rdkit/Code/GraphMol/CIPLabeler/Mancude.cpp

//
//
//  Copyright (C) 2020 Schrödinger, 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.
//

///
/// Utilities to handle atomic numbers in Mancude (maximum number
/// of noncumulative double bonds) rings
///
/// This guarantees that aromatic rings containing heteroatoms
/// are always resolved in the same way
///

#include <list>
#include <vector>

#include "Mancude.h"
#include "CIPMol.h"

namespace RDKit {

namespace CIPLabeler {

namespace {

// Initialize atom types for ring atoms, looking at connectivity,
// bond orders, atomic number and charge.
bool SeedTypes(std::vector<Type> &types, const CIPMol &mol) {
  bool result = false;
  for (const auto &atom : mol.atoms()) {
    const int aidx = atom->getIdx();

    // check ring
    int btypes = atom->getTotalNumHs();
    bool ring = false;
    for (const auto &bond : mol.getBonds(atom)) {
      // Given the possible types we have, we only care
      // for single and double bonds which are in rings.
      switch (mol.getBondOrder(bond)) {
        case 1:
          btypes += 0x00000001;
          break;
        case 2:
          btypes += 0x00000100;
          break;
        default:
          btypes += 0x01000000;
          break;
      }
      if (mol.isInRing(bond)) {
        ring = true;
      }
    }
    if (ring) {
      int q = atom->getFormalCharge();
      switch (atom->getAtomicNum()) {
        case 6:   // C
        case 14:  // Si
        case 32:  // Ge
          if (q == 0 && btypes == 0x0102) {
            types[aidx] = Type::Cv4D3;
          } else if (q == -1 && btypes == 0x0003) {
            types[aidx] = Type::Cv3D3Minus;
            result = true;
          }
          break;
        case 7:   // N
        case 15:  // P
        case 33:  // As
          if (q == 0 && btypes == 0x0101) {
            types[aidx] = Type::Nv3D2;
            result = true;
          } else if (q == -1 && btypes == 0x0002) {
            types[aidx] = Type::Nv2D2Minus;
            result = true;
          } else if (q == +1 && btypes == 0x0102) {
            types[aidx] = Type::Nv4D3Plus;
            result = true;
          }
          break;
        case 8:  // O
          if (q == 1 && btypes == 0x0101) {
            types[aidx] = Type::Ov3D2Plus;
            result = true;
          }
          break;
      }
    }
  }
  return result;
}

// Reset atom types for atoms that have been given a type,
// but cannot be part of a mancude system (more than one
// typed neighbor is required for resonance to be possible)
void RelaxTypes(std::vector<Type> &types, const CIPMol &mol) {
  std::list<Atom *> queue;
  auto counts = std::vector<int>(mol.getNumAtoms());
  for (auto atom : mol.atoms()) {
    const auto aidx = atom->getIdx();
    for (const auto &nbr : mol.getNeighbors(atom)) {
      if (types[nbr->getIdx()] != Type::Other) {
        ++counts[aidx];
      }
    }

    if (counts[aidx] == 1) {
      queue.push_back(atom);
    }
  }

  for (const auto &atom : queue) {
    const auto aidx = atom->getIdx();
    if (types[aidx] != Type::Other) {
      types[aidx] = Type::Other;

      for (auto &nbr : mol.getNeighbors(atom)) {
        auto nbridx = nbr->getIdx();
        --counts[nbridx];
        if (counts[nbridx] == 1) {
          queue.push_back(nbr);
        }
      }
    }
  }
}

// Mark mol atoms in the same resonant part of the mol.
void VisitPart(std::vector<int> &parts, const std::vector<Type> &types,
               int part, Atom *atom, const CIPMol &mol) {
  Atom *next;
  do {
    next = nullptr;
    for (auto &bond : mol.getBonds(atom)) {
      if (!mol.isInRing(bond)) {
        continue;
      }

      auto nbr = bond->getOtherAtom(atom);
      int aidx = nbr->getIdx();

      if (parts[aidx] == 0 && types[aidx] != Type::Other) {
        parts[aidx] = part;
        if (next != nullptr) {
          VisitPart(parts, types, part, nbr, mol);
        } else {
          next = nbr;
        }
      }
    }
    atom = next;
  } while (atom != nullptr);
}

