我需要以任意精度获取一个值的散列值(来自 Boost.Multiprecision);我用 cpp_int
后端。我想出了以下代码:
boost::multiprecision::cpp_int x0 = 1;
const auto seed = std::hash<std::string>{}(x0.str());
我不需要代码尽可能快,但我发现对字符串表示进行哈希处理非常笨拙。
所以我的问题是双重的:
- 保持任意精度,我可以更有效地散列值吗?
- 也许我不应该坚持保持任意精度,我应该转换成一个我可以轻松散列的
double
(不过,我仍然会使用任意精度值进行哈希表所需的比较)?
最佳答案
您可以(ab)使用序列化支持:
Support for serialization comes in two forms: Classes
number
,debug_adaptor
,logged_adaptor
andrational_adaptor
have "pass through" serialization support which requires the underlying backend to be serializable.Backends
cpp_int
,cpp_bin_float
,cpp_dec_float
andfloat128
have full support for Boost.Serialization.
所以,让我拼凑一些可以与 boost 和 std 无序容器一起使用的东西:
template <typename Map>
void test(Map const& map) {
std::cout << "\n" << __PRETTY_FUNCTION__ << "\n";
for(auto& p : map)
std::cout << p.second << "\t" << p.first << "\n";
}
int main() {
using boost::multiprecision::cpp_int;
test(std::unordered_map<cpp_int, std::string> {
{ cpp_int(1) << 111, "one" },
{ cpp_int(2) << 222, "two" },
{ cpp_int(3) << 333, "three" },
});
test(boost::unordered_map<cpp_int, std::string> {
{ cpp_int(1) << 111, "one" },
{ cpp_int(2) << 222, "two" },
{ cpp_int(3) << 333, "three" },
});
}
我们转发相关hash<>
我们自己的实现 hash_impl
使用多精度和序列化的特化:
namespace std {
template <typename backend>
struct hash<boost::multiprecision::number<backend> >
: mp_hashing::hash_impl<boost::multiprecision::number<backend> >
{};
}
namespace boost {
template <typename backend>
struct hash<multiprecision::number<backend> >
: mp_hashing::hash_impl<multiprecision::number<backend> >
{};
}
现在,当然,这引出了一个问题,hash_impl
怎么样?实现了吗?
template <typename T> struct hash_impl {
size_t operator()(T const& v) const {
using namespace boost;
size_t seed = 0;
{
iostreams::stream<hash_sink> os(seed);
archive::binary_oarchive oa(os, archive::no_header | archive::no_codecvt);
oa << v;
}
return seed;
}
};
这看起来很简单。那是因为 Boost 很棒,写了一个 hash_sink
与 Boost Iostreams 一起使用的设备只是以下简单的练习:
namespace io = boost::iostreams;
struct hash_sink {
hash_sink(size_t& seed_ref) : _ptr(&seed_ref) {}
typedef char char_type;
typedef io::sink_tag category;
std::streamsize write(const char* s, std::streamsize n) {
boost::hash_combine(*_ptr, boost::hash_range(s, s+n));
return n;
}
private:
size_t* _ptr;
};
完整演示:
#include <iostream>
#include <iomanip>
#include <boost/archive/binary_oarchive.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_int/serialize.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/stream_buffer.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/functional/hash.hpp>
namespace mp_hashing {
namespace io = boost::iostreams;
struct hash_sink {
hash_sink(size_t& seed_ref) : _ptr(&seed_ref) {}
typedef char char_type;
typedef io::sink_tag category;
std::streamsize write(const char* s, std::streamsize n) {
boost::hash_combine(*_ptr, boost::hash_range(s, s+n));
return n;
}
private:
size_t* _ptr;
};
template <typename T> struct hash_impl {
size_t operator()(T const& v) const {
using namespace boost;
size_t seed = 0;
{
iostreams::stream<hash_sink> os(seed);
archive::binary_oarchive oa(os, archive::no_header | archive::no_codecvt);
oa << v;
}
return seed;
}
};
}
#include <unordered_map>
#include <boost/unordered_map.hpp>
namespace std {
template <typename backend>
struct hash<boost::multiprecision::number<backend> >
: mp_hashing::hash_impl<boost::multiprecision::number<backend> >
{};
}
namespace boost {
template <typename backend>
struct hash<multiprecision::number<backend> >
: mp_hashing::hash_impl<multiprecision::number<backend> >
{};
}
template <typename Map>
void test(Map const& map) {
std::cout << "\n" << __PRETTY_FUNCTION__ << "\n";
for(auto& p : map)
std::cout << p.second << "\t" << p.first << "\n";
}
int main() {
using boost::multiprecision::cpp_int;
test(std::unordered_map<cpp_int, std::string> {
{ cpp_int(1) << 111, "one" },
{ cpp_int(2) << 222, "two" },
{ cpp_int(3) << 333, "three" },
});
test(boost::unordered_map<cpp_int, std::string> {
{ cpp_int(1) << 111, "one" },
{ cpp_int(2) << 222, "two" },
{ cpp_int(3) << 333, "three" },
});
}
打印
void test(const Map&) [with Map = std::unordered_map<boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<> >, std::basic_string<char> >]
one 2596148429267413814265248164610048
three 52494017394792286184940053450822912768476066341437098474218494553838871980785022157364316248553291776
two 13479973333575319897333507543509815336818572211270286240551805124608
void test(const Map&) [with Map = boost::unordered::unordered_map<boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<> >, std::basic_string<char> >]
three 52494017394792286184940053450822912768476066341437098474218494553838871980785022157364316248553291776
two 13479973333575319897333507543509815336818572211270286240551805124608
one 2596148429267413814265248164610048
如您所见,Boost 和标准库的 unordered_map
在实现上的区别以不同的顺序显示相同的哈希值。
关于c++ - 散列任意精度值(boost::multiprecision::cpp_int),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/30097385/