The library only includes vkFFT.h file (in C) and a set of shaders (C-like language compiled to SPIR-V). Vulkan_FFT.cpp is only an example that shows how VkFFT can be used. It also contains the benchmark in it, but it is not a part of the library.
Aah, okay, I where a little confused about the Vulkan_FFT.cpp. It seemed a little weird to have everything in the .h file, and not just the functions you want to expose in the library.
Again, I know no C++ and a very limited amount of C, so don’t put to much value in my comment. You seem to be very fond of switch statements, consider not stuffing to much code into each case. It make the flow hard to follow. Break the case code into functions and call those.
You have a switch with 40 cases, to load the SPIR-V. I feel like there’s a better way to deal with that. Maybe just a strict naming convention, so having the ID is enough to locate the file.
There are surely many different ways to get the job done. VkFFT.h file by itself desn't do any computations btw - it is more like a configurator that launches shaders (sth similar to kernels in CUDA). Having only one header also makes it easier to distribute the library as a code.
I just like the switches as in glsl(shaders) compiler can optimize and unroll them, if the key is provided as a constant. This can provide a non-negligible algorithm speed increase. So over the course of development I came to the point that it is not hard to follow the switches logic for me.