[GitHub] [lucene] neoremind commented on a change in pull request #91: LUCENE-9932: Performance improvement for BKD index building

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neoremind commented on a change in pull request #91:
URL: https://github.com/apache/lucene/pull/91#discussion_r617403651



##########
File path: lucene/core/src/java/org/apache/lucene/util/StableMSBRadixSorter.java
##########
@@ -0,0 +1,91 @@
+/*
+ * 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.util;
+
+/**
+ * Stable radix sorter for variable-length strings. It has to check data beforehand to see if stable
+ * sort can be enabled or not. If not applicable, it works the same way as its parent class {@link
+ * MSBRadixSorter}.
+ *
+ * @lucene.internal
+ */
+public abstract class StableMSBRadixSorter extends MSBRadixSorter {
+
+  protected boolean useStableSort;
+
+  public StableMSBRadixSorter(int maxLength) {
+    super(maxLength);
+  }
+
+  /** Check if stable sort can be enabled or not. */
+  protected boolean isEnableStableSort(int from, int to) {
+    return false;
+  }
+
+  /**
+   * If stable sort is applicable, do some closure logic, which might update some of the internal
+   * variable.
+   */
+  protected void doClosureIfStableSortEnabled() {}
+
+  /** Assign the from-th value to to-th position in another array which used temporarily. */
+  protected void assign(int from, int to) {
+    throw new UnsupportedOperationException("not implement");
+  }
+
+  /** Finalize assign operation, to switch array. */
+  protected void finalizeAssign(int from, int to) {
+    throw new UnsupportedOperationException("not implement");
+  }
+
+  @Override
+  public void sort(int from, int to) {
+    checkRange(from, to);
+    useStableSort = isEnableStableSort(from, to);
+    if (useStableSort) {
+      doClosureIfStableSortEnabled();
+    }
+    sort(from, to, 0, 0);
+  }
+
+  @Override
+  protected void reorder(int from, int to, int[] startOffsets, int[] endOffsets, int k) {
+    if (useStableSort) {
+      stableReorder(from, to, startOffsets, endOffsets, k);
+    } else {
+      super.reorder(from, to, startOffsets, endOffsets, k);
+    }

Review comment:
       Good catch. If I override the method, then the parent class will use stable reorder. Interestingly, I found non-stable sorting runs faster accidentally (I debug the whole day :joy:). So to make the behavior work as expected, I update the code in such way.




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