boost/boost/functional/hash/detail/hash_float_generic.hpp

   1
   2 // Copyright 2005-2009 Daniel James.
   3 // Distributed under the Boost Software License, Version 1.0. (See accompanying
   4 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   5
   6 // A general purpose hash function for non-zero floating point values.
   7
   8 #if !defined(BOOST_FUNCTIONAL_HASH_DETAIL_HASH_FLOAT_GENERIC_HEADER)
   9 #define BOOST_FUNCTIONAL_HASH_DETAIL_HASH_FLOAT_GENERIC_HEADER
  10
  11 #include <boost/functional/hash/detail/float_functions.hpp>
  12 #include <boost/integer/static_log2.hpp>
  13 #include <boost/functional/hash/detail/limits.hpp>
  14
  15 #if defined(_MSC_VER) && (_MSC_VER >= 1020)
  16 # pragma once
  17 #endif
  18
  19 #if defined(BOOST_MSVC)
  20 #pragma warning(push)
  21 #if BOOST_MSVC >= 1400
  22 #pragma warning(disable:6294) // Ill-defined for-loop: initial condition does
  23                               // not satisfy test. Loop body not executed
  24 #endif
  25 #endif
  26
  27 namespace boost
  28 {
  29     namespace hash_detail
  30     {
  31         inline void hash_float_combine(std::size_t& seed, std::size_t value)
  32         {
  33             seed ^= value + (seed<<6) + (seed>>2);
  34         }
  35
  36         template <class T>
  37         inline std::size_t float_hash_impl2(T v)
  38         {
  39             boost::hash_detail::call_frexp<T> frexp;
  40             boost::hash_detail::call_ldexp<T> ldexp;
  41
  42             int exp = 0;
  43
  44             v = frexp(v, &exp);
  45
  46             // A postive value is easier to hash, so combine the
  47             // sign with the exponent and use the absolute value.
  48             if(v < 0) {
  49                 v = -v;
  50                 exp += limits<T>::max_exponent -
  51                     limits<T>::min_exponent;
  52             }
  53
  54             // The result of frexp is always between 0.5 and 1, so its
  55             // top bit will always be 1. Subtract by 0.5 to remove that.
  56             v -= T(0.5);
  57             v = ldexp(v, limits<std::size_t>::digits + 1);
  58             std::size_t seed = static_cast<std::size_t>(v);
  59             v -= seed;
  60
  61             // ceiling(digits(T) * log2(radix(T))/ digits(size_t)) - 1;
  62             std::size_t const length
  63                 = (limits<T>::digits *
  64                         boost::static_log2<limits<T>::radix>::value - 1)
  65                 / limits<std::size_t>::digits;
  66
  67             for(std::size_t i = 0; i != length; ++i)
  68             {
  69                 v = ldexp(v, limits<std::size_t>::digits);
  70                 std::size_t part = static_cast<std::size_t>(v);
  71                 v -= part;
  72                 hash_float_combine(seed, part);
  73             }
  74
  75             hash_float_combine(seed, exp);
  76
  77             return seed;
  78         }
  79
  80         template <class T>
  81         inline std::size_t float_hash_impl(T v)
  82         {
  83             typedef BOOST_DEDUCED_TYPENAME select_hash_type<T>::type type;
  84             return float_hash_impl2(static_cast<type>(v));
  85         }
  86     }
  87 }
  88
  89 #if defined(BOOST_MSVC)
  90 #pragma warning(pop)
  91 #endif
  92
  93 #endif