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# Archives Discussions

Journeyman III

## Simplifying and Optimizing a Recursive Sort Algorithm for OpenCL?

I have a recursive O(n log(n)) (I think) sorting algorithm that should work decently if parallelized. Here is the code using C++ code.

`// Groups the array into two parts: low and high.for(int i = 0; i < LENGTH/2; i++){    auto &objectA = vec;    auto &objectB = vec[(LENGTH+1)/2+i];    if(objectB < objectA)        std::swap(objectA, objectB);}// Exposes the highest half of low and lowest half of high.// The next two statements can be done concurrentlystd::partial_sort (vec.begin() + LENGTH/2, vec.end() - LENGTH/4, vec.end());std::partial_sort (vec.rbegin() + LENGTH/2, vec.rend() - LENGTH/4, vec.rend(),    [](int a, int b){ return b<a; });/* Simplified from the these two sort calls. * Sorts the exposed halves. Makes low and high only have the lowest and highest entries, respectively. *  std::sort (vec.begin() + LENGTH/4, vec.end() - LENGTH/4); * Sorts the low half of the array. *  std::sort (vec.begin(), vec.begin() + LENGTH/2); */std::sort (vec.begin(), vec.end() - LENGTH/4);// Sort the high half of the array.std::sort (vec.end() - LENGTH/2, vec.end());`

There are four recursive calls so I'm getting a headache by looking at it. I don't have  experience simplifying very complex algorithms like. After dealing with the recursion, my main concern is what to do when the arrays get small. Sorting those small arrays could take a serious chunk out of performance.

Any ideas for the algorithm? Any ideas on how I should make this more comprehensible for myself?

EDIT: When replacing the std::sorts with my own implementation, I realized I used a sum instead of a product to calculate the big O.

Message was edited by: Kyle Siefring