include/libtorrent/random_sample.hpp

   1 /*
   2
   3 Copyright (c) 2006, Arvid Norberg & Daniel Wallin
   4 All rights reserved.
   5
   6 Redistribution and use in source and binary forms, with or without
   7 modification, are permitted provided that the following conditions
   8 are met:
   9
  10     * Redistributions of source code must retain the above copyright
  11       notice, this list of conditions and the following disclaimer.
  12     * Redistributions in binary form must reproduce the above copyright
  13       notice, this list of conditions and the following disclaimer in
  14       the documentation and/or other materials provided with the distribution.
  15     * Neither the name of the author nor the names of its
  16       contributors may be used to endorse or promote products derived
  17       from this software without specific prior written permission.
  18
  19 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  20 AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  21 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  22 ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  23 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  24 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  25 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  26 INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  27 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  28 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  29 POSSIBILITY OF SUCH DAMAGE.
  30
  31 */
  32
  33 #ifndef TORRENT_RANDOM_SAMPLE_HPP
  34 #define TORRENT_RANDOM_SAMPLE_HPP
  35
  36 #include <iterator>
  37 #include <cstdlib>
  38
  39 #include "libtorrent/config.hpp"
  40
  41 namespace libtorrent
  42 {
  43
  44         template<class InIter, class OutIter, class Distance>
  45         inline void random_sample_n(InIter start, InIter end
  46                         , OutIter out, Distance n)
  47         {
  48                 Distance t = 0;
  49                 Distance m = 0;
  50                 Distance N = std::distance(start, end);
  51
  52                 TORRENT_ASSERT(N >= n);
  53
  54                 while (m < n)
  55                 {
  56                         if ((std::rand() / (RAND_MAX + 1.f)) * (N - t) >= n - m)
  57                         {
  58                                 ++start;
  59                                 ++t;
  60                         }
  61                         else
  62                         {
  63                                 *out = *start;
  64                                 ++out;
  65                                 ++start;
  66                                 ++t;
  67                                 ++m;
  68                         }
  69                 }
  70         }
  71
  72 }
  73
  74 #endif