ceph/src/rocksdb/third-party/folly/folly/hash/Hash.h

   1 //  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 #include <cstdint>
   9
  10 namespace folly {
  11 namespace hash {
  12
  13 /*
  14  * Thomas Wang 64 bit mix hash function
  15  */
  16
  17 inline uint64_t twang_mix64(uint64_t key) noexcept {
  18   key = (~key) + (key << 21); // key *= (1 << 21) - 1; key -= 1;
  19   key = key ^ (key >> 24);
  20   key = key + (key << 3) + (key << 8); // key *= 1 + (1 << 3) + (1 << 8)
  21   key = key ^ (key >> 14);
  22   key = key + (key << 2) + (key << 4); // key *= 1 + (1 << 2) + (1 << 4)
  23   key = key ^ (key >> 28);
  24   key = key + (key << 31); // key *= 1 + (1 << 31)
  25   return key;
  26 }
  27
  28 } // namespace hash
  29 } // namespace folly