1 // std::tr1::hash definitions, long double bits -*- C++ -*-
2 
3 // Copyright (C) 2010, 2011 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 namespace std _GLIBCXX_VISIBILITY(default)
26 {
27   namespace tr1
28   {
29     // For long double, careful with random padding bits (e.g., on x86,
30     // 10 bytes -> 12 bytes) and resort to frexp.
31     template<>
32       size_t
33       hash<long double>::operator()(long double __val) const
34       {
35 	// 0 and -0 both hash to zero.
36 	if (__val == 0.0L)
37 	  return 0;
38 
39 	int __exponent;
40 	__val = __builtin_frexpl(__val, &__exponent);
41 	__val = __val < 0.0l ? -(__val + 0.5l) : __val;
42 
43 	const long double __mult = __SIZE_MAX__ + 1.0l;
44 	__val *= __mult;
45 
46 	// Try to use all the bits of the mantissa (really necessary only
47 	// on 32-bit targets, at least for 80-bit floating point formats).
48 	const size_t __hibits = (size_t)__val;
49 	__val = (__val - (long double)__hibits) * __mult;
50 
51 	const size_t __coeff = __SIZE_MAX__ / __LDBL_MAX_EXP__;
52 
53 	return __hibits + (size_t)__val + __coeff * __exponent;
54       }
55   }
56 }
57