protected virtual void updateX(float step_time, cWorld world)
        {
            acceleration.X = force.X;
            velocity.X    += acceleration.X * step_time;

            if (acceleration.X < 0.0f)
            {
                velocity.X = AppMath.Max <float>(velocity.X, -this.maxWalkSpeed);
            }
            else if (acceleration.X > 0.0f)
            {
                velocity.X = AppMath.Min <float>(velocity.X, this.maxWalkSpeed);
            }
            else
            // if (isOnGround)
            {
                velocity.X = isOnGround ? velocity.X * Constants.GROUND_SLOW_DOWN_FACTOR
                    : velocity.X * Constants.AIR_SLOW_DOWN_FACTOR;
            }

            velocity.X = Math.Abs(velocity.X) <= 0.05f ? 0.0f : velocity.X;

            float delta = velocity.X * step_time;

            if (delta <= 0.0f)
            {
                float wallRightX;

                if (hasLeftWall2(world, delta, out wallRightX))
                {
                    position.X = wallRightX;
                    velocity.X = 0.0f;
                }
                else
                {
                    position.X += delta;
                }
            }
            else
            {
                float wallLeftX;
                if (hasRightWall2(world, delta, out wallLeftX))
                {
                    position.X = wallLeftX - Bounds.dims.X;
                    velocity.X = 0.0f;
                }
                else
                {
                    position.X += delta;
                }
            }
        }