Collection of unmoving collidable objects.
Batching multiple static objects together into a StaticGroup as opposed to adding them separately to the Space avoids BroadPhase pollution, improving performance.
Inheritance: StaticCollidable
        ///<summary>
        /// Cleans up the pair handler.
        ///</summary>
        public override void CleanUp()
        {
            base.CleanUp();

            staticGroup = null;
            //Child type needs to null out other reference.
        }
 ///<summary>
 /// Constructs a new StaticGroupShape.
 ///</summary>
 ///<param name="collidables">List of collidables in the StaticGroup.</param>
 ///<param name="owner">StaticGroup directly associated with this shape.</param>
 public StaticGroupShape(IList<Collidable> collidables, StaticGroup owner)
 {
     this.StaticGroup = owner;
     CollidableTree = new BoundingBoxTree<Collidable>(collidables);
     //Rather than hooking up a bunch of ShapeChanged events here that don't capture the full capacity of change
     //in our child collidables, we will rely on the user telling the collidable tree to reformat itself directly.
 }
        ///<summary>
        /// Initializes the pair handler.
        ///</summary>
        ///<param name="entryA">First entry in the pair.</param>
        ///<param name="entryB">Second entry in the pair.</param>
        public override void Initialize(BroadPhaseEntry entryA, BroadPhaseEntry entryB)
        {
            //Other member of the pair is initialized by the child.
            staticGroup = entryA as StaticGroup;
            if (staticGroup == null)
            {
                staticGroup = entryB as StaticGroup;
                if (staticGroup == null)
                {
                    throw new Exception("Inappropriate types used to initialize pair.");
                }
            }

            base.Initialize(entryA, entryB);
        }
        /// <summary>
        /// Constructs a new demo.
        /// </summary>
        /// <param name="game">Game owning this demo.</param>
        public MutableStaticGroupTestDemo(DemosGame game)
            : base(game)
        {


            //Creating a bunch of separate StaticMeshes or kinematic Entity objects for an environment can pollute the broad phase.
            //This is because the broad phase implementation doesn't have guarantees about what elements can collide, so it has to
            //traverse the acceleration structure all the way down to pairs to figure it out.  That can get expensive!

            //Individual objects, like StaticMeshes, can have very complicated geometry without hurting the broad phase because the broad phase
            //has no knowledge of the thousands of triangles in the mesh.  The StaticMesh itself knows that the triangles within the mesh
            //never need to collide, so it never needs to test them against each other.

            //Similarly, the StaticGroup can be given a bunch of separate collidables.  The broad phase doesn't directly know about these child collidables-
            //it only sees the StaticGroup.  The StaticGroup knows that the things inside it can't ever collide with each other, so no tests are needed.
            //This avoids the performance problem!

            //To demonstrate, we'll be creating a set of static objects and giving them to a group to manage.
            var collidables = new List<Collidable>();

            //Start with a whole bunch of boxes.  These are entity collidables, but without entities!
            float xSpacing = 6;
            float ySpacing = 6;
            float zSpacing = 6;


            //NOTE: You might notice this demo takes a while to start, especially on the Xbox360.  Do not fear!  That's due to the creation of the graphics data, not the physics.
            //The physics can handle over 100,000 static objects pretty easily.  The graphics, not so much :)
            //Try disabling the game.ModelDrawer.Add() lines and increasing the number of static objects.  
            int xCount = 15;
            int yCount = 7;
            int zCount = 15;


            var random = new Random(5);
            for (int i = 0; i < xCount; i++)
            {
                for (int j = 0; j < yCount; j++)
                {
                    for (int k = 0; k < zCount; k++)
                    {
                        //Create a transform and the instance of the mesh.
                        var collidable = new ConvexCollidable<BoxShape>(new BoxShape((float)random.NextDouble() * 6 + .5f, (float)random.NextDouble() * 6 + .5f, (float)random.NextDouble() * 6 + .5f));

                        //This EntityCollidable isn't associated with an entity, so we must manually tell it where to sit by setting the WorldTransform.
                        //This also updates its bounding box.
                        collidable.WorldTransform = new RigidTransform(
                            new Vector3(i * xSpacing - xCount * xSpacing * .5f, j * ySpacing + 3, k * zSpacing - zCount * zSpacing * .5f),
                            Quaternion.CreateFromAxisAngle(Vector3.Normalize(new Vector3((float)random.NextDouble(), (float)random.NextDouble(), (float)random.NextDouble())), (float)random.NextDouble() * 100));

