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diff --git a/subprojects/viatra-runtime-matchers/src/main/java/tools/refinery/viatra/runtime/matchers/context/IQueryRuntimeContext.java b/subprojects/viatra-runtime-matchers/src/main/java/tools/refinery/viatra/runtime/matchers/context/IQueryRuntimeContext.java
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+/*******************************************************************************
+ * Copyright (c) 2010-2015, Bergmann Gabor, Istvan Rath and Daniel Varro
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ * 
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package tools.refinery.viatra.runtime.matchers.context;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.Optional;
+import java.util.concurrent.Callable;
+
+import tools.refinery.viatra.runtime.matchers.planning.helpers.StatisticsHelper;
+import tools.refinery.viatra.runtime.matchers.tuple.ITuple;
+import tools.refinery.viatra.runtime.matchers.tuple.Tuple;
+import tools.refinery.viatra.runtime.matchers.tuple.TupleMask;
+import tools.refinery.viatra.runtime.matchers.util.Accuracy;
+
+/**
+ * Provides instance model information (relations corresponding to input keys) to query evaluator backends at runtime.
+ * Implementors shall extend {@link AbstractQueryRuntimeContext} instead directly this interface.
+ * 
+ * @author Bergmann Gabor
+ * @noimplement This interface is not intended to be implemented by clients. Extend {@link AbstractQueryRuntimeContext} instead.
+ */
+public interface IQueryRuntimeContext {
+    /** 
+     * Provides metamodel-specific info independent of the runtime instance model.
+     */
+    public IQueryMetaContext getMetaContext();
+    
+    
+    /**
+     * The given callable will be executed, and all model traversals will be delayed until the execution is done. If
+     * there are any outstanding information to be read from the model, a single coalesced model traversal will
+     * initialize the caches and deliver the notifications.
+     * 
+     * <p> Calls may be nested. A single coalesced traversal will happen at the end of the outermost call.
+     * 
+     * <p> <b>Caution: </b> results returned by the runtime context may be incomplete during the coalescing period, to be corrected by notifications sent during the final coalesced traversal. 
+     * For example, if a certain input key is not cached yet, an empty relation may be reported during <code>callable.call()</code>; the cache will be constructed after the call terminates and notifications will deliver the entire content of the relation.  
+     * Non-incremental query backends should therefore never enumerate input keys while coalesced (verify using {@link #isCoalescing()}).
+     * 
+     * @param callable
+     */
+    public abstract <V> V coalesceTraversals(Callable<V> callable) throws InvocationTargetException;    
+    /**
+     * @return true iff currently within a coalescing section (i.e. within the callable of a call to {@link #coalesceTraversals(Callable)}).
+     */
+    public boolean isCoalescing();
+    
+    /**
+     * Returns true if index is available for the given key providing the given service.
+     * @throws IllegalArgumentException if key is not enumerable or an unknown type, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @since 1.4
+     */
+    public boolean isIndexed(IInputKey key, IndexingService service);
+    
+    /**
+     * If the given (enumerable) input key is not yet indexed, the model will be traversed 
+     * (after the end of the outermost coalescing block, see {@link IQueryRuntimeContext#coalesceTraversals(Callable)}) 
+     * so that the index can be built. It is possible that the base indexer will select a higher indexing level merging
+     * multiple indexing requests to an appropriate level.
+     * 
+     * <p><b>Postcondition:</b> After invoking this method, {@link #getIndexed(IInputKey, IndexingService)} for the same key
+     * and service will be guaranteed to return the requested or a highing indexing level as soon as {@link #isCoalescing()} first returns false.
+     * 
+     * <p><b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException if key is not enumerable or an unknown type, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @since 1.4
+     */
+    public void ensureIndexed(IInputKey key, IndexingService service);
+    
+    /**
+     * Returns the number of tuples in the extensional relation identified by the input key seeded with the given mask and tuple.
+     * 
+     * @param key an input key 
+     * @param seedMask
+     *            a mask that extracts those parameters of the input key (from the entire parameter list) that should be
+     *            bound to a fixed value;  must not be null. <strong>Note</strong>: any given index must occur at most once in seedMask.