// Classify mol atoms into different resonant parts of the mol.
int VisitParts(std::vector<int> &parts, const std::vector<Type> &types,
               const CIPMol &mol) {
  int numparts = 0;
  for (auto &atom : mol.atoms()) {
    int aidx = atom->getIdx();
    if (parts[aidx] == 0 && types[aidx] != Type::Other) {
      parts[aidx] = ++numparts;
      VisitPart(parts, types, parts[aidx], atom, mol);
    }
  }
  return numparts;
}

}  // namespace

std::vector<boost::rational<int>> calcFracAtomNums(const CIPMol &mol) {
  const auto num_atoms = mol.getNumAtoms();
  std::vector<boost::rational<int>> fractions;
  fractions.reserve(num_atoms);
  for (const auto &atom : mol.atoms()) {
    fractions.emplace_back(atom->getAtomicNum(), 1);
  }

  // Mark all atoms which are potentially part of a resonance system.
  auto types = std::vector<Type>(num_atoms, Type::Other);
  if (SeedTypes(types, mol)) {
    // Filter out atoms which cannot be resonant because
    // of not having the proper environment.
    RelaxTypes(types, mol);

    // Find resonant systems: parts stores the ids of the
    // systems each atom is involved in.
    auto parts = std::vector<int>(num_atoms);
    int numparts = VisitParts(parts, types, mol);

    auto resparts = std::vector<int>(numparts);
    int numres = 0;

    if (numparts > 0) {
      for (auto i = 0u; i < num_atoms; ++i) {
        if (parts[i] == 0) {
          continue;
        }
        auto atom = mol.getAtom(i);

        // Find resonant structures caused by relocation of a negative charge.
        if (types[i] == Type::Cv3D3Minus || types[i] == Type::Nv2D2Minus) {
          int j = 0;
          for (; j < numres; ++j) {
            if (resparts[j] == parts[i]) {
              break;
            }
          }
          if (j >= numres) {
            resparts[numres] = parts[i];
            ++numres;
          }
        }

        int numerator = 0;
        int denominator = 0;
        for (const auto &nbr : mol.getNeighbors(atom)) {
          if (parts[nbr->getIdx()] == parts[i]) {
            numerator += nbr->getAtomicNum();
            ++denominator;
          }
        }

        // boost::rational does not accept 0 as denominator.
        if (denominator == 0) {
          fractions[i].assign(0, 1);
        } else {
          fractions[i].assign(numerator, denominator);
        }
      }
    }

    // If there are any resonant structures due to negative charges,
    // recalculate the average atomic number considering relocation
    // of the charge through higher order bonds.
    if (numres > 0) {
      for (int j = 0; j < numres; ++j) {
        int numerator = 0;
        int denominator = 0;
        int part = resparts[j];
        for (auto i = 0u; i < num_atoms; ++i) {
          if (parts[i] == part) {
            // boost::rational does not accept 0 as denominator
            if (denominator == 0) {
              fractions[i].assign(0, 1);
            } else {
              fractions[i].assign(numerator, denominator);
            }

            ++denominator;
            auto atom = mol.getAtom(i);
            for (auto &bond : mol.getBonds(atom)) {
              auto nbr = bond->getOtherAtom(atom);
              int bord = mol.getBondOrder(bond);
              if (bord > 1 && parts[nbr->getIdx()] == part) {
                numerator += (bord - 1) * nbr->getAtomicNum();
              }
            }
          }
        }
      }
    }
  }
  return fractions;
}

}  // namespace CIPLabeler
}  // namespace RDKit