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libs/multi_index/example/rearrange.cpp

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+/* Boost.MultiIndex example of use of rearrange facilities.
+ *
+ * Copyright 2003-2008 Joaquin M Lopez Munoz.
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * http://www.boost.org/LICENSE_1_0.txt)
+ *
+ * See http://www.boost.org/libs/multi_index for library home page.
+ */
+
+#if !defined(NDEBUG)
+#define BOOST_MULTI_INDEX_ENABLE_INVARIANT_CHECKING
+#define BOOST_MULTI_INDEX_ENABLE_SAFE_MODE
+#endif
+
+#include <boost/config.hpp>
+#include <boost/detail/iterator.hpp>
+#include <boost/multi_index_container.hpp>
+#include <boost/multi_index/random_access_index.hpp>
+#include <boost/random/binomial_distribution.hpp>
+#include <boost/random/uniform_real.hpp>
+#include <boost/random/mersenne_twister.hpp>
+#include <algorithm>
+#include <iostream>
+#include <iterator>
+#include <vector>
+
+using boost::multi_index_container;
+using namespace boost::multi_index;
+
+/* We model a card deck with a random access array containing
+ * card numbers (from 0 to 51), supplemented with an additional
+ * index which retains the start ordering.
+ */
+
+class deck
+{
+  BOOST_STATIC_CONSTANT(std::size_t,num_cards=52);
+
+  typedef multi_index_container<
+    int,
+    indexed_by<
+      random_access<>, /* base index    */
+      random_access<>  /* "start" index */
+    >
+  >                              container_type;
+  container_type cont;
+
+public:
+  deck()
+  {
+    cont.reserve(num_cards);
+    get<1>(cont).reserve(num_cards);
+    for(std::size_t i=0;i<num_cards;++i)cont.push_back(i);
+  }
+
+  typedef container_type::iterator  iterator;
+  typedef container_type::size_type size_type;
+
+  iterator  begin()const{return cont.begin();}
+  iterator  end()const{return cont.end();}
+  size_type size()const{return cont.size();}
+
+  template<typename InputIterator>
+  void rearrange(InputIterator it)
+  {
+    cont.rearrange(it);
+  }
+
+  void reset()
+  {
+    /* simply rearrange the base index like the start index */
+
+    cont.rearrange(get<1>(cont).begin());
+  }
+
+  std::size_t position(int i)const
+  {
+    /* The position of a card in the deck is calculated by locating
+     * the card through the start index (which is ordered), projecting
+     * to the base index and diffing with the begin position.
+     * Resulting complexity: constant.
+     */
+
+    return project<0>(cont,get<1>(cont).begin()+i)-cont.begin();
+  }
+
+  std::size_t rising_sequences()const
+  {
+    /* Iterate through all cards and increment the sequence count
+     * when the current position is left to the previous.
+     * Resulting complexity: O(n), n=num_cards.
+     */
+
+    std::size_t s=1;
+    std::size_t last_pos=0;
+
+    for(std::size_t i=0;i<num_cards;++i){
+      std::size_t pos=position(i);
+      if(pos<last_pos)++s;
+      last_pos=pos;
+    }
+
+    return s;
+  }
+};
+
+/* A vector of reference_wrappers to deck elements can be used
+ * as a view to the deck container.
+ * We use a special implicit_reference_wrapper having implicit
+ * ctor from its base type, as this simplifies the use of generic
+ * techniques on the resulting data structures.
+ */
+
+template<typename T>
+class implicit_reference_wrapper:public boost::reference_wrapper<T>
+{
+private:
+  typedef boost::reference_wrapper<T> super;
+public:
+  implicit_reference_wrapper(T& t):super(t){}
+};
+
+typedef std::vector<implicit_reference_wrapper<const int> > deck_view;
+
+/* Riffle shuffle is modeled like this: A cut is selected in the deck
+ * following a binomial distribution. Then, cards are randomly selected
+ * from one packet or the other with probability proportional to
+ * packet size.
+ */
+
+template<typename RandomAccessIterator,typename OutputIterator>
+void riffle_shuffle(
+  RandomAccessIterator first,RandomAccessIterator last,
+  OutputIterator out)
+{
+  static boost::mt19937 rnd_gen;
+
+  typedef typename boost::detail::iterator_traits<
+    RandomAccessIterator>::difference_type         difference_type;
+  typedef boost::binomial_distribution<
+    difference_type>                               rnd_cut_select_type;
+  typedef boost::uniform_real<>                    rnd_deck_select_type;
+
+  rnd_cut_select_type  cut_select(last-first);
+  RandomAccessIterator middle=first+cut_select(rnd_gen);
+  difference_type      s0=middle-first;
+  difference_type      s1=last-middle;
+  rnd_deck_select_type deck_select;
+
+  while(s0!=0&&s1!=0){
+    if(deck_select(rnd_gen)<(double)s0/(s0+s1)){
+      *out++=*first++;
+      --s0;
+    }
+    else{
+      *out++=*middle++;
+      --s1;
+    }
+  }
+  std::copy(first,first+s0,out);
+  std::copy(middle,middle+s1,out);
+}
+
+struct riffle_shuffler
+{
+  void operator()(deck& d)const
+  {
+    dv.clear();
+    dv.reserve(d.size());
+    riffle_shuffle(
+      d.begin(),d.end(),std::back_inserter(dv)); /* do the shuffling  */
+    d.rearrange(dv.begin());                     /* apply to the deck */
+  }
+
+private:
+  mutable deck_view dv;
+};
+
+/* A truly random shuffle (up to stdlib implementation quality) using
+ * std::random_shuffle.
+ */
+
+struct random_shuffler
+{
+  void operator()(deck& d)const
+  {
+    dv.clear();
+    dv.reserve(d.size());
+    std::copy(d.begin(),d.end(),std::back_inserter(dv));
+    std::random_shuffle(dv.begin(),dv.end()); /* do the shuffling  */
+    d.rearrange(dv.begin());                  /* apply to the deck */
+  }
+
+private:
+  mutable deck_view dv;
+};
+
+/* Repeat a given shuffling algorithm repeats_num times
+ * and obtain the resulting rising sequences number. Average
+ * for tests_num trials.
+ */
+
+template<typename Shuffler>
+double shuffle_test(
+ unsigned int repeats_num,unsigned int tests_num
+ BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE(Shuffler))
+{
+  deck          d;
+  Shuffler      sh;
+  unsigned long total=0;
+
+  for(unsigned int n=0;n<tests_num;++n){
+    for(unsigned m=0;m<repeats_num;++m)sh(d);
+    total+=d.rising_sequences();
+    d.reset();
+  }
+
+  return (double)total/tests_num;
+}
+
+int main()
+{
+  unsigned rifs_num=0;
+  unsigned tests_num=0;
+
+  std::cout<<"number of riffle shuffles (vg 5):";
+  std::cin>>rifs_num;
+  std::cout<<"number of tests (vg 1000):";
+  std::cin>>tests_num;
+
+  std::cout<<"shuffling..."<<std::endl;
+
+  std::cout<<"riffle shuffling\n"
+             "  avg number of rising sequences: "
+           <<shuffle_test<riffle_shuffler>(rifs_num,tests_num)
+           <<std::endl;
+
+  std::cout<<"random shuffling\n"
+             "  avg number of rising sequences: "
+           <<shuffle_test<random_shuffler>(1,tests_num)
+           <<std::endl;
+
+  return 0;
+}