/// <inheritdoc/>
public override int Contribution(int u, Graph g, ICycle cycle, BitArray cyclic)
{
if (!cyclic[u])
{
return(-1);
}
IAtom atom = g.GetAtom(u);
Element elem = atom.Element;
// the element isn't allow to be aromatic (Daylight spec)
if (!elem.IsAromatic(Element.AromaticSpecification.Daylight) || elem == Element.Unknown)
{
return(-1);
}
// count cyclic and acyclic double bonds
int nCyclic = 0, nAcyclic = 0;
int deg = g.Degree(u) + g.ImplHCount(u);
Edge acyclic = null;
int sum = 0;
foreach (var e in g.GetEdges(u))
{
sum += e.Bond.Order;
if (e.Bond.Order == 2)
{
if (!cyclic[e.Other(u)])
{
nAcyclic++;
acyclic = e;
}
else
{
nCyclic++;
}
}
}
int charge = atom.Charge;
int valence = sum + g.ImplHCount(u);
if (!atom.Element.Verify(valence, charge))
{
return(-1);
}
if (deg > 3)
{
return(-1);
}
if (nCyclic > 1)
{
return(-1);
}
if (nCyclic == 1 && nAcyclic == 1)
{
// [P|N](=O)(=*)* - note arsenic not allowed
if ((elem == Nitrogen || elem == Phosphorus) &&
g.GetAtom(acyclic.Other(u)).Element == Oxygen)
{
return(1);
}
return(-1);
}
else if (nCyclic == 1 && nAcyclic == 0)
{
// any element (except Arsenic) with a single cyclic double bond
// contributes 1 p electron
return(elem != Arsenic ? 1 : -1);
}
else if (nCyclic == 0 && nAcyclic == 1)
{
// a cyclic exo-cyclic double bond - how many electrons determine
// by AcyclicContribution()
return(AcyclicContribution(atom, g.GetAtom(acyclic.Other(u)), charge));
}
else if (nCyclic == 0 && nAcyclic == 0 && charge > -3)
{
// no double bonds - do we have any lone pairs to contribute?
int v = Valence(elem, charge);
if (v - sum >= 2 && charge <= 0)
{
return(2);
}
if (charge == 1 && atom.Element == Carbon)
{
return(0);
}
}
return(-1);
}