@@ -460,7 +460,12 @@ void kekulizeFused(RWMol &mol, const VECT_INT_VECT &arings,
     // number of backTracks) and we still need to backtrack
     // can't kekulize this thing
     std::ostringstream errout;
-    errout << "Can't kekulize mol " << std::endl;
+    errout << "Can't kekulize mol.";
+    errout << "  Atoms remaining:";
+    for (unsigned int i = 0; i < nats; ++i) {
+      if (dBndCands[i]) errout << " " << i;
+    }
+    errout << std::endl;
     std::string msg = errout.str();
     BOOST_LOG(rdErrorLog) << msg << std::endl;
     throw MolSanitizeException(msg);
// $Id$
//
//  Copyright (C) 2001-2009 Greg Landrum and Rational Discovery LLC
//
//   @@ All Rights Reserved @@
//  This file is part of the RDKit.
//  The contents are covered by the terms of the BSD license
//  which is included in the file license.txt, found at the root
//  of the RDKit source tree.
//
#include <GraphMol/RDKitBase.h>
#include <GraphMol/Canon.h>
#include <GraphMol/Rings.h>
#include <GraphMol/SanitException.h>
#include <RDGeneral/RDLog.h>
#include <boost/dynamic_bitset.hpp>

// end of namespace Kekulize
namespace RDKit {
// Local utility namespace
namespace {

// Determine whether or not a molecule is to the left of Carbon
bool isEarlyAtom(int atomicNum) {
  // FIX: this is duplicated from Atom.cpp. It should be defined once!
  return (4 - PeriodicTable::getTable()->getNouterElecs(atomicNum)) > 0;
}

void backTrack(RWMol &mol, INT_INT_DEQ_MAP &options, int lastOpt,
               INT_VECT &done, INT_DEQUE &aqueue,
               boost::dynamic_bitset<> &dBndCands,
               boost::dynamic_bitset<> &dBndAdds) {
  RDUNUSED_PARAM(options);
  // so we made a wrong turn at the lastOpt
  // remove on done list that comes after the lastOpt including itself

  INT_VECT_I ei = std::find(done.begin(), done.end(), lastOpt);
  INT_VECT tdone;
  tdone.insert(tdone.end(), done.begin(), ei);

  INT_VECT_CRI eri = std::find(done.rbegin(), done.rend(), lastOpt);
  ++eri;
  // and push them back onto the stack
  for (INT_VECT_CRI ri = done.rbegin(); ri != eri; ++ri) {
    aqueue.push_front(*ri);
  }

  // remove any double bonds that were add since we passed through lastOpt
  Bond *bnd;
  unsigned int nbnds = mol.getNumBonds();
  for (unsigned int bi = 0; bi < nbnds; bi++) {
    if (dBndAdds[bi]) {
      bnd = mol.getBondWithIdx(bi);
      int aid1 = bnd->getBeginAtomIdx();
      int aid2 = bnd->getEndAtomIdx();
      // if one of these atoms has been dealt with before lastOpt
      // we don't have to chnage the double bond addition
      if ((std::find(tdone.begin(), tdone.end(), aid1) == tdone.end()) &&
          (std::find(tdone.begin(), tdone.end(), aid2) == tdone.end())) {
        // otherwise strip the double bond and set it back to single
        // and add the atoms to candidate for double bonds
        dBndAdds[bi] = 0;
        bnd->setBondType(Bond::SINGLE);
        dBndCands[aid1] = 1;
        dBndCands[aid2] = 1;
      }
    }
  }
  done = tdone;
}

void markDbondCands(RWMol &mol, const INT_VECT &allAtms,
                    boost::dynamic_bitset<> &dBndCands, INT_VECT &questions,
                    INT_VECT &done) {
  // ok this function does more than mark atoms that are candidates for
  // double bonds during kekulization
  // - check that an non aromatic atom does not have any aromatic bonds
  // - marks all aromatic bonds to single bonds
  // - marks atoms that can take a double bond

