Add set operation functions about arrayfire HOT 7 CLOSED

Boost.Compute has these algorithms implemented for OpenCL:

• unique()
• set_union()
• set_intersection()

Let me know if you need help integrating them.

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@kylelutz I think ArrayFire's behavior for set operations is slightly different from Boost.Compute. Let me have a look and see if they can be made compatible.

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@kylelutz Do set_union and set_intersection require the inputs to be sorted ?

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Yeah, they have the same behavior and requirements as the standard C++ algorithms (std::set_union() / std::set_intersection()).

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@kylelutz All the set operations have two APIs like this:

AFAPI array setunique(const array& input);
AFAPI void setunique(array& values, array& indices, array& locations, const array& input);

ArrayFire does not expect the inputs to be sorted, so the input needs to be sorted internally. We can extend the API to include a default parameter bool isSorted=false. This way user can optionally turn off the internal sort.

For the first API we can use std (for CPU), thrust (for CUDA) and Boost.Compute (for OpenCL).
For the second API we can write the code using existing ArrayFire functions.

If you are interested in contributing code, I can create the skeleton and implement the CUDA and CPU backends and leave the OpenCL backend for you.

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I'd be happy to! Probably won't be too much work.

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@kylelutz I just pushed the skeleton and CPU implementation to the "set" branch. Let me know if you want to work on the OpenCL backend for these three functions.

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