[GitHub] [arrow] felipecrv commented on a diff in pull request #35814: GH-35360: Take offset into account in ScalarHashImpl::ArrayHash()

["felipecrv (via GitHub)" <gi...@apache.org>]

felipecrv commented on code in PR #35814:
URL: https://github.com/apache/arrow/pull/35814#discussion_r1210302073


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cpp/src/arrow/util/hashing.cc:
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@@ -0,0 +1,167 @@
+// 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.
+
+#include "arrow/util/hashing.h"
+#include "arrow/util/macros.h"
+
+namespace arrow {
+namespace internal {
+
+namespace {
+
+/// \brief Rotate-right, 64-bit.
+///
+/// This compiles to a single instruction on CPUs that have rotation instructions.
+///
+/// \pre n must be in the range [1, 64].
+#define ROTR64(v, n) (((v) >> (n)) | ((v) << (64 - (n))))
+
+/// \brief A hash function for bitmaps that can handle offsets and lengths in
+/// terms of number of bits. The hash only depends on the bits actually hashed.
+///
+/// This implementation is based on 64-bit versions of MurmurHash2 by Austin Appleby.
+///
+/// The key (a bitmap) is read as a sequence of 64-bit words before the trailing
+/// bytes. So if the input is 64-bit aligned, all memory accesses are aligned.
+///
+/// It's the caller's responsibility to ensure that bits_offset + num_bits are
+/// readable from the bitmap.
+///
+/// \param key The pointer to the bitmap.
+/// \param seed The seed for the hash function (useful when chaining hash functions).
+/// \param bits_offset The offset in bits relative to the start of the bitmap.
+/// \param num_bits The number of bits after the offset to be hashed.
+uint64_t MurmurHashBitmap64A(const uint8_t* key, uint64_t seed, uint64_t bits_offset,

Review Comment:
   This is MurmurHash modified to take bits starting on non-byte boundaries.



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