  bool hasAromaticOrDummyAtom = false;
  for (INT_VECT_CI adx = allAtms.begin(); adx != allAtms.end(); ++adx) {
    if (mol.getAtomWithIdx(*adx)->getIsAromatic() ||
        !mol.getAtomWithIdx(*adx)->getAtomicNum()) {
      hasAromaticOrDummyAtom = true;
      break;
    }
  }
  // if there's not at least one atom in the ring that's
  // marked as being aromatic or a dummy,
  // there's no point in continuing:
  if (!hasAromaticOrDummyAtom) return;

  std::vector<Bond *> makeSingle;

  for (INT_VECT_CI adx = allAtms.begin(); adx != allAtms.end(); ++adx) {
    Atom *at = mol.getAtomWithIdx(*adx);

    if (!at->getIsAromatic() && at->getAtomicNum()) {
      done.push_back(*adx);
      // make sure all the bonds on this atom are also non aromatic
      // i.e. can't have aromatic bond onto a non-aromatic atom
      RWMol::OEDGE_ITER beg, end;
      boost::tie(beg, end) = mol.getAtomBonds(at);
      while (beg != end) {
        // ok we can't have an aromatic atom
        if (mol[*beg]->getIsAromatic()) {
          std::ostringstream errout;
          errout << "Aromatic bonds on non aromatic atom " << at->getIdx();
          std::string msg = errout.str();
          BOOST_LOG(rdErrorLog) << msg << std::endl;
          throw MolSanitizeException(msg);
        }
        ++beg;
      }
      continue;
    }

    // count the number of neighbors connected with single,
    // double, or aromatic bonds. Along the way, mark
    // bonds that we will later mark as being single:
    int sbo = 0;
    RWMol::OEDGE_ITER beg, end;
    boost::tie(beg, end) = mol.getAtomBonds(at);
    while (beg != end) {
      Bond *bond = mol[*beg].get();
      if (bond->getIsAromatic() && (bond->getBondType() == Bond::SINGLE ||
                                    bond->getBondType() == Bond::DOUBLE ||
                                    bond->getBondType() == Bond::AROMATIC)) {
        ++sbo;
        // mark this bond to be marked single later
        // we don't want to do right now because it can screw-up the
        // valence calculation to determine the number of hydrogens below
        makeSingle.push_back(bond);
      } else {
        sbo += (int)bond->getValenceContrib(at);
      }
      ++beg;
    }

    if (!at->getAtomicNum()) {
      // dummies always start as candidates to have a double bond:
      dBndCands[*adx] = 1;
      // but they don't have to have one, so mark them as questionable:
      questions.push_back(*adx);
    } else {
      // for non dummies, it's a bit more work to figure out if they
      // can take a double bond:

      sbo += at->getTotalNumHs();
      int dv = PeriodicTable::getTable()->getDefaultValence(at->getAtomicNum());
      int chrg = at->getFormalCharge();
      if (isEarlyAtom(at->getAtomicNum())) chrg *= -1;  // fix for GitHub #65
      // special case for carbon - see GitHub #539
      if (at->getAtomicNum() == 6 && chrg > 0) chrg = -chrg;
      dv += chrg;
      int tbo = at->getTotalValence();
      int nRadicals = at->getNumRadicalElectrons();
      int totalDegree = at->getDegree() + at->getImplicitValence();

      const INT_VECT &valList =
          PeriodicTable::getTable()->getValenceList(at->getAtomicNum());
      unsigned int vi = 1;

      while (tbo > dv && vi < valList.size() && valList[vi] > 0) {
        dv = valList[vi] + chrg;
        ++vi;
      }

      // std::cerr<<"  kek: "<<at->getIdx()<<" tbo:"<<tbo<<" sbo:"<<sbo<<"
      // dv:"<<dv<<" totalDegree:"<<totalDegree<<"
      // nRadicals:"<<nRadicals<<std::endl;
      if (totalDegree + nRadicals >= dv) {
        // if our degree + nRadicals exceeds the default valence,
        // there's no way we can take a double bond, just continue.
        continue;
      }