+     * @param seed
+     *            the tuple of fixed values restricting the match set to be considered, in the same order as given in
+     *            parameterSeedMask, so that for each considered match tuple,
+     *            projectedParameterSeed.equals(parameterSeedMask.transform(match)) should hold. Must not be null.
+     *  
+     * @return the number of tuples in the model for the given key and seed
+     * 
+     * <p><b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException if key is not enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @since 1.7
+     */
+    public int countTuples(IInputKey key, TupleMask seedMask, ITuple seed);
+    
+    
+    /**
+     * Gives an estimate of the number of different groups the tuples of the given relation are projected into by the given mask
+     * (e.g. for an identity mask, this means the full relation size). The estimate must meet the required accuracy.
+     * 
+     * <p> Must accept any input key, even non-enumerables or those not recognized by this runtime context. 
+     * If there is insufficient information to provide an answer up to the required precision, {@link Optional#empty()} is returned.  
+     * 
+     * <p> PRE: {@link TupleMask#isNonrepeating()} must hold for the group mask.
+     * 
+     * @return if available, an estimate of the cardinality of the projection of the given extensional relation, with the desired accuracy.
+     * 
+     * @since 2.1
+     */
+    public Optional<Long> estimateCardinality(IInputKey key, TupleMask groupMask, Accuracy requiredAccuracy);
+    
+    
+    /**
+     * Gives an estimate of the average size of different groups the tuples of the given relation are projected into by the given mask
+     * (e.g. for an identity mask, this means 1, while for an empty mask, the result is the full relation size). 
+     * The estimate must meet the required accuracy.
+     * 
+     * <p> Must accept any input key, even non-enumerables or those not recognized by this runtime context. 
+     * If there is insufficient information to provide an answer up to the required precision, {@link Optional#empty()} may be returned.
+     * 
+     * <p> For an empty relation, zero is acceptable as an exact answer.
+     * 
+     * <p> PRE: {@link TupleMask#isNonrepeating()} must hold for the group mask.
+     * 
+     * @return if available, an estimate of the average size of each projection group of the given extensional relation, with the desired accuracy.
+     * 
+     * @since 2.1
+     */
+    public default Optional<Double> estimateAverageBucketSize(IInputKey key, TupleMask groupMask, Accuracy requiredAccuracy) {
+        if (key.isEnumerable()) {
+            return StatisticsHelper.estimateAverageBucketSize(groupMask, requiredAccuracy, 
+                (mask, accuracy) -> this.estimateCardinality(key, mask, accuracy));
+        } else return groupMask.isIdentity() ? Optional.of(1.0) : Optional.empty();
+    }
+    
+    
+    /**
+     * Returns the tuples in the extensional relation identified by the input key, optionally seeded with the given tuple.
+     * 
+     * @param key an input key
+     * @param seedMask
+     *            a mask that extracts those parameters of the input key (from the entire parameter list) that should be
+     *            bound to a fixed value;  must not be null. <strong>Note</strong>: any given index must occur at most once in seedMask.
+     * @param seed
+     *            the tuple of fixed values restricting the match set to be considered, in the same order as given in
+     *            parameterSeedMask, so that for each considered match tuple,
+     *            projectedParameterSeed.equals(parameterSeedMask.transform(match)) should hold. Must not be null. 
+     * @return the tuples in the model for the given key and seed
+     * 
+     * <p><b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException if key is not enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @since 1.7
+     */
+    public Iterable<Tuple> enumerateTuples(IInputKey key, TupleMask seedMask, ITuple seed);
+    
+    /**
+     * Simpler form of {@link #enumerateTuples(IInputKey, TupleMask, Tuple)} in the case where all values of the tuples
+     * are bound by the seed except for one.
+     * 
+     * <p>
+     * Selects the tuples in the extensional relation identified by the input key, optionally seeded with the given
+     * tuple, and then returns the single value from each tuple which is not bound by the ssed mask.
+     * 
+     * @param key
+     *            an input key
+     * @param seedMask
+     *            a mask that extracts those parameters of the input key (from the entire parameter list) that should be
+     *            bound to a fixed value; must not be null. <strong>Note</strong>: any given index must occur at most
+     *            once in seedMask, and seedMask must include all parameters in any arbitrary order except one.
