http://www.youtube.com/watch?v=54m6CWGbkWU

A simple demo project which uses Ogre 3D, Lua, multiple actors, A* Pathfinding,
and a component-based game object system with a naive rule-based
activation/deactivation scheme.

This is an implementation of the ideas and concepts outlined in this article
http://cowboyprogramming.com/2007/01/05/evolve-your-heirachy
this thread
http://www.gamedev.net/topic/463508-outboard-component-based-entity-system-architecture/page__whichpage__3#3089913
and this short paper
http://www.josericardojunior.com/docs/DGOCSMBC_SBG08_1.pdf titled "Dynamic Game Object Component System for Mutable Behaviour Characters"

A component-based approach for Game Object definition promises the mixing
and matching of various aspects of said Game Object to form new ones,
which is hard to accomplish in an inheritence-based approach. Components
are generalized by extracting the different aspects of the Game object and
hiding them behind predefined interfaces. As long as these interfaces are
implemented, in this project's case: setup(), postSetup() and update(),
components are free to be mixed with others to contribute its special
functionality to an actor. The different components I've found for
this project are: Position, Movement, Targeting, Target, Map, MapItem,
RandomMapSpawn, ai.Seeking, ai.Idle, Visual, Sleepable, MouseInput, and
KeyboardInput.

A simple definition of a frog hunting flies might look like this:

Frog = class(Actor)
Frog.aspects = {
    { MapItem, { id=2 } },
    { Visual, {
        mesh="frog.mesh",
        material="Frog"
        scale={ 0.5, 0.5, 0.5 }
    }},
    { Targeting, { targets = { "fly" } } },
    { ai.Seeking, {
        sleep=0.75,
        activate=function(self)
            return self:getActor():getAspect(Targeting):distanceScore() < 2 and
                   self:getActor():getAspect(Hunger):isCritical()
        end
    }},
    Sleepable,
    { Hunger,  { level=0, increase_rate=0.5, critical=150 } }
}

This defines a frog which is mapped to the tile id 2 on the map and
is visually represented using the mesh "frog.mesh", a "Frog" material
and a half point scale on all three dimensions. It targets any Actors
specified as Target named "fly", and will hunt it using the ai.Seeking
behaviour if the distance between the frog and the fly is close enough,
and is hungry enough to do so.

All the actor definitions for this project can be checked out under
bin/linux2/actors/sample.lua with the Animal actor being the most "complex"
one for the project, using two ai.Seeking and one ai.Idle component.

Instead of explicitly maintaining Finite State Machines in order to
implement AI Actors, the component mechanism can be used to mix and
match different behaviours depending on the environment and the actor's
individual state. This is done by specifiying an activation rule for AI
components, which when evaluated as true, will activate the component. As
long it is active, its update function will run and take over the decision
making for its owner (the actor). This way multiple actor behaviour can be
activated and run concurrently at the same time. It is the programmers job
to define non-conflicting AI behavour to facilitate mixing and matching of
those behaviour. This is an implementation of the idea mentioned in this
short paper http://www.josericardojunior.com/docs/DGOCSMBC_SBG08_1.pdf
titled "Dynamic Game Object Component System for Mutable Behaviour
Characters".

Currently the rule matching algorithm is a naive one, in that ALL the
activation rules are checked on each iteration. An improved version would
include the Rete algorithm (Forgy,1982) to improve matching speed.

All of the above and an A* Pathfinding module are implemented in Lua using
a thin scripting layer to ease the binding process. The C++ side implements
the ground infrastructure to have an Ogre 3D Application up and running.

The file structure is composed of the following:

C++

    src/app.cpp                       - Ogre 3D Application src/app.cpp
    src/lua/lua_game.cpp              - Object Binding: SceneNode, ManualObject and Method Binding (addEntity, addLine)
                                        to the Lua side
    src/script.cpp                    - Helper submodule to ease the binding process (no external dependencies)

    ---------------------------------------------------------------------

Lua

    bin/linux2/autoexec.lua

        -- entry point called from the Ogre 3D Application

    bin/linux2/astar.lua

        -- A* Pathfinding Implementation in Lua

    bin/linux2/scheduler.lua

        -- simple Lua's coroutine-based task scheduler with sleeping support

    bin/linux2/actor.lua

        -- base Actor and ActorManager definition

    bin/linux2/aspects/base.lua

        -- base Aspect and AspectManager definition

    bin/linux2/aspects/position.lua

        -- general component to handle positioning of actors. synchronizes with the Ogre 3D SceneNode stored under the visual aspect on the same actor.

    bin/linux2/aspects/movement.lua

        -- general component to handle movement. Supplies the move() interface. Used by the ai.Seeking Aspect attached to the same actor.

    bin/linux2/aspects/visual.lua

        -- general component to determine the visual aspect of the actor. Uses Ogre 3d's exposed objects to accomplish this (SceneNode, addEntity)

    bin/linux2/aspects/input.lua

        -- supplies two components: MouseInput and KeyboardInput. Attaching these components to an actor enables the respective input mechanisms for that actor.

    bin/linux2/aspects/hunger.lua

        -- adds the concept of Hunger to the attached actor. Supplies the interface isCritical() to determine whether the actor is hungry or not.

    bin/linux2/aspects/target.lua

        -- defines two components: Target and Targeting. Attaching the first to an actor makes that actor a target for those actors with the Targeting components attached. Targets are identified by a simple string name. The two imortant interfaces on Targeting are: distanceScore(), which determines the distance between the current actor and its target, and drawPath(), which draws a line between the actor and its target.

    bin/linux2/aspects/sleep.lua

        -- attached Actors are able to go into "sleep" mode, skipping the update loop for a set amount of time (in seconds). Main interface is sleep(), alongside wakeUp(), and isAwake()

    bin/linux2/aspects/map.lua

        -- defines Map, MapItem, and RandomMapSpawn. Map is component to transform a 2D Grid into a 3D Maze. Attaching a MapItem to an Actor, makes it show up on a map identified by a tile id. RandomMapSpawn exposes an interface to randomly spawn MapItem mapped actors on running Maps.

    bin/linux2/aspects/ai/base.lua

        -- base game AI component with rule-based activation/deactivation

    bin/linux2/aspects/ai/seeking.lua

        -- depends on the components: movement, targeting, and sleep to provide a "seeking" behaviour for an actor, when activated.

    bin/linux2/aspects/ai/idle.lua

        -- depends on the sleep component: sleep for a specified amount of time on activation.

    bin/linux2/actors/sample.lua

        -- all actor definitions using the above aspects/components for a simple "penguin in a maze" simulation