eqtests: add abstract oracle based on test word generation (see #41)

eqtests/basic-eqtests/src/main/java/de/learnlib/eqtests/basic/AbstractTestWordEQOracle.java

@@ -0,0 +1,124 @@
+/* Copyright (C) 2013-2017 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.eqtests.basic;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.Objects;
+import java.util.stream.Stream;
+
+import javax.annotation.Nullable;
+
+import com.google.common.base.Preconditions;
+import com.google.common.collect.Streams;
+import de.learnlib.api.EquivalenceOracle;
+import de.learnlib.api.MembershipOracle;
+import de.learnlib.oracles.DefaultQuery;
+import net.automatalib.automata.concepts.Output;
+import net.automatalib.commons.util.collections.BatchingIterator;
+import net.automatalib.words.Word;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * An abstract equivalence oracle that takes care of query batching and hypothesis checking and allows extending classes
+ * to solely focus on test word generation by implementing {@link #generateTestWords(Output, Collection)}.
+ * <p>
+ * Being {@link Stream stream}-based, this oracle encourages the lazy computation of counterexamples, so that all
+ * counterexamples do not have to be computed upfront, but only as long as the first valid counterexample is found.
+ *
+ * @param <A>
+ *         hypothesis type
+ * @param <I>
+ *         input symbol type
+ * @param <D>
+ *         output (domain) type
+ *
+ * @author frohme
+ */
+public abstract class AbstractTestWordEQOracle<A extends Output<I, D>, I, D> implements EquivalenceOracle<A, I, D> {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(AbstractTestWordEQOracle.class);
+
+    private final MembershipOracle<I, D> membershipOracle;
+    private final int batchSize;
+
+    public AbstractTestWordEQOracle(MembershipOracle<I, D> membershipOracle) {
+        this(membershipOracle, 1);
+    }
+
+    public AbstractTestWordEQOracle(MembershipOracle<I, D> membershipOracle, int batchSize) {
+        Preconditions.checkArgument(batchSize > 0);
+
+        this.membershipOracle = membershipOracle;
+        this.batchSize = batchSize;
+    }
+
+    @Nullable
+    @Override
+    public DefaultQuery<I, D> findCounterExample(A hypothesis, Collection<? extends I> inputs) {
+        // Fail fast on empty inputs
+        if (inputs.isEmpty()) {
+            LOGGER.warn("Passed empty set of inputs to equivalence oracle; no counterexample can be found!");
+            return null;
+        }
+
+        final Stream<Word<I>> testWordStream = generateTestWords(hypothesis, inputs);
+        final Stream<DefaultQuery<I, D>> queryStream = testWordStream.map(DefaultQuery<I, D>::new);
+        final Stream<DefaultQuery<I, D>> answeredQueryStream = answerQueries(queryStream);
+
+        final Stream<DefaultQuery<I, D>> ceStream = answeredQueryStream.filter(query -> {
+            D hypOutput = hypothesis.computeOutput(query.getInput());
+            return !Objects.equals(hypOutput, query.getOutput());
+        });
+
+        return ceStream.findFirst().orElse(null);
+    }
+
+    /**
+     * Generate the stream of test words that should be used for the current equivalence check cycle.
+     *
+     * @param hypothesis
+     *         the current hypothesis of the learning algorithm
+     * @param inputs
+     *         the collection of inputs to consider
+     *
+     * @return the stream of test words used for equivalence testing
+     *
+     * @see EquivalenceOracle#findCounterExample(Object, Collection)
+     */
+    protected abstract Stream<Word<I>> generateTestWords(A hypothesis, Collection<? extends I> inputs);
+
+    private Stream<DefaultQuery<I, D>> answerQueries(final Stream<DefaultQuery<I, D>> stream) {
+        if (isBatched()) {
+            /*
+             * FIXME: currently necessary because of a bug in the JDK
+             * see https://bugs.openjdk.java.net/browse/JDK-8075939
+             */
+            return Streams.stream(Streams.stream(new BatchingIterator<>(stream.iterator(), this.batchSize))
+                                         .peek(membershipOracle::processQueries)
+                                         .flatMap(List::stream)
+                                         .iterator());
+        } else {
+            return stream.peek(membershipOracle::processQuery);
+        }
+    }
+
+    private boolean isBatched() {
+        return this.batchSize > 1;
+    }
+
+}