      // we're a candidate if our total current bond order + nRadicals + 1
      // matches the valence state
      // (including nRadicals here was SF.net issue 3349243)
      if (dv == (sbo + 1 + nRadicals)) {
        dBndCands[*adx] = 1;
      } else if (!nRadicals && at->getNoImplicit() && dv == (sbo + 2)) {
        // special case: there is currently no radical on the atom, but if
        // if we allow one then this is a candidate:
        dBndCands[*adx] = 1;
      }
    }
  }  // loop over all atoms in the fused system

  // now turn all the aromatic bond in this fused system to single
  for (std::vector<Bond *>::iterator bi = makeSingle.begin();
       bi != makeSingle.end(); ++bi) {
    (*bi)->setBondType(Bond::SINGLE);
  }
}

bool kekulizeWorker(RWMol &mol, const INT_VECT &allAtms,
                    boost::dynamic_bitset<> dBndCands,
                    boost::dynamic_bitset<> dBndAdds, INT_VECT done,
                    unsigned int maxBackTracks) {
  INT_DEQUE astack;
  INT_INT_DEQ_MAP options;
  int lastOpt = -1;

  // ok the algorithm goes something like this
  // - start with an atom that has been marked aromatic before
  // - check if it can have a double bond
  // - add its neighbors to the stack
  // - check if one of its neighbors can also have a double bond
  // - if yes add a double bond.
  // - if multiple neighbors can have double bonds - add them to a
  //   options stack we may have to retrace out path if we chose the
  //   wrong neighbor to add the double bond
  // - if double bond added update the candidates for double bond
  // - move to the next atom on the stack and repeat the process
  // - if an atom that can have multiple a double bond has no
  //   neighbors that can take double bond - we made a mistake
  //   earlier by picking a wrong candidate for double bond
  // - in this case back track to where we made the mistake

  int curr;
  INT_DEQUE btmoves;
  unsigned int numBT = 0;  // number of back tracks so far
  while ((done.size() < allAtms.size()) || (astack.size() > 0)) {
    // pick a curr atom to work with
    if (astack.size() > 0) {
      curr = astack.front();
      astack.pop_front();
    } else {
      for (INT_VECT_CI ai = allAtms.begin(); ai != allAtms.end(); ++ai) {
        if (std::find(done.begin(), done.end(), (*ai)) == done.end()) {
          curr = (*ai);
          break;
        }
      }
    }
    done.push_back(curr);

    // loop over the neighbors if we can add double bonds or
    // simply push them onto the stack
    INT_DEQUE opts;
    bool cCand = false;
    if (dBndCands[curr]) {
      cCand = true;
    }
    int ncnd;
    // if we are here because of backtracking
    if (options.find(curr) != options.end()) {
      opts = options[curr];
      CHECK_INVARIANT(opts.size() > 0, "");
    } else {
      RWMol::ADJ_ITER nbrIdx, endNbrs;
      boost::tie(nbrIdx, endNbrs) =
          mol.getAtomNeighbors(mol.getAtomWithIdx(curr));
      while (nbrIdx != endNbrs) {
        // ignore if the neighbor has already been dealt with before
        if (std::find(done.begin(), done.end(), static_cast<int>(*nbrIdx)) !=
            done.end()) {
          ++nbrIdx;
          continue;
        }
        // ignore if the neighbor is not part of the fused system
        if (std::find(allAtms.begin(), allAtms.end(),
                      static_cast<int>(*nbrIdx)) == allAtms.end()) {
          ++nbrIdx;
          continue;
        }

        // if the neighbor is not on the stack add it
        if (std::find(astack.begin(), astack.end(),
                      static_cast<int>(*nbrIdx)) == astack.end()) {
          astack.push_back(rdcast<int>(*nbrIdx));
        }

