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Index Sort: Example and Test
`

    # include <cppad/utility/index_sort.hpp>
    # include <cppad/utility/vector.hpp>
    # include <valarray>
    # include <vector>

    namespace{
         // class that uses < to compare a pair of size_t values
         class Key {
         public:
              size_t first_;
              size_t second_;
              //
              Key(void)
              { }
              //
              Key(size_t first, size_t second)
              : first_(first), second_(second)
              { }
              //
              bool operator<(const Key& other) const
              {     if( first_ == other.first_ )
                        return second_ < other.second_;
                   return first_ < other.first_;
              }
         };

         template <class VectorKey, class VectorSize>
         bool vector_case(void)
         {     bool ok = true;
              size_t i, j;
              size_t first[]  =  { 4, 4, 3, 3, 2, 2, 1, 1};
              size_t second[] = { 0, 1, 0, 1, 0, 1, 0, 1};
              size_t size     = sizeof(first) / sizeof(first[0]);

              VectorKey keys(size);
              for(i = 0; i < size; i++)
                   keys[i] = Key(first[i], second[i]);

              VectorSize ind(size);
              CppAD::index_sort(keys, ind);

              // check that all the indices are different
              for(i = 0; i < size; i++)
              {     for(j = 0; j < size; j++)
                        ok &= (i == j) | (ind[i] != ind[j]);
              }

              // check for increasing order
              for(i = 0; i < size-1; i++)
              {     if( first[ ind[i] ] == first[ ind[i+1] ] )
                        ok &= second[ ind[i] ] <= second[ ind[i+1] ];
                   else     ok &= first[ ind[i] ] < first[ ind[i+1] ];
              }

              return ok;
         }
    }

    bool index_sort(void)
    {     bool ok = true;

         // some example simple vector template classes
         ok &= vector_case<  std::vector<Key>,  std::valarray<size_t> >();
         ok &= vector_case< std::valarray<Key>, CppAD::vector<size_t> >();
         ok &= vector_case< CppAD::vector<Key>,   std::vector<size_t> >();

         return ok;
    }

`

Input File: example/utility/index_sort.cpp