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This artifact provides the implementation of the L* learning algorithm described in the paper "Learning Regular Sets from Queries and Counterexamples" (https://doi.org/10.1016/0890-5401(87)90052-6) by Dana Angluin including variations and optimizations thereof such as the versions based on "On the Learnability of Infinitary Regular Sets" (https://dx.doi.org/10.1006/inco.1995.1070) by Oded Maler and Amir Pnueli or "Inference of finite automata using homing sequences" (http://dx.doi.org/10.1006/inco.1993.1021) by Ronald L. Rivest and Robert E. Schapire.

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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.lstar.closing;

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

import de.learnlib.datastructure.observationtable.ObservationTable;
import de.learnlib.datastructure.observationtable.Row;
import de.learnlib.oracle.MembershipOracle;
import net.automatalib.alphabet.Alphabet;
import net.automatalib.common.util.comparison.CmpUtil;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Collection of predefined observation table closing strategies.
 *
 * @see ClosingStrategy
 */
public final class ClosingStrategies {

    /**
     * Closing strategy that randomly selects one representative row to close from each equivalence class.
     */
    public static final ClosingStrategy<@Nullable Object, @Nullable Object> CLOSE_RANDOM = new CloseRandomStrategy();

    /**
     * Closing strategy that selects the first row from each equivalence class as representative.
     */
    public static final ClosingStrategy<@Nullable Object, @Nullable Object> CLOSE_FIRST =
            new ClosingStrategy<@Nullable Object, @Nullable Object>() {

                @Override
                public  List> selectClosingRows(List>> unclosedClasses,
                                                                ObservationTable table,
                                                                MembershipOracle oracle) {
                    List> result = new ArrayList<>(unclosedClasses.size());
                    for (List> clazz : unclosedClasses) {
                        result.add(clazz.get(0));
                    }
                    return result;
                }

                @Override
                public String toString() {
                    return "CloseFirst";
                }
            };

    /**
     * Closing strategy that selects the shortest row of each equivalence class (more precisely: a row which's prefix
     * has minimal length in the respective class) as representative.
     */
    public static final ClosingStrategy<@Nullable Object, @Nullable Object> CLOSE_SHORTEST =
            new ClosingStrategy<@Nullable Object, @Nullable Object>() {

                @Override
                public  List> selectClosingRows(List>> unclosedClasses,
                                                                ObservationTable table,
                                                                MembershipOracle oracle) {

                    List> result = new ArrayList<>();
                    for (List> clazz : unclosedClasses) {
                        Row shortest = null;
                        int shortestLen = Integer.MAX_VALUE;
                        for (Row row : clazz) {
                            int prefixLen = row.getLabel().length();
                            if (shortest == null || prefixLen < shortestLen) {
                                shortest = row;
                                shortestLen = prefixLen;
                            }
                        }
                        assert shortest != null;
                        result.add(shortest);
                    }
                    return result;
                }

                @Override
                public String toString() {
                    return "CloseShortest";
                }
            };

    /**
     * Closing strategy that selects the lexicographically minimal row (wrt. its prefix) of each equivalence class as
     * representative.
     */
    public static final ClosingStrategy<@Nullable Object, @Nullable Object> CLOSE_LEX_MIN =
            new ClosingStrategy<@Nullable Object, @Nullable Object>() {

                @Override
                public  List> selectClosingRows(List>> unclosedClasses,
                                                                ObservationTable table,
                                                                MembershipOracle oracle) {
                    List> result = new ArrayList<>(unclosedClasses.size());
                    Alphabet alphabet = table.getInputAlphabet();
                    for (List> clazz : unclosedClasses) {
                        Row lexMin = null;
                        for (Row row : clazz) {
                            if (lexMin == null) {
                                lexMin = row;
                            } else if (CmpUtil.lexCompare(row.getLabel(), lexMin.getLabel(), alphabet) < 0) {
                                lexMin = row;
                            }
                        }
                        assert lexMin != null;
                        result.add(lexMin);
                    }
                    return result;
                }

                @Override
                public String toString() {
                    return "CloseLexMin";
                }
            };

    private ClosingStrategies() {
        // prevent instantiation
    }

    @SuppressWarnings("unchecked")
    public static ClosingStrategy<@Nullable Object, @Nullable Object>[] values() {
        return new ClosingStrategy[] {CLOSE_RANDOM, CLOSE_FIRST, CLOSE_SHORTEST, CLOSE_LEX_MIN};
    }

}