code: double getChainPerimeter(int chain) {
  		int chainIndex = getChainIndex_(chain);
  		double v = m_chainPerimeters.read(chainIndex);
  		if (NumberUtils.isNaN(v)) {
  			updateChainAreaAndPerimeter_(chain);
  			v = m_chainPerimeters.read(chainIndex);
  		}
  
  		return v;
  	}
docs: universe chain.