WFMath  0.3.12
shuffle.h
00001 // randgen.h (random number functions)
00002 //
00003 //  The WorldForge Project
00004 //  Copyright (C) 2002  The WorldForge Project
00005 //
00006 //  This program is free software; you can redistribute it and/or modify
00007 //  it under the terms of the GNU General Public License as published by
00008 //  the Free Software Foundation; either version 2 of the License, or
00009 //  (at your option) any later version.
00010 //
00011 //  This program is distributed in the hope that it will be useful,
00012 //  but WITHOUT ANY WARRANTY; without even the implied warranty of
00013 //  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00014 //  GNU General Public License for more details.
00015 //
00016 //  You should have received a copy of the GNU General Public License
00017 //  along with this program; if not, write to the Free Software
00018 //  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00019 //
00020 //  For information about WorldForge and its authors, please contact
00021 //  the Worldforge Web Site at http://www.worldforge.org.
00022 
00023 // Author: Ron Steinke
00024 // Created: 2002-5-23
00025 
00026 #ifndef WFMATH_SHUFFLE_H
00027 #define WFMATH_SHUFFLE_H
00028 
00029 #include <wfmath/MersenneTwister.h>
00030 
00031 #include <vector>
00032 
00033 namespace WFMath {
00034 
00036 
00039 template<class C>
00040 void Shuffle(std::vector<C>& v) // need vector for random access
00041 {
00042   typedef typename std::vector<C>::size_type size_type;
00043   size_type pos = v.size();
00044 
00045   if(!pos) // handle size() == 0 nicely
00046     return;
00047 
00048   // This swaps each element with one of the ones before
00049   // it, starting with the last element. Essentially,
00050   // this generates an operation from the permutation
00051   // group of size() elements, and applies it to the
00052   // vector. Note that the loop only executes size() - 1
00053   // times, as element 0 has nothing to swap with.
00054   while(--pos) {
00055     size_type new_pos = MTRand::instance.randInt(pos); // 0 <= new_pos <= pos
00056     if(new_pos == pos)
00057       continue;
00058     C tmp = v[pos];
00059     v[pos] = v[new_pos];
00060     v[new_pos] = tmp;
00061   }
00062 }
00063 
00064 } // namespace WFMath
00065 
00066 #endif  // WFMATH_SHUFFLE_H