[英]std::bitset hash function algorithm
Does anybody know what algorithm thw hash function for bitset is using, 有人知道bitset使用的哈希函数是什么算法,
this is from website : http://en.cppreference.com/w/cpp/utility/bitset/hash 这是来自网站: http : //en.cppreference.com/w/cpp/utility/bitset/hash
#include <iostream>
#include <bitset>
#include <functional>
int main()
{
std::bitset<4> b1(1);
std::bitset<4> b2(2);
std::bitset<4> b3(b2);
std::bitset<4> b4(8);
std::cout<<b4<<'\n';
std::hash<std::bitset<4>> hash_fn;
size_t h1 = hash_fn(b1);
size_t h2 = hash_fn(b2);
size_t h3 = hash_fn(b4);
std::cout << h1 << '\n';
std::cout << h2 << '\n';
std::cout << h3 << '\n';
}
and the output is 而输出是
1000
4334672815104069193
16667047557902998627
2258353126044249582
http://en.cppreference.com/w/cpp/utility/bitset/hash http://en.cppreference.com/w/cpp/utility/bitset/hash
Also why doesn't it convert the bits to unsigend long and generate a hash value? 另外为什么不将这些位转换为unsigend long并生成一个哈希值?
As noted by Igor , C++ standard does not specify algorithm, it only requires that hash value depends only on object and will be same for the duration of the program: http://eel.is/c++draft/hash.requirements 正如Igor所指出的 ,C ++标准没有指定算法, 它只 要求哈希值仅依赖于对象,并且在程序的持续时间内是相同的: http : //eel.is/c++draft/hash.requirements
20.5.3.4 Hash requirements [hash.requirements] 1 A type H meets the Hash requirements if:
20.5.3.4哈希要求[hash.requirements] 1如果出现以下情况,则H类型满足哈希要求:
- (1.1) it is a function object type,
(1.1)它是一个函数对象类型,
- (1.2) it satisfies the requirements of CopyConstructible and Destructible, and
(1.2)它满足CopyConstructible和Destructible的要求,并且
- (1.3) the expressions shown in Table 29 are valid and have the indicated semantics.
(1.3)表29中所示的表达式是有效的并且具有指示的语义。
2 Given Key is an argument type for function objects of type H, in Table 29 h is a value of type (possibly const) H, u is an lvalue of type Key, and k is a value of a type convertible to (possibly const) Key.
2给定Key是类型H的函数对象的参数类型,在表29中,h是类型的值(可能是const)H,u是Key类型的左值,k是可转换为的类型的值(可能是const) )关键。
Table 29 — Hash requirements
表29 - 哈希要求
- Expression Return type Requirement
表达式返回类型要求
- h(k) size_t The value returned shall depend only on the argument k for the duration of the program.
h(k)size_t返回的值仅取决于程序持续时间的参数k。 [ Note: Thus all evaluations of the expression h(k) with the same value for k yield the same result for a given execution of the program.
[注意:因此,对于给定的程序执行,对具有相同k值的表达式h(k)的所有求值产生相同的结果。 — end note ] [ Note: For two different values t1 and t2, the probability that h(t1) and h(t2) compare equal should be very small, approaching 1.0 / numeric_limits::max().
- 结束注释] [注意:对于两个不同的值t1和t2,h(t1)和h(t2)比较的概率应该非常小,接近1.0 / numeric_limits :: max()。 — end note ]
- 结束说明]
- h(u) size_t Shall not modify u.
h(u)size_t不得修改你。
Gcc's libstdc++ implementation of bitset uses std::hash: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/debug/bitset Gcc的bitset的libstdc ++实现使用std :: hash: https : //github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/debug/bitset
#if __cplusplus >= 201103L
// DR 1182.
/// std::hash specialization for bitset.
template<size_t _Nb>
struct hash<__debug::bitset<_Nb>>
: public __hash_base<size_t, __debug::bitset<_Nb>>
{
size_t
operator()(const __debug::bitset<_Nb>& __b) const noexcept
{ return std::hash<_GLIBCXX_STD_C::bitset<_Nb>>()(__b._M_base()); }
};
#endif
https://github.com/gcc-mirror/gcc/blob/1cb6c2eb3b8361d850be8e8270c597270a1a7967/libstdc%2B%2B-v3/include/std/bitset#L1561 https://github.com/gcc-mirror/gcc/blob/1cb6c2eb3b8361d850be8e8270c597270a1a7967/libstdc%2B%2B-v3/include/std/bitset#L1561
// DR 1182.
/// std::hash specialization for bitset.
template<size_t _Nb>
struct hash<_GLIBCXX_STD_C::bitset<_Nb>>
: public __hash_base<size_t, _GLIBCXX_STD_C::bitset<_Nb>>
{
size_t
operator()(const _GLIBCXX_STD_C::bitset<_Nb>& __b) const noexcept
{
const size_t __clength = (_Nb + __CHAR_BIT__ - 1) / __CHAR_BIT__;
return std::_Hash_impl::hash(__b._M_getdata(), __clength);
}
};
LLVM's libcxx uses own implementation for bitset, xoring all words: https://github.com/llvm-mirror/libcxx/blob/2c4b8af9aada61d83610330416eb8a39a8aa5494/include/bitset#L417 LLVM的libcxx使用自己的bitset实现,xoring所有单词: https : //github.com/llvm-mirror/libcxx/blob/2c4b8af9aada61d83610330416eb8a39a8aa5494/include/bitset#L417
template <size_t _Size>
struct _LIBCPP_TEMPLATE_VIS hash<bitset<_Size> >
: public unary_function<bitset<_Size>, size_t>
{
_LIBCPP_INLINE_VISIBILITY
size_t operator()(const bitset<_Size>& __bs) const _NOEXCEPT
{return __bs.__hash_code();}
};
template <size_t _N_words, size_t _Size>
inline
size_t
__bitset<_N_words, _Size>::__hash_code() const _NOEXCEPT
{
size_t __h = 0;
for (size_type __i = 0; __i < _N_words; ++__i)
__h ^= __first_[__i];
return __h;
}
and simpler variant for 1 word bitset: 1字bitset的更简单的变体:
inline
size_t
__bitset<1, _Size>::__hash_code() const _NOEXCEPT
{
return __first_;
}
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