                        collidables.Add(collidable);
                    }
                }
            }


            //Now create a bunch of instanced meshes too.
            xSpacing = 6;
            ySpacing = 6;
            zSpacing = 6;

            xCount = 10;
            yCount = 2;
            zCount = 10;

            Vector3[] vertices;
            int[] indices;
            ModelDataExtractor.GetVerticesAndIndicesFromModel(game.Content.Load<Model>("fish"), out vertices, out indices);
            var meshShape = new InstancedMeshShape(vertices, indices);

            for (int i = 0; i < xCount; i++)
            {
                for (int j = 0; j < yCount; j++)
                {
                    for (int k = 0; k < zCount; k++)
                    {
                        //Create a transform and the instance of the mesh.
                        var transform = new AffineTransform(
                            new Vector3((float)random.NextDouble() * 6 + .5f, (float)random.NextDouble() * 6 + .5f, (float)random.NextDouble() * 6 + .5f),
                             Quaternion.CreateFromAxisAngle(Vector3.Normalize(new Vector3((float)random.NextDouble(), (float)random.NextDouble(), (float)random.NextDouble())), (float)random.NextDouble() * 100),
                            new Vector3(i * xSpacing - xCount * xSpacing * .5f, j * ySpacing + 50, k * zSpacing - zCount * zSpacing * .5f));
                        var mesh = new InstancedMesh(meshShape, transform);
                        //Making the triangles one-sided makes collision detection a bit more robust, since the backsides of triangles won't try to collide with things
                        //and 'pull' them back into the mesh.
                        mesh.Sidedness = TriangleSidedness.Counterclockwise;
                        collidables.Add(mesh);
                    }
                }
            }

            var ground = new ConvexCollidable<BoxShape>(new BoxShape(200, 1, 200));
            ground.WorldTransform = new RigidTransform(new Vector3(0, -3, 0), Quaternion.Identity);
            collidables.Add(ground);

            var group = new StaticGroup(collidables);
            var removed = new RawList<Collidable>();
            var contained = new RawList<Collidable>();
            group.Shape.CollidableTree.CollectLeaves(contained);
            for (int i = 0; i < 100000; ++i)
            {
                for (int collidableIndex = contained.Count - 1; collidableIndex >= 0; --collidableIndex)
                {
                    if (random.NextDouble() < 0.6)
                    {
                        group.Shape.Remove(contained[collidableIndex]);
                        removed.Add(contained[collidableIndex]);
                        contained.FastRemoveAt(collidableIndex);
                    }
                }

                for (int collidableIndex = removed.Count - 1; collidableIndex >= 0; --collidableIndex)
                {
                    if (random.NextDouble() < 0.4)
                    {
                        group.Shape.Add(removed[collidableIndex]);
                        contained.Add(removed[collidableIndex]);
                        removed.FastRemoveAt(collidableIndex);
                    }
                }
            }

            for (int i = 0; i < contained.Count; ++i)
            {
                game.ModelDrawer.Add(contained[i]);
            }
            Space.Add(group);




            //Create a bunch of dynamic boxes to drop on the staticswarm.
            xCount = 8;
            yCount = 3;
            zCount = 8;
            xSpacing = 3f;
            ySpacing = 5f;
            zSpacing = 3f;
            for (int i = 0; i < xCount; i++)
                for (int j = 0; j < zCount; j++)
                    for (int k = 0; k < yCount; k++)
                    {
                        Space.Add(new Box(new Vector3(
                                                 xSpacing * i - (xCount - 1) * xSpacing / 2f,
                                                 100 + k * (ySpacing),
                                                 2 + zSpacing * j - (zCount - 1) * zSpacing / 2f),
                                             1, 1, 1, 10));
                    }




            game.Camera.Position = new Vector3(0, 60, 90);
        }
        /// <summary>
        /// Constructs a new demo.
        /// </summary>
        /// <param name="game">Game owning this demo.</param>
        public CharacterStressierTestDemo(DemosGame game)
            : base(game)
        {
            //Load in mesh data and create the group.
            Vector3[] staticTriangleVertices;
            int[] staticTriangleIndices;

