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//               The Next Generation Logic Library                       //
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//  Copyright 2015-20xx Christoph Zengler                                //
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package org.logicng.transformations.cnf;

import org.logicng.datastructures.ubtrees.UBTree;
import org.logicng.formulas.FType;
import org.logicng.formulas.Formula;
import org.logicng.formulas.FormulaTransformation;
import org.logicng.formulas.Literal;
import org.logicng.transformations.Subsumption;

import java.util.ArrayList;
import java.util.List;
import java.util.SortedSet;

/**
 * This transformation performs subsumption on a given CNF and returns a new CNF.
 * I.e. performs as many subsumptions as possible.  A subsumption in a CNF means,
 * that e.g. a clause {@code A | B | C} is subsumed by another clause {@code A | B}
 * and can therefore be deleted for an equivalent CNF.
 * @version 2.3.0
 * @since 1.5.0
 */
public final class CNFSubsumption extends Subsumption implements FormulaTransformation {

    private static final CNFSubsumption INSTANCE = new CNFSubsumption();

    /**
     * @deprecated In the next version, the standard constructor will be replaced by a private constructor.
     * In order to instantiate an object of this class, use the {@link #get()} method.
     */
    @Deprecated
    public CNFSubsumption() {
        // Intentionally left empty
    }

    /**
     * Returns the singleton instance of this function.
     * @return an instance of this function
     */
    public static CNFSubsumption get() {
        return INSTANCE;
    }

    @Override
    public Formula apply(final Formula formula, final boolean cache) {
        if (!formula.isCNF()) {
            throw new IllegalArgumentException("CNF subsumption can only be applied to formulas in CNF");
        }
        if (formula.type().precedence() >= FType.LITERAL.precedence() || formula.type() == FType.OR) {
            return formula;
        }
        assert formula.type() == FType.AND;
        final UBTree ubTree = generateSubsumedUBTree(formula);
        final List clauses = new ArrayList<>();
        for (final SortedSet literals : ubTree.allSets()) {
            clauses.add(formula.factory().clause(literals));
        }
        return formula.factory().cnf(clauses);
    }
}