- some may use
> look-up tables, some may be controlled by neural networks, etc. etc.
>
> When I originally wrote this in C, I just used an array of pointers to
> agents. In C++ I did something similar, defining an agent class of which
> all implementations were subclasses.
>
> I'm just wondering what might be the best and most idiomatic way to do
> this in D. My likely agent design will be a "policy class" style,
>
> struct Agent(alias X, alias Y, ...)
> {
> mixin X;
> mixin Y;
> // etc...
> }
>
> ... with template mixins to implement the individual components that
> vary by agent type. The public interface will be identical for all agents.
>
> What I can't work out is how to store a collection of these agents in an
> array or similarly efficient container. Can anyone advise?

Sounds like you need an 'interface' and classes that implement it. The following code uses simple string mixins:

import std.stdio;

interface Agent
{
    double trade();
}

class SmartAgent : Agent
{
    double trade()
    {
        writeln("Trading smartly");
        return 42;
    }
}

class ConfigurableAgent(string X, string Y) : Agent
{
    mixin(X);
    mixin(Y);

    double trade()
    {
        writefln("Trading with type '%s' and '%s'", typeid(x), typeid(y));
        return x + y;
    }
}

void main()
{
    Agent[] agents;
    agents ~= new SmartAgent();
    agents ~= new ConfigurableAgent!("int x;", "int y;");
    agents ~= new ConfigurableAgent!("double x;", "long y;");

    foreach (agent; agents) {
        agent.trade();
    }
}

Ali

On 20/06/12 22:14, Ali Çehreli wrote:
> void main()
> {
>     Agent[] agents;
>     agents ~= new SmartAgent();
>     agents ~= new ConfigurableAgent!("int x;", "int y;");
>     agents ~= new ConfigurableAgent!("double x;", "long y;");
>
>     foreach (agent; agents) {
>         agent.trade();
>     }
> }

Ahh, I hadn't realized that I could create an array whose "type" was the interface -- though it makes sense.  Thanks very much!

Is there any way to do this with structs instead of classes?  I ask because in my experience the difference in performance is dramatic.

Joseph Rushton Wakeling:

> Is there any way to do this with structs instead of classes?  I ask because in my experience the difference in performance is dramatic.

std.variant.Algebraic?

Bye,
bearophile

On 20/06/12 23:00, bearophile wrote:
> std.variant.Algebraic?

Interesting thought.  I fear it's a no-go as you'd have to specify up-front all the possible agent choices, but I'll have a play ...

Thanks very much! :-)