add InterningMembershipOracle

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Added
 
 * LearnLib now supports JPMS modules. All artifacts now provide a `module-info` descriptor except of the distribution artifacts (for Maven-less environments) which only provide an `Automatic-Module-Name` due to non-modular dependencies. Note that while this is a Java 9+ feature, LearnLib still supports Java 8 byte code for the remaining class files.
+* Added an `InterningMembershipOracle` (including refinements) to the `learnlib-cache` artifact that interns query responses to reduce memory consumption of large data structures. This exports the internal concepts of the DHC learner (which no longer interns query responses automatically).
 
 ### Changed
 

build-parent/pom.xml

@@ -63,6 +63,7 @@ limitations under the License.
                             <exclude>de/learnlib/filter/reuse/**/Reuse*Builder.class</exclude>
 
                             <!-- generated refinements for oracles -->
+                            <exclude>de/learnlib/filter/cache/**/*Interning*.class</exclude>
                             <exclude>de/learnlib/filter/statistic/**/DFA*.class</exclude>
                             <exclude>de/learnlib/filter/statistic/**/Mealy*.class</exclude>
                             <exclude>de/learnlib/filter/statistic/**/Moore*.class</exclude>

filters/cache/pom.xml

@@ -69,6 +69,12 @@ limitations under the License.
             <artifactId>slf4j-api</artifactId>
         </dependency>
 
+        <!-- build -->
+        <dependency>
+            <groupId>de.learnlib.tooling</groupId>
+            <artifactId>annotations</artifactId>
+        </dependency>
+
         <!-- only required in java doc and tests -->
         <dependency>
             <groupId>de.learnlib</groupId>

filters/cache/src/main/java/de/learnlib/filter/cache/InterningMembershipOracle.java

@@ -0,0 +1,101 @@
+/* Copyright (C) 2013-2024 TU Dortmund University
+ * 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.filter.cache;
+
+import java.lang.ref.WeakReference;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.WeakHashMap;
+
+import de.learnlib.oracle.MembershipOracle;
+import de.learnlib.oracle.MembershipOracle.MealyMembershipOracle;
+import de.learnlib.oracle.MembershipOracle.MooreMembershipOracle;
+import de.learnlib.query.DefaultQuery;
+import de.learnlib.query.Query;
+import de.learnlib.tooling.annotation.refinement.GenerateRefinement;
+import de.learnlib.tooling.annotation.refinement.Generic;
+import de.learnlib.tooling.annotation.refinement.Interface;
+import de.learnlib.tooling.annotation.refinement.Mapping;
+import net.automatalib.word.Word;
+
+/**
+ * A {@link MembershipOracle} that interns query outputs. May be used to reduce memory consumption of data structures
+ * that store a lot of query responses. Typically, this oracle only makes sense for output types that are not already
+ * interned by the JVM (such as {@link Boolean}s in case of {@link DFAMembershipOracle}s).
+ */
+@GenerateRefinement(name = "InterningMealyMembershipOracle",
+                    packageName = "de.learnlib.filter.cache.mealy",
+                    generics = {@Generic(value = "I", desc = "input symbol type"),
+                                @Generic(value = "O", desc = "output symbol type")},
+                    parentGenerics = {@Generic("I"), @Generic(clazz = Word.class, generics = "O")},
+                    interfaces = @Interface(clazz = MealyMembershipOracle.class,
+                                            generics = {@Generic("I"), @Generic("O")}),
+                    typeMappings = @Mapping(from = MembershipOracle.class,
+                                            to = MealyMembershipOracle.class,
+                                            generics = {@Generic("I"), @Generic("O")}))
+@GenerateRefinement(name = "InterningMooreMembershipOracle",
+                    packageName = "de.learnlib.filter.cache.moore",
+                    generics = {@Generic(value = "I", desc = "input symbol type"),
+                                @Generic(value = "O", desc = "output symbol type")},
+                    parentGenerics = {@Generic("I"), @Generic(clazz = Word.class, generics = "O")},
+                    interfaces = @Interface(clazz = MooreMembershipOracle.class,
+                                            generics = {@Generic("I"), @Generic("O")}),
+                    typeMappings = @Mapping(from = MembershipOracle.class,
+                                            to = MooreMembershipOracle.class,
+                                            generics = {@Generic("I"), @Generic("O")}))
+public class InterningMembershipOracle<I, D> implements MembershipOracle<I, D> {
+
+    private final MembershipOracle<I, D> delegate;
+    private final Map<D, WeakReference<D>> cache;
+
+    public InterningMembershipOracle(MembershipOracle<I, D> delegate) {
+        this.delegate = delegate;
+        this.cache = new WeakHashMap<>();
+    }
+
+    @Override
+    public void processQueries(Collection<? extends Query<I, D>> queries) {
+        final List<DefaultQuery<I, D>> delegates = new ArrayList<>(queries.size());
+
+        for (Query<I, D> q : queries) {
+            delegates.add(new DefaultQuery<>(q));
+        }
+
+        this.delegate.processQueries(delegates);
+
+        final Iterator<? extends Query<I, D>> origIter = queries.iterator();
+        final Iterator<DefaultQuery<I, D>> delegateIter = delegates.iterator();
+
+        while (origIter.hasNext() && delegateIter.hasNext()) {
+            final Query<I, D> origNext = origIter.next();
+            final DefaultQuery<I, D> delegateNext = delegateIter.next();
+            final D delegateOutput = delegateNext.getOutput();
+
+            // Since the GC may free our references during the lookup, repeat until we have a (non-null) cache hit.
+            D origOutput;
+            do {
+                origOutput = cache.computeIfAbsent(delegateOutput, k -> new WeakReference<>(delegateOutput)).get();
+            } while (origOutput == null);
+
+            origNext.answer(origOutput);
+        }
+
+        assert !origIter.hasNext() && !delegateIter.hasNext();
+    }
+}

