Retain Duplicates with Set Intersection in CUDA - cuda

Question

I'm using CUDA and THRUST to perform paired set operations. I would like to retain duplicates, however. For example:
int keys[6] = {1, 1, 1, 3, 4, 5, 5};
int vals[6] = {1, 2, 3, 4, 5, 6, 7};
int comp[2] = {1, 5};
thrust::set_intersection_by_key(keys, keys + 6, comp, comp + 2, vals, rk, rv);
Desired result
rk[1, 1, 1, 5, 5]
rv[1, 2, 3, 6, 7]
Actual Result
rk[1, 5]
rv[5, 7]
I want all of the vals where the corresponding key is contained in comp.
Is there any way to achieve this using thrust, or do I have to write my own kernel or thrust function?
I'm using this function: set_intersection_by_key.

Answer 1

Quoting from the thrust documentation:
The generalization is that if an element appears m times in [keys_first1, keys_last1) and n times in [keys_first2, keys_last2) (where m may be zero), then it appears min(m,n) times in the keys output range
Since comp does only contain each key once, n=1 and therefore min(m,1) = 1.
In order to get "all of the vals where the corresponding key is contained in comp", you can use the approach of my answer to a similar problem.
Similarly, the example code does the following steps:
Get the largest element of d_comp. This assumes that d_comp is already sorted.
Create vector d_map of size largest_element+1. Copy 1 to all positions of the entries of d_comp in d_map.
Copy all entries from d_vals for which there is a 1 entry in d_map into d_result.
#include <thrust/device_vector.h>
#include <thrust/iterator/constant_iterator.h>
#include <thrust/iterator/permutation_iterator.h>
#include <thrust/functional.h>
#include <thrust/copy.h>
#include <thrust/scatter.h>
#include <iostream>
#define PRINTER(name) print(#name, (name))
void print(const char* name, const thrust::device_vector<int>& v)
{
std::cout << name << ":\t";
thrust::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, "\t"));
std::cout << std::endl;
}
int main()
{
int keys[] = {1, 1, 1, 3, 4, 5, 5};
int vals[] = {1, 2, 3, 4, 5, 6, 7};
int comp[] = {1, 5};
const int size_data = sizeof(keys)/sizeof(keys[0]);
const int size_comp = sizeof(comp)/sizeof(comp[0]);
// copy data to GPU
thrust::device_vector<int> d_keys (keys, keys+size_data);
thrust::device_vector<int> d_vals (vals, vals+size_data);
thrust::device_vector<int> d_comp (comp, comp+size_comp);
PRINTER(d_keys);
PRINTER(d_vals);
PRINTER(d_comp);
int largest_element = d_comp.back();
thrust::device_vector<int> d_map(largest_element+1);
thrust::constant_iterator<int> one(1);
thrust::scatter(one, one+size_comp, d_comp.begin(), d_map.begin());
PRINTER(d_map);
thrust::device_vector<int> d_result(size_data);
using namespace thrust::placeholders;
int final_size = thrust::copy_if(d_vals.begin(),
d_vals.end(),
thrust::make_permutation_iterator(d_map.begin(), d_keys.begin()),
d_result.begin(),
_1
) - d_result.begin();
d_result.resize(final_size);
PRINTER(d_result);
return 0;
}
output:
d_keys: 1 1 1 3 4 5 5
d_vals: 1 2 3 4 5 6 7
d_comp: 1 5
d_map: 0 1 0 0 0 1
d_result: 1 2 3 6 7