[GitHub] [druid] jon-wei commented on a change in pull request #10304: Add vectorization for druid-histogram extension

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jon-wei commented on a change in pull request #10304:
URL: https://github.com/apache/druid/pull/10304#discussion_r478782925



##########
File path: extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/ApproximateHistogramFoldingBufferAggregatorInternal.java
##########
@@ -0,0 +1,99 @@
+/*
+ * 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.druid.query.aggregation.histogram;
+
+import javax.annotation.Nullable;
+import java.nio.ByteBuffer;
+
+/**
+ * A helper class used by {@link ApproximateHistogramFoldingBufferAggregator} and
+ * {@link ApproximateHistogramFoldingVectorAggregator} for aggregation operations on byte buffers.
+ * Getting the object from value selectors is outside this class.
+ */
+final class ApproximateHistogramFoldingBufferAggregatorInternal
+{
+  private final int resolution;
+  private final float upperLimit;
+  private final float lowerLimit;
+
+  private float[] tmpBufferA;
+  private long[] tmpBufferB;
+
+  public ApproximateHistogramFoldingBufferAggregatorInternal(
+      int resolution,
+      float lowerLimit,
+      float upperLimit
+  )
+  {
+    this.resolution = resolution;
+    this.lowerLimit = lowerLimit;
+    this.upperLimit = upperLimit;
+
+    tmpBufferA = new float[resolution];
+    tmpBufferB = new long[resolution];
+  }
+
+  public void init(ByteBuffer buf, int position)
+  {
+    ApproximateHistogram h = new ApproximateHistogram(resolution, lowerLimit, upperLimit);
+
+    ByteBuffer mutationBuffer = buf.duplicate();
+    mutationBuffer.position(position);
+    // use dense storage for aggregation
+    h.toBytesDense(mutationBuffer);
+  }
+
+  public void aggregate(ByteBuffer buf, int position, @Nullable ApproximateHistogram hNext)
+  {
+    if (hNext == null) {
+      return;
+    }
+    ByteBuffer mutationBuffer = buf.duplicate();
+    mutationBuffer.position(position);
+
+    ApproximateHistogram h0 = ApproximateHistogram.fromBytesDense(mutationBuffer);
+    foldFast(h0, hNext);
+
+    mutationBuffer.position(position);
+    h0.toBytesDense(mutationBuffer);
+  }
+
+  public void foldFast(ApproximateHistogram left, ApproximateHistogram right)
+  {
+    //TODO: do these have to set in every call
+    left.setLowerLimit(lowerLimit);

Review comment:
       I think it's necessary, the `fromBytesDense` call ends up using this constructor which uses a default lower/upper limit:
   
   ```
    public ApproximateHistogram(int binCount, float[] positions, long[] bins, float min, float max)
     {
       this(
           positions.length,        //size
           positions,               //positions
           bins,                    //bins
           binCount,                //binCount
           min,                     //min
           max,                     //max
           sumBins(bins, binCount), //count
           Float.NEGATIVE_INFINITY, //lowerLimit
           Float.POSITIVE_INFINITY  //upperLimit
       );
     }
   ```




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