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New algorithm for hash floating point numbers.

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[SVN r40264]
This commit is contained in:
Daniel James
2007-10-21 08:36:47 +00:00
parent a039550901
commit f7877f6738
1 changed files with 26 additions and 6 deletions
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32
include/boost/functional/detail/hash_float.hpp
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@@ -15,6 +15,7 @@
#endif
#include <boost/functional/detail/float_functions.hpp>
#include <boost/integer/static_log2.hpp>
#include <boost/cstdint.hpp>
#include <boost/limits.hpp>
#include <boost/assert.hpp>
@@ -82,23 +83,42 @@ namespace boost
}
#else
template <class T>
inline std::size_t float_hash_impl(T v)
{
int exp = 0;
v = boost::hash_detail::call_frexp(v, &exp);
std::size_t seed = 0;
// A postive value is easier to hash, so combine the
// sign with the exponent.
if(v < 0) {
v = -v;
exp += std::numeric_limits<T>::max_exponent -
std::numeric_limits<T>::min_exponent;
}
// The result of frexp is always between 0.5 and 1, so its
// top bit will always be 1. Subtract by 0.5 to remove that.
v -= T(0.5);
v = boost::hash_detail::call_ldexp(v,
std::numeric_limits<std::size_t>::digits + 1);
std::size_t seed = static_cast<std::size_t>(v);
v -= seed;
// ceiling(digits(T) * log2(radix(T))/ digits(size_t)) - 1;
std::size_t const length
= (std::numeric_limits<T>::digits +
std::numeric_limits<int>::digits - 1)
/ std::numeric_limits<int>::digits;
= (std::numeric_limits<T>::digits *
boost::static_log2<std::numeric_limits<T>::radix>::value
- 1)
/ std::numeric_limits<std::size_t>::digits;
for(std::size_t i = 0; i != length; ++i)
{
v = boost::hash_detail::call_ldexp(v, std::numeric_limits<int>::digits);
int const part = static_cast<int>(v);
v = boost::hash_detail::call_ldexp(v,
std::numeric_limits<std::size_t>::digits);
std::size_t part = static_cast<std::size_t>(v);
v -= part;
hash_float_combine(seed, part);
}