            var playgroundModel = game.Content.Load<Model>("playground");
            //This is a little convenience method used to extract vertices and indices from a model.
            //It doesn't do anything special; any approach that gets valid vertices and indices will work.
            ModelDataExtractor.GetVerticesAndIndicesFromModel(playgroundModel, out staticTriangleVertices, out staticTriangleIndices);
            var meshShape = new InstancedMeshShape(staticTriangleVertices, staticTriangleIndices);
            var meshes = new List<Collidable>();

            var xSpacing = 400;
            var ySpacing = 400;
            var xCount = 11;
            var yCount = 11;
            for (int i = 0; i < xCount; i++)
            {
                for (int j = 0; j < yCount; j++)
                {
                    var staticMesh = new InstancedMesh(meshShape, new AffineTransform(Matrix3x3.Identity, new Vector3(-xSpacing * (xCount - 1) / 2 + i * xSpacing, 0, -ySpacing * (yCount - 1) / 2 + j * ySpacing)));
                    staticMesh.Sidedness = TriangleSidedness.Counterclockwise;
                    Space.Add(staticMesh);
                    //meshes.Add(staticMesh);
                    game.ModelDrawer.Add(staticMesh);
                }
            }
            //var group = new StaticGroup(meshes);
            //Space.Add(group);

            //To demonstrate, we'll be creating a set of static objects and giving them to a group to manage.
            var collidables = new List<Collidable>();

            //Start with a whole bunch of boxes.  These are entity collidables, but without entities!
            xSpacing = 25;
            ySpacing = 16;
            float zSpacing = 25;

            xCount = 25;
            yCount = 7;
            int zCount = 25;

            var random = new Random();
            for (int i = 0; i < xCount; i++)
            {
                for (int j = 0; j < yCount; j++)
                {
                    for (int k = 0; k < zCount; k++)
                    {
                        //Create a transform and the instance of the mesh.
                        var collidable = new ConvexCollidable<BoxShape>(new BoxShape((float)random.NextDouble() * 25 + 5.5f, (float)random.NextDouble() * 25 + 5.5f, (float)random.NextDouble() * 25 + 5.5f));

                        //This EntityCollidable isn't associated with an entity, so we must manually tell it where to sit by setting the WorldTransform.
                        //This also updates its bounding box.
                        collidable.WorldTransform = new RigidTransform(
                            new Vector3(i * xSpacing - xCount * xSpacing * .5f, j * ySpacing + -50, k * zSpacing - zCount * zSpacing * .5f),
                            Quaternion.CreateFromAxisAngle(Vector3.Normalize(new Vector3((float)random.NextDouble(), (float)random.NextDouble(), (float)random.NextDouble())), (float)random.NextDouble() * 100));

                        collidables.Add(collidable);
                        game.ModelDrawer.Add(collidable);
                    }
                }
            }
            var group = new StaticGroup(collidables);
            Space.Add(group);

            //Now drop the characters on it!
            var numColumns = 16;
            var numRows = 16;
            var numHigh = 16;
            float separation = 24;

            for (int i = 0; i < numRows; i++)
                for (int j = 0; j < numColumns; j++)
                    for (int k = 0; k < numHigh; k++)
                    {
                        var character = new CharacterController();
                        character.Body.Position =
                            new Vector3(
                            separation * i - numRows * separation / 2,
                            50f + k * separation,
                            separation * j - numColumns * separation / 2);

                        characters.Add(character);

                        Space.Add(character);
                    }

            game.Camera.Position = new Vector3(0, 10, 40);

            //Dump some boxes on top of the characters for fun.
            numColumns = 16;
            numRows = 16;
            numHigh = 8;
            separation = 24;
            for (int i = 0; i < numRows; i++)
                for (int j = 0; j < numColumns; j++)
                    for (int k = 0; k < numHigh; k++)
                    {
                        var toAdd = new Box(
                            new Vector3(
                            separation * i - numRows * separation / 2,
                            52f + k * separation,
                            separation * j - numColumns * separation / 2),
                            0.8f, 0.8f, 0.8f, 15);
                        toAdd.PositionUpdateMode = BEPUphysics.PositionUpdating.PositionUpdateMode.Continuous;

                        Space.Add(toAdd);
                    }
        }