Showcase how a user can manage SSDV ordinal maps

lucene/facet/src/java/org/apache/lucene/facet/sortedset/SsdvReaderStatesManager.java

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+/*
+ * 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.
+ */
+package org.apache.lucene.facet.sortedset;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.Map;
+import java.util.Set;
+import org.apache.lucene.facet.FacetsConfig;
+import org.apache.lucene.search.IndexSearcher;
+
+/**
+ * This class demonstrates how Lucene could help a user build SSDV ordinal maps eagerly, on refresh.
+ * This class offers:
+ *
+ * <p>1. A centralized store of SSDV reader states.
+ *
+ * <p>2. A method to rebuild all reader states the user needs.
+ */
+public class SsdvReaderStatesManager {
+  /**
+   * The only attribute is a mapping of SSDV fields to reader states. Each reader state will be a
+   * {@link DefaultSortedSetDocValuesReaderState} with an ordinal map for one of the fields.
+   */
+  private Map<String, SortedSetDocValuesReaderState> fieldToReaderState;
+
+  /** No arguments in the constructor. */
+  SsdvReaderStatesManager() {}
+
+  /**
+   * The application code would have a set of fields the user wants to facet on. They pass this set
+   * here once, on application start-up.
+   */
+  public void registerFields(Set<String> fields) {
+    fields.forEach(field -> fieldToReaderState.put(field, null));
+  }
+
+  /**
+   * Here we create corresponding reader states for all the fields. Each time the user does a
+   * refresh, they would also call this method, which ensures they will be using up-to-date ordinal
+   * maps until the next refresh.
+   */
+  public void loadReaderStates(IndexSearcher searcher, FacetsConfig facetsConfig) {
+    fieldToReaderState.replaceAll(
+        (field, readerState) -> {
+          try {
+            return new DefaultSortedSetDocValuesReaderState(
+                searcher.getIndexReader(), field, facetsConfig);
+          } catch (IOException e) {
+            throw new UncheckedIOException(e);
+          }
+        });
+  }
+
+  /**
+   * When the user wants to facet on a field, they call getReaderState on that field and pass the
+   * resulting reader state to the {@link SortedSetDocValuesFacetCounts} constructor.
+   */
+  public SortedSetDocValuesReaderState getReaderState(String field) {
+    return fieldToReaderState.get(field);
+  }
+}