        // check if the neighbor is also a candidate for a double bond
        // the refinement that we'll make to the candidate check we've already
        // done is to make sure that the bond is either flagged as aromatic
        // or involves a dummy atom. This was Issue 3525076.
        // This fix is not really 100% of the way there: a situation like
        // that for Issue 3525076 but involving a dummy atom in the cage
        // could lead to the same failure. The full fix would require
        // a fairly detailed analysis of all bonds in the molecule to determine
        // which of them is eligible to be converted.
        if (cCand && dBndCands[*nbrIdx] &&
            (mol.getBondBetweenAtoms(curr, *nbrIdx)->getIsAromatic() ||
             mol.getAtomWithIdx(curr)->getAtomicNum() == 0 ||
             mol.getAtomWithIdx(*nbrIdx)->getAtomicNum() == 0)) {
          opts.push_back(rdcast<int>(*nbrIdx));
        }  // end of curr atoms can have a double bond
        ++nbrIdx;
      }  // end of looping over neighbors
    }
    // now add a double bond from current to one of the neighbors if we can
    if (cCand) {
      if (opts.size() > 0) {
        ncnd = opts.front();
        opts.pop_front();
        Bond *bnd = mol.getBondBetweenAtoms(curr, ncnd);
        bnd->setBondType(Bond::DOUBLE);

        // remove current and the neighbor from the dBndCands list
        dBndCands[curr] = 0;
        dBndCands[ncnd] = 0;

        // add them to the list of bonds to which have been made double
        dBndAdds[bnd->getIdx()] = 1;

        // if this is an atom we previously visted and picked we
        // simply tried a different option now, overwrite the options
        // stored for this atoms
        if (options.find(curr) != options.end()) {
          if (opts.size() == 0) {
            options.erase(curr);
            btmoves.pop_back();
            if (btmoves.size() > 0) {
              lastOpt = btmoves.back();
            } else {
              lastOpt = -1;
            }
          } else {
            options[curr] = opts;
          }
        } else {
          // this is new atoms we are trying and have other
          // neighbors as options to add double bond store this to
          // the options stack, we may have made a mistake in
          // which one we chose and have to return here
          if (opts.size() > 0) {
            lastOpt = curr;
            btmoves.push_back(lastOpt);
            options[curr] = opts;
          }
        }

      }  // end of adding a double bond
      else {
        // we have an atom that should be getting a double bond
        // but none of the neighbors can take one. Most likely
        // because of a wrong choice earlier so back track
        if ((lastOpt >= 0) && (numBT < maxBackTracks)) {
          // std::cerr << "PRE BACKTRACK" << std::endl;
          // mol.debugMol(std::cerr);
          backTrack(mol, options, lastOpt, done, astack, dBndCands, dBndAdds);
          // std::cerr << "POST BACKTRACK" << std::endl;
          // mol.debugMol(std::cerr);
          numBT++;
        } else {
          return false;
        }
      }  // end of else try to backtrack
    }    // end of curr atom atom being a cand for double bond
  }      // end of while we are not done with all atoms
  return true;
}

class QuestionEnumerator {
 public:
  QuestionEnumerator(const INT_VECT &questions)
      : d_questions(questions), d_pos(1){};
  INT_VECT next() {
    INT_VECT res;
    if (d_pos >= (0x1u << d_questions.size())) {
      return res;
    }
    for (unsigned int i = 0; i < d_questions.size(); ++i) {
      if (d_pos & (0x1u << i)) {
        res.push_back(d_questions[i]);
      }
    }
    ++d_pos;
    return res;
  };

 private:
  INT_VECT d_questions;
  unsigned int d_pos;
};

