private void CalculateFactors(ISet <Clause> parentFactors)
{
nonTrivialFactors = new HashedSet <Clause>();
IDictionary <Variable, ITerm> theta = new Dictionary <Variable, ITerm>();
var lits = new List <Literal>();
for (var i = 0; i < 2; i++)
{
lits.Clear();
if (i == 0)
{
// Look at the positive literals
lits.AddRange(positiveLiterals);
}
else
{
// Look at the negative literals
lits.AddRange(negativeLiterals);
}
for (int x = 0; x < lits.Count; x++)
{
for (int y = x + 1; y < lits.Count; y++)
{
var litX = lits[x];
var litY = lits[y];
theta.Clear();
IDictionary <Variable, ITerm> substitution = _unifier.Unify(
litX.AtomicSentence, litY.AtomicSentence, theta);
if (substitution != null)
{
var posLits = new List <Literal>();
var negLits = new List <Literal>();
if (i == 0)
{
posLits.Add(_substVisitor.Subst(substitution, litX));
}
else
{
negLits.Add(_substVisitor.Subst(substitution, litX));
}
posLits.AddRange(from pl in this.positiveLiterals
where pl != litX && pl != litY
select _substVisitor.Subst(substitution, pl));
negLits.AddRange(from nl in this.negativeLiterals
where nl != litX && nl != litY
select _substVisitor.Subst(substitution, nl));
// Ensure the non trivial factor is standardized apart
_standardizeApart.GetStandardizeApartResult(posLits, negLits, _saIndexical);
Clause c = new Clause(posLits, negLits);
c.SetProofStep(new ProofStepClauseFactor(c, this));
if (this.Immutable)
{
c.Immutable = true;
}
if (!this.IsStandardizedApartCheckRequired())
{
c.SetStandardizedApartCheckNotRequired();
}
if (parentFactors == null || !parentFactors.Contains(c))
{
c.CalculateFactors(this.nonTrivialFactors);
this.nonTrivialFactors.UnionWith(c.GetFactors());
}
}
}
}
}
factors = new HashedSet <Clause>();
// Need to add self, even though a non-trivial
// factor. See: slide 30
// http://logic.stanford.edu/classes/cs157/2008/lectures/lecture10.pdf
// for example of incompleteness when
// trivial factor not included.
factors.Add(this);
factors.UnionWith(nonTrivialFactors);
}
// Note: Applies binary resolution rule and factoring
// Note: returns a set with an empty clause if both clauses
// are empty, otherwise returns a set of binary resolvents.
public ISet <Clause> BinaryResolvents(Clause othC)
{
ISet <Clause> resolvents = new HashedSet <Clause>();
// Resolving two empty clauses
// gives you an empty clause
if (IsEmpty() && othC.IsEmpty())
{
resolvents.Add(new Clause());
return(resolvents);
}
// Ensure Standardized Apart
// Before attempting binary resolution
othC = this.SaIfRequired(othC);
var allPosLits = new List <Literal>();
var allNegLits = new List <Literal>();
allPosLits.AddRange(this.positiveLiterals);
allPosLits.AddRange(othC.positiveLiterals);
allNegLits.AddRange(this.negativeLiterals);
allNegLits.AddRange(othC.negativeLiterals);
var trPosLits = new List <Literal>();
var trNegLits = new List <Literal>();
var copyRPosLits = new List <Literal>();
var copyRNegLits = new List <Literal>();
for (int i = 0; i < 2; i++)
{
trPosLits.Clear();
trNegLits.Clear();
if (i == 0)
{
// See if this clauses positives
// unify with the other clauses
// negatives
trPosLits.AddRange(this.positiveLiterals);
trNegLits.AddRange(othC.negativeLiterals);
}
else
{
// Try the other way round now
trPosLits.AddRange(othC.positiveLiterals);
trNegLits.AddRange(this.negativeLiterals);
}
// Now check to see if they resolve
IDictionary <Variable, ITerm> copyRBindings = new Dictionary <Variable, ITerm>();
foreach (var pl in trPosLits)
{
foreach (var nl in trNegLits)
{
copyRBindings.Clear();
if (null != _unifier.Unify(pl.AtomicSentence, nl.AtomicSentence, copyRBindings))
{
copyRPosLits.Clear();
copyRNegLits.Clear();
var found = false;
foreach (var l in allPosLits)
{
if (!found && pl.Equals(l))
{
found = true;
continue;
}
copyRPosLits.Add(_substVisitor.Subst(copyRBindings, l));
}
found = false;
foreach (Literal l in allNegLits)
{
if (!found && nl.Equals(l))
{
found = true;
continue;
}
copyRNegLits.Add(_substVisitor.Subst(copyRBindings, l));
}
// Ensure the resolvents are standardized apart
var renameSubstitituon = _standardizeApart
.GetStandardizeApartResult(copyRPosLits, copyRNegLits, _saIndexical);
var c = new Clause(copyRPosLits, copyRNegLits);
c.SetProofStep(new ProofStepClauseBinaryResolvent(c, this, othC, copyRBindings, renameSubstitituon));
if (this.Immutable)
{
c.Immutable = true;
}
if (!this.IsStandardizedApartCheckRequired())
{
c.SetStandardizedApartCheckNotRequired();
}
resolvents.Add(c);
}
}
}
}
return(resolvents);
}
