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This artifact provides the implementation of the TTT algorithm as described in the paper "The TTT Algorithm: A Redundancy-Free Approach to Active Automata Learning" (https://doi.org/10.1007/978-3-319-11164-3_26) by Malte Isberner, Falk Howar, and Bernhard Steffen.
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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.algorithm.ttt.mealy;

import com.github.misberner.buildergen.annotations.GenerateBuilder;
import de.learnlib.acex.AcexAnalyzer;
import de.learnlib.acex.MealyOutInconsPrefixTransformAcex;
import de.learnlib.acex.OutInconsPrefixTransformAcex;
import de.learnlib.algorithm.LearningAlgorithm;
import de.learnlib.algorithm.ttt.base.AbstractBaseDTNode;
import de.learnlib.algorithm.ttt.base.AbstractTTTLearner;
import de.learnlib.algorithm.ttt.base.BaseTTTDiscriminationTree;
import de.learnlib.algorithm.ttt.base.OutputInconsistency;
import de.learnlib.algorithm.ttt.base.TTTState;
import de.learnlib.algorithm.ttt.base.TTTTransition;
import de.learnlib.oracle.MembershipOracle;
import de.learnlib.query.DefaultQuery;
import de.learnlib.util.mealy.MealyUtil;
import net.automatalib.alphabet.Alphabet;
import net.automatalib.automaton.transducer.MealyMachine;
import net.automatalib.word.Word;
import net.automatalib.word.WordBuilder;

public class TTTLearnerMealy extends AbstractTTTLearner, I, Word>
        implements LearningAlgorithm.MealyLearner {

    @GenerateBuilder(defaults = AbstractTTTLearner.BuilderDefaults.class)
    public TTTLearnerMealy(Alphabet alphabet, MembershipOracle> oracle, AcexAnalyzer analyzer) {
        super(alphabet,
              oracle,
              new TTTHypothesisMealy<>(alphabet),
              new BaseTTTDiscriminationTree<>(oracle, TTTDTNodeMealy::new),
              analyzer);
    }

    @Override
    @SuppressWarnings("unchecked")
    public MealyMachine getHypothesisModel() {
        return (TTTHypothesisMealy) hypothesis;
    }

    @Override
    protected TTTTransition> createTransition(TTTState> state, I sym) {
        TTTTransitionMealy trans = new TTTTransitionMealy<>(state, sym);
        trans.output = query(state, Word.fromLetter(sym)).firstSymbol();
        return trans;
    }

    @Override
    @SuppressWarnings("unchecked")
    protected boolean refineHypothesisSingle(DefaultQuery> ceQuery) {
        DefaultQuery> shortenedCeQuery =
                MealyUtil.shortenCounterExample((TTTHypothesisMealy) hypothesis, ceQuery);
        return shortenedCeQuery != null && super.refineHypothesisSingle(shortenedCeQuery);
    }

    @Override
    protected OutInconsPrefixTransformAcex> deriveAcex(OutputInconsistency> outIncons) {
        TTTState> source = outIncons.srcState;
        Word suffix = outIncons.suffix;

        OutInconsPrefixTransformAcex> acex = new MealyOutInconsPrefixTransformAcex<>(suffix,
                                                                                                oracle,
                                                                                                w -> getDeterministicState(
                                                                                                        source,
                                                                                                        w).getAccessSequence());

        acex.setEffect(0, outIncons.targetOut);
        Word lastHypOut = computeHypothesisOutput(getAnySuccessor(source, suffix.prefix(-1)), suffix.suffix(1));
        acex.setEffect(suffix.length() - 1, lastHypOut);
        return acex;
    }

    @Override
    protected Word succEffect(Word effect) {
        return effect.subWord(1);
    }

    @Override
    protected OutputInconsistency> findOutputInconsistency() {
        OutputInconsistency> best = null;

        for (TTTState> state : hypothesis.getStates()) {
            AbstractBaseDTNode> node = state.getDTLeaf();
            while (!node.isRoot()) {
                Word expectedOut = node.getParentOutcome();
                node = node.getParent();
                Word suffix = node.getDiscriminator();
                Word hypOut = computeHypothesisOutput(state, suffix);
                int mismatchIdx = MealyUtil.findMismatch(expectedOut, hypOut);
                if (mismatchIdx != MealyUtil.NO_MISMATCH && (best == null || mismatchIdx <= best.suffix.length())) {
                    best = new OutputInconsistency<>(state,
                                                     suffix.prefix(mismatchIdx + 1),
                                                     expectedOut.prefix(mismatchIdx + 1));
                }
            }
        }
        return best;
    }

    @Override
    protected Word predictSuccOutcome(TTTTransition> trans,
                                         AbstractBaseDTNode> succSeparator) {
        TTTTransitionMealy mtrans = (TTTTransitionMealy) trans;
        if (succSeparator == null) {
            return Word.fromLetter(mtrans.output);
        }
        Word subtreeLabel = succSeparator.subtreeLabel(trans.getDTTarget());
        assert subtreeLabel != null;
        return subtreeLabel.prepend(mtrans.output);
    }

    @Override
    protected Word computeHypothesisOutput(TTTState> state, Word suffix) {
        TTTState> curr = state;

        WordBuilder wb = new WordBuilder<>(suffix.length());

        for (I sym : suffix) {
            TTTTransitionMealy trans = (TTTTransitionMealy) hypothesis.getInternalTransition(curr, sym);
            wb.append(trans.output);
            curr = getAnyTarget(trans);
        }

        return wb.toWord();
    }

    @Override
    protected AbstractBaseDTNode> createNewNode(AbstractBaseDTNode> parent,
                                                           Word parentOutput) {
        return new TTTDTNodeMealy<>(parent, parentOutput);
    }
}