libstdc++
random_number.h
Go to the documentation of this file.
00001 // -*- C++ -*-
00002 
00003 // Copyright (C) 2007, 2008, 2009 Free Software Foundation, Inc.
00004 //
00005 // This file is part of the GNU ISO C++ Library.  This library is free
00006 // software; you can redistribute it and/or modify it under the terms
00007 // of the GNU General Public License as published by the Free Software
00008 // Foundation; either version 3, or (at your option) any later
00009 // version.
00010 
00011 // This library is distributed in the hope that it will be useful, but
00012 // WITHOUT ANY WARRANTY; without even the implied warranty of
00013 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00014 // General Public License for more details.
00015 
00016 // Under Section 7 of GPL version 3, you are granted additional
00017 // permissions described in the GCC Runtime Library Exception, version
00018 // 3.1, as published by the Free Software Foundation.
00019 
00020 // You should have received a copy of the GNU General Public License and
00021 // a copy of the GCC Runtime Library Exception along with this program;
00022 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
00023 // <http://www.gnu.org/licenses/>.
00024 
00025 /** @file parallel/random_number.h
00026  *  @brief Random number generator based on the Mersenne twister.
00027  *  This file is a GNU parallel extension to the Standard C++ Library.
00028  */
00029 
00030 // Written by Johannes Singler.
00031 
00032 #ifndef _GLIBCXX_PARALLEL_RANDOM_NUMBER_H
00033 #define _GLIBCXX_PARALLEL_RANDOM_NUMBER_H 1
00034 
00035 #include <parallel/types.h>
00036 #include <tr1/random>
00037 #include <limits>
00038 
00039 namespace __gnu_parallel
00040 {
00041   /** @brief Random number generator, based on the Mersenne twister. */
00042   class _RandomNumber
00043   {
00044   private:
00045     std::tr1::mt19937   _M_mt;
00046     uint64_t            _M_supremum;
00047     uint64_t            _M_rand_sup;
00048     double              _M_supremum_reciprocal;
00049     double              _M_rand_sup_reciprocal;
00050 
00051     // Assumed to be twice as long as the usual random number.
00052     uint64_t              __cache;
00053 
00054     // Bit results.
00055     int __bits_left;
00056     
00057     static uint32_t
00058     __scale_down(uint64_t __x,
00059 #if _GLIBCXX_SCALE_DOWN_FPU
00060                uint64_t /*_M_supremum*/, double _M_supremum_reciprocal)
00061 #else
00062                uint64_t _M_supremum, double /*_M_supremum_reciprocal*/)
00063 #endif
00064         {
00065 #if _GLIBCXX_SCALE_DOWN_FPU
00066           return uint32_t(__x * _M_supremum_reciprocal);
00067 #else
00068           return static_cast<uint32_t>(__x % _M_supremum);
00069 #endif
00070         }
00071 
00072   public:
00073     /** @brief Default constructor. Seed with 0. */
00074     _RandomNumber()
00075     : _M_mt(0), _M_supremum(0x100000000ULL),
00076       _M_rand_sup(1ULL << std::numeric_limits<uint32_t>::digits),
00077       _M_supremum_reciprocal(double(_M_supremum) / double(_M_rand_sup)),
00078       _M_rand_sup_reciprocal(1.0 / double(_M_rand_sup)),
00079       __cache(0), __bits_left(0) { }
00080 
00081     /** @brief Constructor.
00082      *  @param __seed Random __seed.
00083      *  @param _M_supremum Generate integer random numbers in the
00084      *                  interval @c [0,_M_supremum). */
00085     _RandomNumber(uint32_t __seed, uint64_t _M_supremum = 0x100000000ULL)
00086     : _M_mt(__seed), _M_supremum(_M_supremum),
00087       _M_rand_sup(1ULL << std::numeric_limits<uint32_t>::digits),
00088       _M_supremum_reciprocal(double(_M_supremum) / double(_M_rand_sup)),
00089       _M_rand_sup_reciprocal(1.0 / double(_M_rand_sup)),
00090       __cache(0), __bits_left(0) { }
00091 
00092     /** @brief Generate unsigned random 32-bit integer. */
00093     uint32_t
00094     operator()()
00095     { return __scale_down(_M_mt(), _M_supremum, _M_supremum_reciprocal); }
00096 
00097     /** @brief Generate unsigned random 32-bit integer in the
00098         interval @c [0,local_supremum). */
00099     uint32_t
00100     operator()(uint64_t local_supremum)
00101     {
00102       return __scale_down(_M_mt(), local_supremum,
00103                         double(local_supremum * _M_rand_sup_reciprocal));
00104     }
00105 
00106     /** @brief Generate a number of random bits, run-time parameter.
00107      *  @param bits Number of bits to generate. */
00108     unsigned long
00109     __genrand_bits(int __bits)
00110     {
00111       unsigned long __res = __cache & ((1 << __bits) - 1);
00112       __cache = __cache >> __bits;
00113       __bits_left -= __bits;
00114       if (__bits_left < 32)
00115         {
00116           __cache |= ((uint64_t(_M_mt())) << __bits_left);
00117           __bits_left += 32;
00118         }
00119       return __res;
00120     }
00121 };
00122 
00123 } // namespace __gnu_parallel
00124 
00125 #endif /* _GLIBCXX_PARALLEL_RANDOM_NUMBER_H */