createEmptyMovieClip overrides existing MovieClip with the same depth

[relrelb]

In Free Rider 2, the bike and player disappear when hit.
After digging into this, I think I found the root cause of the problem:

class vehicle {
    ...
    function vehicle() {
        this.mymc = _root.createEmptyMovieClip("vehiclemc", 3);
        ...
    }
    ...
}

class bmx extends vehicle {
    function bmx() {
        super();
        ...
    }
    ...
    function createRagDoll() {
        _root.gui.message("Ouch! [Press Enter]",16764108);
        ...
        _root.player = new splitBike(this,this.getStick());
    }
    ...
}

class ragdoll extends vehicle {
    function ragdoll(pStick, pmc) {
        super();
        this.mymc = pmc;
        ...
    }
    ...
}

At the beginning, a new bmx is created. When hit, bmx.createRagDoll is called, which in turn creates a new ragdoll (during the creation of a new splitBike).
Since both bmx and ragdoll inherit from vehicle, the call to _root.createEmptyMovieClip("vehiclemc", 3) occurs multiple times.
The second call removes the MovieClip of bmx, just because a new MovieClip is created with the same depth.
The removal happens in add_child_from_avm.

The inherent problem here is that MovieClips store their children in a BTreeMap indexed by their depth, so children with the same depth cannot co-exist.
I tried to convert the BTreeMap to a simple Vec, but it was just too much to rewrite.

[Herschel]

Not sure what's going on in this specific case--I'll have to take a look--but in general creating a movie on an already occupied depth removes the previous clip. A simple example:

var mc = createEmptyMovieClip("mc", 0);
mc.lineStyle(3, 0);
mc.lineTo(100, 100);

// mc2 is placed on the same depth as mc, so mc is destroyed
var mc2 = createEmptyMovieClip("mc2", 0);
mc2.lineStyle(3, 0xff0000);
mc2.lineTo(100, 200);
// Only mc2 is visible

[Herschel]

Seems like this is a case of the original reference mymc auto-updating to the newly created clip. The clip does indeed get replaced when it uses the same depth, but since it uses the same path _root.vehiclemc, the mymc reference automatically points to the new clip. You can see it work if you manually update the references at the end of splitBike:

     pBike.mymc = _root.vehiclemc;
      this.myRagdoll.mymc = _root.vehiclemc;

Closing in favor of #1513.