libstdc++-v3/src/c++11/hash_c++0x.cc

   1 // std::hash definitions -*- C++ -*-
   2
   3 // Copyright (C) 2010-2018 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the
   7 // terms of the GNU General Public License as published by the
   8 // Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // This library is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 #if __cplusplus < 201103L
  26 # error "hash_c++0x.cc must be compiled with -std=gnu++0x"
  27 #endif
  28
  29 #include <type_traits>
  30 #include <bits/functional_hash.h>
  31
  32 namespace std _GLIBCXX_VISIBILITY(default)
  33 {
  34   _GLIBCXX_PURE size_t
  35   hash<long double>::operator()(long double __val) const noexcept
  36   {
  37     // 0 and -0 both hash to zero.
  38     if (__val == 0.0L)
  39       return 0;
  40
  41     int __exponent;
  42     __val = __builtin_frexpl(__val, &__exponent);
  43     __val = __val < 0.0l ? -(__val + 0.5l) : __val;
  44
  45     const long double __mult = __SIZE_MAX__ + 1.0l;
  46     __val *= __mult;
  47
  48     // Try to use all the bits of the mantissa (really necessary only
  49     // on 32-bit targets, at least for 80-bit floating point formats).
  50     const size_t __hibits = (size_t)__val;
  51     __val = (__val - (long double)__hibits) * __mult;
  52
  53     const size_t __coeff = __SIZE_MAX__ / __LDBL_MAX_EXP__;
  54
  55     return __hibits + (size_t)__val + __coeff * __exponent;
  56   }
  57 }