Github user ponkin commented on a diff in the pull request:
https://github.com/apache/spark/pull/15917#discussion_r88440366
--- Diff:
common/sketch/src/main/java/org/apache/spark/util/sketch/Murmur3_128.java ---
@@ -0,0 +1,135 @@
+/*
+ * 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.spark.util.sketch;
+
+/**
+ * 128-bit Murmur3 hasher.
+ * Best perfomance is on x86_64 platform
+ * Based on implementation
https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp
+ */
+final class Murmur3_128 {
--- End diff --
This decision come from fact that RoaringBitmap supports only int
indexes(and can have only Int.MaxValue bits), since we use RoaringBitmap[] to
represent vector way larger than Int.MaxValue, we need good hash function to
generate long hash values. This implementation is based on
[smhasher](https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp)
and [solr
implementation](https://github.com/yonik/java_util/blob/master/src/util/hash/MurmurHash3.java)
which are both freely available I suppose.
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