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//               The Next Generation Logic Library                       //
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//  Copyright 2015-20xx Christoph Zengler                                //
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//  Licensed under the Apache License, Version 2.0 (the "License");      //
//  you may not use this file except in compliance with the License.     //
//  You may obtain a copy of the License at                              //
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//  http://www.apache.org/licenses/LICENSE-2.0                           //
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//  permissions and limitations under the License.                       //
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package org.logicng.predicates;

import static org.logicng.formulas.cache.PredicateCacheEntry.IS_DNF;
import static org.logicng.predicates.TermPredicate.minterm;

import org.logicng.datastructures.Tristate;
import org.logicng.formulas.Formula;
import org.logicng.formulas.FormulaPredicate;

/**
 * DNF predicate.  Indicates whether a formula is in DNF or not.
 * @version 2.3.0
 * @since 1.0
 */
public final class DNFPredicate implements FormulaPredicate {

    private final static DNFPredicate INSTANCE = new DNFPredicate();

    /**
     * Private empty constructor.  Singleton class.
     */
    private DNFPredicate() {
        // Intentionally left empty
    }

    /**
     * Returns the singleton of the predicate.
     * @return the predicate instance
     */
    public static DNFPredicate get() {
        return INSTANCE;
    }

    @Override
    public boolean test(final Formula formula, final boolean cache) {
        final Tristate cached = formula.predicateCacheEntry(IS_DNF);
        if (cached != Tristate.UNDEF) {
            return cached == Tristate.TRUE;
        }
        boolean result;
        switch (formula.type()) {
            case FALSE:
            case TRUE:
            case LITERAL:
                return true;
            case NOT:
            case IMPL:
            case EQUIV:
            case PBC:
                return false;
            case OR:
                result = true;
                for (final Formula op : formula) {
                    if (!minterm().test(op, false)) {
                        result = false;
                    }
                }
                break;
            case AND:
                result = minterm().test(formula, false);
                break;
            default:
                throw new IllegalArgumentException("Cannot compute DNF predicate on " + formula.type());
        }
        if (cache) {
            formula.setPredicateCacheEntry(IS_DNF, result);
        }
        return result;
    }

    @Override
    public String toString() {
        return this.getClass().getSimpleName();
    }
}