/// <summary>
/// Solves the clauses and sets the assignements.
/// </summary>
/// <param name="solution">The solution object that needs to be populated.</param>
private void Solve(MHSSolution solution)
{
// unit literal propogation
List <Clause> unit_clauses = solution.GetActiveUnitClauses();
while (unit_clauses.Any())
{
UnitLiteralPropogation(solution, unit_clauses);
unit_clauses = solution.GetActiveUnitClauses();
}
// find MHS for all positive clauses
List <Clause> positive_clauses = solution.GetActivePositiveClauses();
while (positive_clauses.Any())
{
ApplyMHS(solution, positive_clauses);
positive_clauses = solution.GetActivePositiveClauses();
}
// set everything else to false
foreach (string variable in solution.VariableLookup.Keys)
{
if (!solution.Assignments.ContainsKey(variable))
{
solution.SetAssignment(variable, false);
}
}
}
/// <summary>
/// Identifies the assignment that satisfies maximum number of clauses.
/// </summary>
/// <param name="solution">The solution object that needs to be populated.</param>
/// <param name="clauses">The clauses.</param>
private void ApplyMHS(MHSSolution solution, List <Clause> clauses)
{
IEnumerable <string> variables = clauses.SelectMany(x => x.Literals)
.Select(x => x.Variable).Distinct();
string chosen = string.Empty;
double max_total_weight = 0;
foreach (string variable in variables)
{
if (!solution.Assignments.ContainsKey(variable))
{
int unknown_clauses = clauses.Count(x => x.Literals.Select(y => y.Variable).Contains(variable));
double total_weight = unknown_clauses * weights[variable];
if (total_weight > max_total_weight)
{
max_total_weight = total_weight;
chosen = variable;
}
}
}
solution.SetAssignment(chosen, clauses.First().Literals.First().Value);
}
/// <summary>
/// Propogates the unit literals to the other clauses.
/// </summary>
/// <param name="solution">The solution object that needs to be populated.</param>
/// <param name="clauses">The clauses.</param>
private void UnitLiteralPropogation(MHSSolution solution, List <Clause> clauses)
{
foreach (Clause clause in clauses)
{
foreach (Literal literal in clause.Literals)
{
if (!solution.Assignments.ContainsKey(literal.Variable))
{
solution.SetAssignment(literal.Variable, literal.Value);
return;
}
}
}
}
/// <summary>
/// Solves the clauses and returns a solution.
/// </summary>
/// <param name="status">The solver status.</param>
/// <returns>The solution.</returns>
public Dictionary <string, bool> Solve(out SolverStatus status)
{
try
{
MHSSolution solution = new MHSSolution(clauses);
Solve(solution);
status = SolverStatus.Satisfiable;
return(solution.Assignments);
}
catch (SatisfiabilityError)
{
status = SolverStatus.Unsatisfiable;
return(null);
}
}
