// L-32 MCS 360 Fri 5 Nov 2010 : partition_with_iterator.cpp

/* This program tests the partition of a vector of pairs. */

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <vector>
using namespace std;

vector< pair<int,char> > random_vector ( int n );
// returns a vector of n random 2-digit numbers
// associated with lower case letters

void write_vector
 ( vector< pair<int,char> >::const_iterator first,
   vector< pair<int,char> >::const_iterator last );
// writes from *first to *last, *last included

vector< pair<int,char> >::iterator partition
 ( vector< pair<int,char> >::iterator lower,
   vector< pair<int,char> >::iterator upper );
// precondition: lower != upper
// takes *lower as pivot and interchanges elements so that
// v[i].first <= v[pivot].first for all i < pivot, and
// v[i].first > v[pivot].first for all i > pivot,
// where lower <= pivot <= upper

int main()
{
   cout << "give n : ";
   int n; cin >> n;

   srand(time(0));
   vector< pair<int,char> > v = random_vector(n);
   cout << n << " random items : ";
   write_vector(v.begin(),v.end()-1); cout << endl;

   vector< pair<int,char> >::iterator pivot = v.begin();
   if(n > 1) pivot = partition(v.begin(),v.end()-1);

   cout << "the partitioned vector : " << endl;
   cout << "-> first half : ";
   write_vector(v.begin(),pivot-1); cout << endl;
   cout << "-> the pivot : ";
   write_vector(pivot,pivot); cout << endl;
   cout << "-> second half : ";
   write_vector(pivot+1,v.end()-1); cout << endl;

   return 0;
}

vector< pair<int,char> > random_vector ( int n )
{
   vector< pair<int,char> > v;

   for(int i=0; i<n; i++)
   {
      pair<int,char> p;
      p.first = 10 + (rand() % 90);
      p.second = 'a' + rand() % 26;
      v.push_back(p);
   }

   return v;
}

void write_vector
 ( vector< pair<int,char> >::const_iterator first,
   vector< pair<int,char> >::const_iterator last )
{
   vector< pair<int,char> >::const_iterator i;

   for(i=first; i!=(last+1); i++)
      cout << "(" << i->first
           << "," << i->second
           << ")";
}

vector< pair<int,char> >::iterator partition
 ( vector< pair<int,char> >::iterator lower,
   vector< pair<int,char> >::iterator upper )
{
   vector< pair<int,char> >::iterator pivot;
   vector< pair<int,char> >::iterator up = lower+1;
   vector< pair<int,char> >::iterator down = upper;

   pair<int,char> x = *lower;

   while(up != down)
   {
      while((up != down) && (up->first <= x.first)) up++;
      while((up != down) && (down->first > x.first)) down--;

      if(up == down) break;

      pair<int,char> tmp = *up;
      *up = *down; *down = tmp;
   }
   if(up->first > x.first) up--;
   *lower = *up; *up = x;
   pivot = up;

   return pivot;
}