bool permuteDummiesAndKekulize(RWMol &mol, const INT_VECT &allAtms,
                               boost::dynamic_bitset<> dBndCands,
                               INT_VECT &questions,
                               unsigned int maxBackTracks) {
  boost::dynamic_bitset<> atomsInPlay(mol.getNumAtoms());
  for (INT_VECT_CI ai = allAtms.begin(); ai != allAtms.end(); ++ai) {
    atomsInPlay[*ai] = 1;
  }
  bool kekulized = false;
  QuestionEnumerator qEnum(questions);
  while (!kekulized && questions.size()) {
    boost::dynamic_bitset<> dBndAdds(mol.getNumBonds());
    INT_VECT done;
#if 1
    // reset the state: all aromatic bonds are remarked to single:
    for (RWMol::BondIterator bi = mol.beginBonds(); bi != mol.endBonds();
         ++bi) {
      if ((*bi)->getIsAromatic() && (*bi)->getBondType() != Bond::SINGLE &&
          atomsInPlay[(*bi)->getBeginAtomIdx()] &&
          atomsInPlay[(*bi)->getEndAtomIdx()]) {
        (*bi)->setBondType(Bond::SINGLE);
      }
    }
#endif
    // pick a new permutation of the questionable atoms:
    const INT_VECT &switchOff = qEnum.next();
    if (!switchOff.size()) break;
    boost::dynamic_bitset<> tCands = dBndCands;
    for (INT_VECT_CI it = switchOff.begin(); it != switchOff.end(); ++it) {
      tCands[*it] = 0;
    }
#if 0
        std::cerr<<"permute: ";
        for (boost::dynamic_bitset<>::size_type i = 0; i < tCands.size(); ++i){
          std::cerr << tCands[i];
        }
        std::cerr<<std::endl;
#endif
    // try kekulizing again:
    kekulized =
        kekulizeWorker(mol, allAtms, tCands, dBndAdds, done, maxBackTracks);
  }
  return kekulized;
}

void kekulizeFused(RWMol &mol, const VECT_INT_VECT &arings,
                   unsigned int maxBackTracks) {
  // get all the atoms in the ring system
  INT_VECT allAtms;
  Union(arings, allAtms);

  // get all the atoms that are candidates to receive a double bond
  // also mark atoms in the fused system that are not aromatic to begin with
  // as done. Mark all the bonds that are part of the aromatic system
  // to be single bonds
  INT_VECT done;
  INT_VECT questions;
  unsigned int nats = mol.getNumAtoms();
  unsigned int nbnds = mol.getNumBonds();
  boost::dynamic_bitset<> dBndCands(nats);
  boost::dynamic_bitset<> dBndAdds(nbnds);

  markDbondCands(mol, allAtms, dBndCands, questions, done);
#if 0
      std::cerr << "candidates: ";
      for(int i=0;i<nats;++i) std::cerr << dBndCands[i];
      std::cerr << std::endl;
#endif

  bool kekulized;
  kekulized =
      kekulizeWorker(mol, allAtms, dBndCands, dBndAdds, done, maxBackTracks);
  if (!kekulized && questions.size()) {
    // we failed, but there are some dummy atoms we can try permuting.
    kekulized = permuteDummiesAndKekulize(mol, allAtms, dBndCands, questions,
                                          maxBackTracks);
  }
  if (!kekulized) {
    // we exhausted all option (or crossed the allowed
    // number of backTracks) and we still need to backtrack
    // can't kekulize this thing
    std::ostringstream errout;
    errout << "Can't kekulize mol " << std::endl;
    std::string msg = errout.str();
    BOOST_LOG(rdErrorLog) << msg << std::endl;
    throw MolSanitizeException(msg);
  }
}
}  // end of utility namespace

namespace MolOps {
void Kekulize(RWMol &mol, bool markAtomsBonds, unsigned int maxBackTracks) {
  // there's no point doing kekulization if there are no aromatic bonds:
  bool foundAromatic = false;
  for (ROMol::BondIterator bi = mol.beginBonds();
       bi != mol.endBonds() && !foundAromatic; ++bi) {
    if ((*bi)->getIsAromatic()) foundAromatic = true;
  }

  // before everything do implicit valence calculation and store them
  // we will repeat after kekulization and compare for the sake of error
  // checking
  INT_VECT valences;
  int numAtoms = mol.getNumAtoms();
  valences.reserve(numAtoms);
  for (ROMol::AtomIterator ai = mol.beginAtoms(); ai != mol.endAtoms(); ++ai) {
    (*ai)->calcImplicitValence(false);
    valences.push_back((*ai)->getTotalValence());
    if (!foundAromatic && (*ai)->getIsAromatic()) foundAromatic = true;
  }
  if (!foundAromatic) return;

