78 lines
1.8 KiB
C++
78 lines
1.8 KiB
C++
///////////////////////////////////////////////////////////////
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// Copyright 2012 John Maddock. Distributed under the Boost
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// Software License, Version 1.0. (See accompanying file
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// LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt
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#include <boost/multiprecision/cpp_int.hpp>
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#include <boost/random.hpp>
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#include <boost/functional/hash.hpp>
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#include <unordered_set>
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#include <city.h>
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//[hash1
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/*`
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All of the types in this library support hashing via boost::hash or std::hash.
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That means we can use multiprecision types directly in hashed containers such as std::unordered_set:
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*/
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//]
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void t1()
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{
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//[hash2
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using namespace boost::multiprecision;
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using namespace boost::random;
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mt19937 mt;
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uniform_int_distribution<uint256_t> ui;
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std::unordered_set<uint256_t> set;
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// Put 1000 random values into the container:
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for(unsigned i = 0; i < 1000; ++i)
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set.insert(ui(mt));
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//]
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}
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//[hash3
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/*`
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Or we can define our own hash function, for example in this case based on
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Google's CityHash:
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*/
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struct cityhash
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{
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std::size_t operator()(const boost::multiprecision::uint256_t& val)const
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{
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// create a hash from all the limbs of the argument, this function is probably x64 specific,
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// and requires that we access the internals of the data type:
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std::size_t result = CityHash64(reinterpret_cast<const char*>(val.backend().limbs()), val.backend().size() * sizeof(val.backend().limbs()[0]));
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// modify the returned hash based on sign:
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return val < 0 ? ~result : result;
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}
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};
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//]
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void t2()
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{
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//[hash4
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/*`As before insert some values into a container, this time using our custom hasher:*/
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std::unordered_set<uint256_t, cityhash> set2;
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for(unsigned i = 0; i < 1000; ++i)
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set2.insert(ui(mt));
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//]
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}
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int main()
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{
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t1();
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t2();
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return 0;
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}
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