random movement around non uniform shape

[WWFC]

hi all,

i'm hoping that someone can help me out here.

attached is an .fla file that has a number of movieclips that move around at random, this part works great. but the movement is only applicable to a rectangle/uniformed area - defined in the action script on frame 1.

function getdistance (x, y, x1, y1) {
    var run, rise;
    run = x1-x;
    rise = y1-y;
    return (hyp(run, rise));
}
function hyp (a, b) {
    return (Math.sqrt(a*a+b*b));
}
MovieClip.prototype.reset = function () { 
var dist, norm, movie_height, movie_width;


//   movie_height: refers to the height of your movie
//   movie_width: refers to the width of your movie

//--------------------------------------------

movie_height = 150;
movie_width = 300;

//--------------------------------------------


speed = Math.random()*4+2;
targx = Math.random()*(movie_width-_width);
targy = Math.random()*(movie_height-_height);
dist = _root.getdistance(_x, _y, targx, targy);

norm = speed/dist;
diffx = (targx-_x)*norm;
diffy = (targy-_y)*norm;
};


MovieClip.prototype.move = function () { 
var cycle;



// cycle: specifies speed of the movement. The smaller 
//         number, the faster the objects move.

//--------------------------------------------

cycle = 200;

//--------------------------------------------

if (_root.getdistance(_x, _y, targx, targy)>speed) {x += diffx;y += diffy;
} else {x = targx;
y = targy;
if (!this.t) {
t = getTimer();
}if (getTimer()-t>cycle) {reset();
t = 0;
}
}
_x = x;
_y = y;
} 

and each clip that moves randomly has this script applied...

onClipEvent (enterFrame) {
    move();
} 

what i want the clips to do is move around within the area of the big mc named "area" in the middle of the stage - by which i mean move anywhere within the boundaries of the shape of the "area" but never outside.

i remember i did something similar many many moons ago - but can't find any reference/files/scripting for what it was that i did to make it work. not sure if the phrase "shape flag" is something that is on the right lines? it was so long ago!

does anyone know what i need to add/edit/change in the AS shown above to achieve the desired effect please?

be really grateful for any pointer, tips or help that anyone feels generous to offer me

fingers crossed...