+     * @param seed
+     *            the tuple of fixed values restricting the match set to be considered, in the same order as given in
+     *            parameterSeedMask, so that for each considered match tuple,
+     *            projectedParameterSeed.equals(parameterSeedMask.transform(match)) should hold. Must not be null.
+     * @return the objects in the model for the given key and seed
+     * 
+     *         <p>
+     *         <b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException
+     *             if key is not enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @since 1.7
+     */
+    public Iterable<? extends Object> enumerateValues(IInputKey key, TupleMask seedMask, ITuple seed);
+    
+    /**
+     * Simpler form of {@link #enumerateTuples(IInputKey, TupleMask, Tuple)} in the case where all values of the tuples
+     * are bound by the seed.
+     * 
+     * <p>
+     * Returns whether the given tuple is in the extensional relation identified by the input key.
+     * 
+     * <p>
+     * Note: this call works for non-enumerable input keys as well.
+     * 
+     * @param key
+     *            an input key
+     * @param seed
+     *            the tuple of fixed values restricting the match set to be considered, in the same order as given in
+     *            parameterSeedMask, so that for each considered match tuple,
+     *            projectedParameterSeed.equals(parameterSeedMask.transform(match)) should hold. Must not be null.
+     * @return true iff there is at least a single tuple contained in the relation that corresponds to the seed tuple
+     * @since 2.0
+     */
+    public boolean containsTuple(IInputKey key, ITuple seed);
+
+    
+    /**
+     * Subscribes for updates in the extensional relation identified by the input key, optionally seeded with the given tuple.
+     * <p> This should be called after invoking 
+     * 
+     * @param key an input key
+     * @param seed can be null or a tuple with matching arity; 
+     *  if non-null, only those updates in the model are notified about 
+     *  that match the seed at positions where the seed is non-null. 
+     * @param listener will be notified of future changes
+     * 
+     * <p><b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException if key is not enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     */
+    public void addUpdateListener(IInputKey key, Tuple seed, IQueryRuntimeContextListener listener);
+    
+    /**
+     * Unsubscribes from updates in the extensional relation identified by the input key, optionally seeded with the given tuple.
+     * 
+     * @param key an input key
+     * @param seed can be null or a tuple with matching arity; 
+     *  if non-null, only those updates in the model are notified about 
+     *  that match the seed at positions where the seed is non-null. 
+     * @param listener will no longer be notified of future changes
+     * 
+     * <p><b>Precondition:</b> the given key is enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     * @throws IllegalArgumentException if key is not enumerable, see {@link IQueryMetaContext#isEnumerable(IInputKey)}.
+     */
+    public void removeUpdateListener(IInputKey key, Tuple seed, IQueryRuntimeContextListener listener);
+    /*
+     TODO: uniqueness
+     */
+    
+    /**
+     * Wraps the external element into the internal representation that is to be used by the query backend 
+     * <p> model element -> internal object.
+     * <p> null must be mapped to null.
+     */
+    public Object wrapElement(Object externalElement);
+
+    /**
+     * Unwraps the internal representation of the element into its original form  
+     * <p> internal object -> model element
+     * <p> null must be mapped to null.
+     */
+    public Object unwrapElement(Object internalElement);
+
+    /**
+     * Unwraps the tuple of elements into the internal representation that is to be used by the query backend
+     * <p> model elements -> internal objects
+     * <p> null must be mapped to null.
+    */
+    public Tuple wrapTuple(Tuple externalElements);
+
+    /**
+     * Unwraps the tuple of internal representations of elements into their original forms
+     * <p> internal objects -> model elements
+     * <p> null must be mapped to null.
+    */
+    public Tuple unwrapTuple(Tuple internalElements);
+
+    /**
+     * Starts wildcard indexing for the given service. After this call, no registration is required for this {@link IndexingService}.
+     * a previously set wildcard level cannot be lowered, only extended.
+     * @since 1.4
+     */
+    public void ensureWildcardIndexing(IndexingService service);
+    
+    /**
+     * Execute the given runnable after traversal. It is guaranteed that the runnable is executed as soon as
+     * the indexing is finished. The callback is executed only once, then is removed from the callback queue.
+     * @param traversalCallback
+     * @throws InvocationTargetException 
+     * @since 1.4
+     */
+    public void executeAfterTraversal(Runnable runnable) throws InvocationTargetException;
+}