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Tweak the float hashing algorithm to perform fewer calls to ldexp for some

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types and possibly generate a better hash value.


[SVN r34273]
This commit is contained in:
Daniel James
2006-06-11 15:29:09 +00:00
parent a4d9b52094
commit cae97c67d8
1 changed files with 22 additions and 11 deletions
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33
include/boost/functional/detail/hash_float.hpp
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@@ -18,7 +18,6 @@
#include <boost/integer/static_log2.hpp>
#include <boost/limits.hpp>
#include <boost/assert.hpp>
#include <errno.h>
// Don't use fpclassify or _fpclass for stlport.
#if !defined(__SGI_STL_PORT) && !defined(_STLPORT_VERSION)
@@ -49,22 +48,34 @@ namespace boost
inline std::size_t float_hash_impl(T v)
{
int exp = 0;
errno = 0;
v = boost::hash_detail::call_frexp(v, &exp);
if(errno) return 0;
std::size_t seed = 0;
// 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.
if(v >= 0) {
v = boost::hash_detail::call_frexp(v, &exp) - 0.5;
}
else {
v = -boost::hash_detail::call_frexp(v, &exp) - 0.5;
exp = ~exp;
}
BOOST_ASSERT(0 <= v && v < 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 *
boost::static_log2<std::numeric_limits<T>::radix>::value +
std::numeric_limits<int>::digits - 1)
/ std::numeric_limits<int>::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);
}
@@ -116,7 +127,7 @@ namespace boost
return 0;
}
#else
return float_hash_impl(v);
return v == 0 ? 0 : float_hash_impl(v);
#endif
}
}