  // A bit on the state of the molecule at this point
  // - aromatic and non aromatic atoms and bonds may be mixed up

  // - for all aromatic bonds it is assumed that that both the following
  //   are true:
  //       - getIsAromatic returns true
  //       - getBondType return aromatic
  // - all aromatic atoms return true for "getIsAromatic"

  // first find the all the simple rings in the molecule
  VECT_INT_VECT arings;
  if (mol.getRingInfo()->isInitialized()) {
    arings = mol.getRingInfo()->atomRings();
  } else {
    MolOps::findSSSR(mol, arings);
  }

  VECT_INT_VECT brings;
  brings = mol.getRingInfo()->bondRings();
  // RingUtils::convertToBonds(arings, brings, mol);

  // make a the neighbor map for the rings i.e. a ring is a
  // neighbor to another candidate ring if it shares at least
  // one bond
  // useful to figure out fused systems
  INT_INT_VECT_MAP neighMap;
  RingUtils::makeRingNeighborMap(brings, neighMap);

  int curr = 0;
  int cnrs = rdcast<int>(arings.size());
  boost::dynamic_bitset<> fusDone(cnrs);
  while (curr < cnrs) {
    INT_VECT fused;
    RingUtils::pickFusedRings(curr, neighMap, fused, fusDone);
    VECT_INT_VECT frings;
    for (INT_VECT_CI ci = fused.begin(); ci != fused.end(); ++ci) {
      frings.push_back(arings[*ci]);
    }
    kekulizeFused(mol, frings, maxBackTracks);
    int rix;
    for (rix = 0; rix < cnrs; rix++) {
      if (!fusDone[rix]) {
        curr = rix;
        break;
      }
    }
    if (rix == cnrs) {
      break;
    }
  }

  if (markAtomsBonds) {
    // if we want the atoms and bonds to be marked non-aromatic do
    // that here.
    for (ROMol::BondIterator bi = mol.beginBonds(); bi != mol.endBonds();
         ++bi) {
      (*bi)->setIsAromatic(false);
    }
    for (ROMol::AtomIterator ai = mol.beginAtoms(); ai != mol.endAtoms();
         ++ai) {
      if ((*ai)->getIsAromatic()) {
        if (!mol.getRingInfo()->numAtomRings((*ai)->getIdx())) {
          std::ostringstream errout;
          errout << "non-ring atom " << (*ai)->getIdx() << " marked aromatic";
          std::string msg = errout.str();
          BOOST_LOG(rdErrorLog) << msg << std::endl;
          throw MolSanitizeException(msg);
        }
        (*ai)->setIsAromatic(false);
        // make sure "explicit" Hs on things like pyrroles don't hang around
        // this was Github Issue 141
        if (((*ai)->getAtomicNum() == 7 || (*ai)->getAtomicNum() == 15) &&
            (*ai)->getFormalCharge() == 0 && (*ai)->getNumExplicitHs() == 1) {
          (*ai)->setNoImplicit(false);
          (*ai)->setNumExplicitHs(0);
          (*ai)->updatePropertyCache(false);
        }
      }
    }
  }

  // ok some error checking here force a implicit valence
  // calculation that should do some error checking by itself. In
  // addition compare them to what they were before kekulizing
  int i = 0;
  for (ROMol::AtomIterator ai = mol.beginAtoms(); ai != mol.endAtoms(); ++ai) {
    int val = (*ai)->getTotalValence();
    if (val != valences[i]) {
      std::ostringstream errout;
      errout << "Kekulization somehow screwed up valence on " << (*ai)->getIdx()
             << ": " << val << "!=" << valences[i] << std::endl;
      std::string msg = errout.str();
      BOOST_LOG(rdErrorLog) << msg << std::endl;
      throw MolSanitizeException(msg);
    }
    i++;
  }
}
}  // end of namespace MolOps
}  // end of namespace RDKit

I find the "atoms remaining" confusing, are these the unkekulized atoms? I almost would like using _supplementalSmilesLabel to to actually indicate the atoms in the smiles string.