ceph/src/boost/libs/multiprecision/example/hashing_examples.cpp

   1 ///////////////////////////////////////////////////////////////
   2 //  Copyright 2012 John Maddock. Distributed under the Boost
   3 //  Software License, Version 1.0. (See accompanying file
   4 //  LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt
   5
   6 #include <boost/multiprecision/cpp_int.hpp>
   7 #include <boost/random.hpp>
   8 #include <boost/functional/hash.hpp>
   9 #include <unordered_set>
  10 #include <city.h>
  11
  12 //[hash1
  13
  14 /*`
  15 All of the types in this library support hashing via boost::hash or std::hash.
  16 That means we can use multiprecision types directly in hashed containers such as std::unordered_set:
  17 */
  18 //]
  19
  20 void t1()
  21 {
  22    //[hash2
  23    using namespace boost::multiprecision;
  24    using namespace boost::random;
  25
  26    mt19937 mt;
  27    uniform_int_distribution<uint256_t> ui;
  28
  29    std::unordered_set<uint256_t> set;
  30    // Put 1000 random values into the container:
  31    for(unsigned i = 0; i < 1000; ++i)
  32       set.insert(ui(mt));
  33
  34    //]
  35 }
  36
  37 //[hash3
  38
  39 /*`
  40 Or we can define our own hash function, for example in this case based on
  41 Google's CityHash:
  42 */
  43
  44 struct cityhash
  45 {
  46    std::size_t operator()(const boost::multiprecision::uint256_t& val)const
  47    {
  48       // create a hash from all the limbs of the argument, this function is probably x64 specific,
  49       // and requires that we access the internals of the data type:
  50       std::size_t result = CityHash64(reinterpret_cast<const char*>(val.backend().limbs()), val.backend().size() * sizeof(val.backend().limbs()[0]));
  51       // modify the returned hash based on sign:
  52       return val < 0 ? ~result : result;
  53    }
  54 };
  55
  56 //]
  57
  58 void t2()
  59 {
  60 //[hash4
  61
  62 /*`As before insert some values into a container, this time using our custom hasher:*/
  63
  64    std::unordered_set<uint256_t, cityhash> set2;
  65    for(unsigned i = 0; i < 1000; ++i)
  66       set2.insert(ui(mt));
  67
  68 //]
  69 }
  70
  71 int main()
  72 {
  73    t1();
  74    t2();
  75    return 0;
  76 }
  77