Add factory methods for IndexWriterConfig.

This adds factory methods for `IndexWriterConfig` that are optimized for
a few different use-cases. Configuring an `IndexWriterConfig` is quite
an expert task, the goal is to give sensible defaults for some common
use-cases. Furthermore, javadocs explain how these configs differ from
defaults, which would help users know what to tune if they want to
further tune their configs.

lucene/misc/src/java/org/apache/lucene/misc/index/IndexWriterConfigs.java

@@ -0,0 +1,95 @@
+/*
+ * 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.misc.index;
+
+import org.apache.lucene.index.IndexWriter;
+import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.LogByteSizeMergePolicy;
+import org.apache.lucene.index.SimpleMergedSegmentWarmer;
+import org.apache.lucene.index.TieredMergePolicy;
+import org.apache.lucene.util.InfoStream;
+
+/** Factory methods for {@link IndexWriterConfig}s with sensible defaults for various use-cases. */
+public class IndexWriterConfigs {
+
+  // Prevent instantiation
+  private IndexWriterConfigs() {}
+
+  /**
+   * Create a new {@link IndexWriterConfig} for time-based data. It internally configures a {@link
+   * LogByteSizeMergePolicy}, whose policy of only merging adjacent segments helps keep the time
+   * range overlap between segments very low. This in-turn makes filtering on timestamp ranges
+   * faster.
+   */
+  public static IndexWriterConfig forTimeBasedData() {
+    IndexWriterConfig config = new IndexWriterConfig();
+    config.setMergePolicy(new LogByteSizeMergePolicy());
+    return config;
+  }
+
+  /**
+   * Create a new {@link IndexWriterConfig} that optimized for fast near-realtime search. Internally
+   * it:
+   *
+   * <ul>
+   *   <li>configures a {@link SimpleMergedSegmentWarmer},
+   *   <li>configures 8 {@link TieredMergePolicy#setSegmentsPerTier(double) segments per tier},
+   *   <li>configures a {@link TieredMergePolicy#setFloorSegmentMB(double) floor segment size} of
+   *       20MB,
+   *   <li>enables {@link BPIndexReorderer BP reordering} with default parameters for segments above
+   *       100k docs and half the max segment size.
+   * </ul>
+   */
+  public static IndexWriterConfig forFastNRTSearch() {
+    IndexWriterConfig config = new IndexWriterConfig();
+    config.setMergedSegmentWarmer(new SimpleMergedSegmentWarmer(InfoStream.NO_OUTPUT));
+    TieredMergePolicy tmp = new TieredMergePolicy();
+    tmp.setSegmentsPerTier(8);
+    tmp.setMaxMergeAtOnce(8);
+    tmp.setFloorSegmentMB(20); // 10x the default of 2MB
+    BPIndexReorderer reorderer = new BPIndexReorderer();
+    BPReorderingMergePolicy bpmp = new BPReorderingMergePolicy(tmp, reorderer);
+    bpmp.setMinNaturalMergeNumDocs(100_000);
+    bpmp.setMinNaturalMergeRatioFromBiggestSegment(0.5f);
+    config.setMergePolicy(bpmp);
+    return config;
+  }
+
+  /**
+   * Create a new {@link IndexWriterConfig} that optimized for fast search on read-only indexes. It
+   * expects the index to be created and then {@link IndexWriter#forceMerge(int) force-merged}
+   * before serving searches. Internally it:
+   *
+   * <ul>
+   *   <li>configures 32 {@link TieredMergePolicy#setSegmentsPerTier(double) segments per tier},
+   *   <li>disables {@link IndexWriterConfig#setMaxFullFlushMergeWaitMillis(long) merge on refresh},
+   *   <li>enables {@link BPIndexReorderer BP reordering} on forced merges with default parameters.
+   * </ul>
+   */
+  public static IndexWriterConfig forFastReadOnlySearch() {
+    IndexWriterConfig config = new IndexWriterConfig();
+    config.setMaxFullFlushMergeWaitMillis(0L);
+    TieredMergePolicy tmp = new TieredMergePolicy();
+    tmp.setSegmentsPerTier(32);
+    tmp.setMaxMergeAtOnce(32);
+    BPIndexReorderer reorderer = new BPIndexReorderer();
+    BPReorderingMergePolicy bpmp = new BPReorderingMergePolicy(tmp, reorderer);
+    bpmp.setMinNaturalMergeNumDocs(Integer.MAX_VALUE);
+    config.setMergePolicy(bpmp);
+    return config;
+  }
+}