filters/cache/src/main/java/module-info.java

@@ -39,6 +39,7 @@
 
     // only required by documentation
     requires static de.learnlib.oracle.parallelism;
+    requires static de.learnlib.tooling.annotation;
 
     exports de.learnlib.filter.cache;
     exports de.learnlib.filter.cache.dfa;

filters/cache/src/test/java/de/learnlib/filter/cache/InterningMembershipOracleTest.java

@@ -0,0 +1,80 @@
+/* Copyright (C) 2013-2024 TU Dortmund University
+ * 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.filter.cache;
+
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+
+import de.learnlib.oracle.SingleQueryOracle.SingleQueryOracleMealy;
+import de.learnlib.query.DefaultQuery;
+import net.automatalib.common.util.Pair;
+import net.automatalib.word.Word;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+public class InterningMembershipOracleTest {
+
+    @Test
+    public void testInterning() {
+        final DefaultQuery<Character, Word<Character>> q1 = new DefaultQuery<>(Word.epsilon(), Word.fromString("qwe"));
+        final DefaultQuery<Character, Word<Character>> q2 = new DefaultQuery<>(Word.fromString("asd"), Word.epsilon());
+
+        final Oracle<Character> oracle = new Oracle<>();
+        final InterningMembershipOracle<Character, Word<Character>> interning = new InterningMembershipOracle<>(oracle);
+
+        interning.processQueries(Arrays.asList(q1, q2));
+
+        final Word<Character> o1 = q1.getOutput();
+
+        Assert.assertSame(q1.getOutput(), o1);
+        Assert.assertSame(q2.getOutput(), o1);
+
+        // repeated queries
+        interning.processQueries(Arrays.asList(q1, q2));
+
+        Assert.assertSame(q1.getOutput(), o1);
+        Assert.assertSame(q2.getOutput(), o1);
+
+        // check executed queries
+        Assert.assertEquals(oracle.count, 4);
+        Assert.assertEquals(oracle.queries,
+                            Set.of(Pair.of(Word.epsilon(), Word.fromString("qwe")),
+                                   Pair.of(Word.fromString("asd"), Word.epsilon())));
+        // check that oracle actually returns different objects
+        Assert.assertNotSame(oracle.answerQuery(q1.getInput()), o1);
+        Assert.assertEquals(oracle.answerQuery(q1.getInput()), o1);
+    }
+
+    private static class Oracle<I> implements SingleQueryOracleMealy<I, Character> {
+
+        private int count;
+        private final Set<Pair<Word<I>, Word<I>>> queries;
+
+        Oracle() {
+            this.count = 0;
+            this.queries = new HashSet<>();
+        }
+
+        @Override
+        public Word<Character> answerQuery(Word<I> prefix, Word<I> suffix) {
+            count++;
+            queries.add(Pair.of(prefix, suffix));
+            return Word.fromString("abcdef");
+        }
+    }
+
+}