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diff --git a/lucene-java-3.5.0/lucene/contrib/facet/src/java/org/apache/lucene/facet/search/FacetsAccumulator.java b/lucene-java-3.5.0/lucene/contrib/facet/src/java/org/apache/lucene/facet/search/FacetsAccumulator.java
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+package org.apache.lucene.facet.search;
+
+import java.io.IOException;
+import java.util.List;
+
+import org.apache.lucene.index.IndexReader;
+
+import org.apache.lucene.facet.search.params.FacetSearchParams;
+import org.apache.lucene.facet.search.params.FacetRequest;
+import org.apache.lucene.facet.search.results.FacetResult;
+import org.apache.lucene.facet.taxonomy.TaxonomyReader;
+
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+/**
+ * Driver for Accumulating facets of faceted search requests over given
+ * documents.
+ * 
+ * @lucene.experimental
+ */
+public abstract class FacetsAccumulator {
+
+  /**
+   * Default threshold for using the complements optimization.
+   * If accumulating facets for a document set larger than this ratio of the index size than 
+   * perform the complement optimization.
+   * @see #setComplementThreshold(double) for more info on the complements optimization.  
+   */
+  public static final double DEFAULT_COMPLEMENT_THRESHOLD = 0.6;
+
+  /**
+   * Passing this to {@link #setComplementThreshold(double)} will disable using complement optimization.
+   */
+  public static final double DISABLE_COMPLEMENT = Double.POSITIVE_INFINITY; // > 1 actually
+
+  /**
+   * Passing this to {@link #setComplementThreshold(double)} will force using complement optimization.
+   */
+  public static final double FORCE_COMPLEMENT = 0; // <=0  
+
+  private double complementThreshold = DEFAULT_COMPLEMENT_THRESHOLD;  
+
+  protected final TaxonomyReader taxonomyReader;
+  protected final IndexReader indexReader;
+  protected FacetSearchParams searchParams;
+
+  private boolean allowLabeling = true;
+
+  public FacetsAccumulator(FacetSearchParams searchParams,
+                            IndexReader indexReader,
+                            TaxonomyReader taxonomyReader) {
+    this.indexReader = indexReader;
+    this.taxonomyReader = taxonomyReader;
+    this.searchParams = searchParams;
+  }
+
+  /**
+   * Accumulate facets over given documents, according to facet requests in effect.
+   * @param docids documents (and their scores) for which facets are Accumulated.
+   * @return Accumulated facets.  
+   * @throws IOException on error.
+   */
+  // internal API note: it was considered to move the docids into the constructor as well, 
+  // but this prevents nice extension capabilities, especially in the way that 
+  // Sampling Accumulator works with the (any) delegated accumulator.
+  public abstract List<FacetResult> accumulate(ScoredDocIDs docids) throws IOException;
+
+  /**
+   * @return the complement threshold
+   * @see #setComplementThreshold(double)
+   */
+  public double getComplementThreshold() {
+    return complementThreshold;
+  }
+
+  /**
+   * Set the complement threshold.
+   * This threshold will dictate whether the complements optimization is applied.
+   * The optimization is to count for less documents. It is useful when the same 
+   * FacetSearchParams are used for varying sets of documents. The first time 
+   * complements is used the "total counts" are computed - counting for all the 
+   * documents in the collection. Then, only the complementing set of documents
+   * is considered, and used to decrement from the overall counts, thereby 
+   * walking through less documents, which is faster.
+   * <p>
+   * Note that this optimization is only available when searching an index
+   * whose {@link IndexReader} implements both 
+   * {@link IndexReader#directory()} and {@link IndexReader#getVersion()} 
+   * otherwise the optimization is silently disabled regardless of
+   * the complement threshold settings.
+   * <p>
+   * For the default settings see {@link #DEFAULT_COMPLEMENT_THRESHOLD}.
+   * <p>
+   * To forcing complements in all cases pass {@link #FORCE_COMPLEMENT}.
+   * This is mostly useful for testing purposes, as forcing complements when only 
+   * tiny fraction of available documents match the query does not make sense and 
+   * would incur performance degradations.
+   * <p>
+   * To disable complements pass {@link #DISABLE_COMPLEMENT}.
+   * @param complementThreshold the complement threshold to set
+   */
+  public void setComplementThreshold(double complementThreshold) {
+    this.complementThreshold = complementThreshold;
+  }
+
+  /**
+   * Check if labeling is allowed for this accumulator.
+   * <p>
+   * By default labeling is allowed.
+   * This allows one accumulator to invoke other accumulators for accumulation
+   * but keep to itself the responsibility of labeling.
+   * This might br handy since labeling is a costly operation. 
+   * @return true of labeling is allowed for this accumulator
+   * @see #setAllowLabeling(boolean)
+   */
+  protected boolean isAllowLabeling() {
+    return allowLabeling;
+  }
+
+  /**
+   * Set whether labeling is allowed for this accumulator.
+   * @param allowLabeling new setting for allow labeling
+   * @see #isAllowLabeling()
+   */
+  protected void setAllowLabeling(boolean allowLabeling) {
+    this.allowLabeling = allowLabeling;
+  }
+
+  /** check if all requests are complementable */
+  protected boolean mayComplement() {
+    for (FacetRequest freq:searchParams.getFacetRequests()) {
+      if (!freq.supportsComplements()) {
+        return false;
+      }
+    }
+    return true;
+  }
+}
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