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A collection of equivalence oracles

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/* Copyright (C) 2013-2023 TU Dortmund
 * This file is part of LearnLib, http://www.learnlib.de/.
 *
 * 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
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package de.learnlib.oracle.equivalence.vpa;

import java.util.Collection;

import de.learnlib.oracle.EquivalenceOracle;
import de.learnlib.query.DefaultQuery;
import net.automatalib.alphabet.VPAlphabet;
import net.automatalib.automaton.vpa.OneSEVPA;
import net.automatalib.util.automaton.vpa.OneSEVPAs;
import net.automatalib.word.Word;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An equivalence oracle based on the computation of a separating word for a given hypothesis and a previously known
 * target system.
 */
public class SimulatorEQOracle implements EquivalenceOracle, I, Boolean> {

    private final OneSEVPA reference;

    public SimulatorEQOracle(OneSEVPA reference) {
        this.reference = reference;
    }

    @Override
    public @Nullable DefaultQuery findCounterExample(OneSEVPA hypothesis,
                                                                 Collection inputs) {
        if (!(inputs instanceof VPAlphabet)) {
            throw new IllegalArgumentException("Inputs are not a visibly push-down alphabet");
        }

        @SuppressWarnings("unchecked")
        final VPAlphabet alphabet = (VPAlphabet) inputs;

        final Word sep = OneSEVPAs.findSeparatingWord(reference, hypothesis, alphabet);

        if (sep == null) {
            return null;
        }

        return new DefaultQuery<>(sep, reference.computeOutput(sep));
    }
}