/**
   * identify additional groups that are not directly attached to amino acids.
   *
   * @param mc {@link ModifiedCompound}.
   * @param chain a {@link Chain}.
   * @return a list of added groups.
   */
  private void identifyAdditionalAttachments(
      ModifiedCompound mc, List<Group> ligands, Map<String, Chain> mapChainIdChain) {
    if (ligands.isEmpty()) {
      return;
    }

    // TODO: should the additional groups only be allowed to the identified
    // ligands or both amino acids and ligands? Currently only on ligands
    // ligands to amino acid bonds for same modification of unknown category
    // will be combined in mergeModComps()
    // TODO: how about chain-chain links?
    List<Group> identifiedGroups = new ArrayList<Group>();
    for (StructureGroup num : mc.getGroups(false)) {
      Group group;
      try {
        // String numIns = "" + num.getResidueNumber();
        // if (num.getInsCode() != null) {
        //	numIns += num.getInsCode();
        // }
        ResidueNumber resNum = new ResidueNumber();
        resNum.setChainId(num.getChainId());
        resNum.setSeqNum(num.getResidueNumber());
        resNum.setInsCode(num.getInsCode());
        // group = chain.getGroupByPDB(numIns);
        group = mapChainIdChain.get(num.getChainId()).getGroupByPDB(resNum);
      } catch (StructureException e) {
        e.printStackTrace();
        // should not happen
        continue;
      }
      identifiedGroups.add(group);
    }

    int start = 0;

    int n = identifiedGroups.size();
    while (n > start) {
      for (Group group1 : ligands) {
        for (int i = start; i < n; i++) {
          Group group2 = identifiedGroups.get(i);
          if (!identifiedGroups.contains(group1)) {
            List<Atom[]> linkedAtoms =
                StructureUtil.findAtomLinkages(group1, group2, false, bondLengthTolerance);
            if (!linkedAtoms.isEmpty()) {
              for (Atom[] atoms : linkedAtoms) {
                mc.addAtomLinkage(
                    StructureUtil.getStructureAtomLinkage(atoms[0], false, atoms[1], false));
              }
              identifiedGroups.add(group1);
              break;
            }
          }
        }
      }

      start = n;
      n = identifiedGroups.size();
    }
  }
  /**
   * Record unidentifiable atom linkages in a chain. Only linkages between two residues or one
   * residue and one ligand will be recorded.
   */
  private void recordUnidentifiableAtomLinkages(
      List<ModifiedCompound> modComps, List<Group> ligands) {

    // first put identified linkages in a map for fast query
    Set<StructureAtomLinkage> identifiedLinkages = new HashSet<StructureAtomLinkage>();
    for (ModifiedCompound mc : modComps) {
      identifiedLinkages.addAll(mc.getAtomLinkages());
    }

    // record
    // cross link
    int nRes = residues.size();
    for (int i = 0; i < nRes - 1; i++) {
      Group group1 = residues.get(i);
      for (int j = i + 1; j < nRes; j++) {
        Group group2 = residues.get(j);
        List<Atom[]> linkages =
            StructureUtil.findAtomLinkages(group1, group2, true, bondLengthTolerance);
        for (Atom[] atoms : linkages) {
          StructureAtomLinkage link =
              StructureUtil.getStructureAtomLinkage(atoms[0], true, atoms[1], true);
          unidentifiableAtomLinkages.add(link);
        }
      }
    }

    // attachment
    int nLig = ligands.size();
    for (int i = 0; i < nRes; i++) {
      Group group1 = residues.get(i);
      for (int j = 0; j < nLig; j++) {
        Group group2 = ligands.get(j);
        if (group1.equals(group2)) { // overlap between residues and ligands
          continue;
        }
        List<Atom[]> linkages =
            StructureUtil.findAtomLinkages(group1, group2, false, bondLengthTolerance);
        for (Atom[] atoms : linkages) {
          StructureAtomLinkage link =
              StructureUtil.getStructureAtomLinkage(atoms[0], true, atoms[1], false);
          unidentifiableAtomLinkages.add(link);
        }
      }
    }
  }