$ cat t1008.cu
#include <thrust/device_vector.h>
#include <thrust/transform.h>
#include <thrust/merge.h>
#include <thrust/iterator/zip_iterator.h>

struct multiply_transformation : public thrust::binary_function<thrust::tuple<int, int>, thrust::tuple<int, int>, thrust::tuple<int, int> >
{
    __host__ __device__
        thrust::tuple<int, int> operator()(thrust::tuple<int, int> d_left, thrust::tuple<int, int> d_right)
    {
        if (thrust::get<0>(d_left) == thrust::get<1>(d_right)) {
            return thrust::make_tuple(thrust::get<0>(d_left), thrust::get<1>(d_left) * thrust::get<1>(d_right));
        }
        return thrust::make_tuple(-1, -1);
    }
};

int main(){
int h_leftCount = 1;
int h_rightCount = 1;
thrust::device_vector<int> d_leftCountKeys(h_leftCount);
thrust::device_vector<int> d_rightCountKeys(h_rightCount);
thrust::device_vector<int> d_leftCounts(h_leftCount);
thrust::device_vector<int> d_rightCounts(h_rightCount);

thrust::device_vector<int> d_mergedKeys(h_leftCount + h_rightCount);
thrust::device_vector<int> d_mergedValues(h_leftCount + h_rightCount);
thrust::merge_by_key(d_leftCountKeys.begin(), d_leftCountKeys.begin() + h_leftCount,
    d_rightCountKeys.begin(), d_rightCountKeys.begin() + h_rightCount,
    d_leftCounts.begin(), d_rightCounts.begin(), d_mergedKeys.begin(), d_mergedValues.begin());

typedef thrust::tuple<thrust::device_vector<int>::iterator, thrust::device_vector<int>::iterator > ZipTuple;

thrust::zip_iterator<ZipTuple> d_zippedCounts(thrust::make_tuple(d_mergedKeys.begin(), d_mergedValues.begin()));

thrust::zip_iterator<ZipTuple> d_zippedCountsOffset(d_zippedCounts + 1);


multiply_transformation transformOperator;
typedef thrust::tuple<int, int> IntTuple;
thrust::device_vector<IntTuple> d_transformedResult(h_leftCount + h_rightCount);
thrust::transform(d_zippedCounts, d_zippedCounts + h_leftCount + h_rightCount - 1, d_zippedCountsOffset, d_transformedResult.begin(), transformOperator);

}
$ nvcc -o t1008 t1008.cu
$ ./t1008
$