Re: [boost] FFT proposal for gsoc 2021

Eduardo Quintana via Boost said: (by the date of Fri, 02 Apr 2021 08:18:07 +0000)

the Gsoc proposal. That you can find here: https://github.com/Lagrang3/gsoc-2021/releases/download/v2.0/gsoc.pdf

Hi, I only have a couple of remarks after reading the proposal.

Also dft can be computed for a limited number of real number representation types: float and double.

Not true. fftw supports also long double and float128, but not boost::mpfr or boost::cpp_bin_float. See e.g. debian packages: libfftw3-long3 , libfftw3-quad3. And a super short example: #include <iostream> #include #include int main() { using Real = boost::float128_t; using Complex = std::complex<Real>; std::vector<Complex> in {{1,-1}, { 0 , 0 }, { 1 , 0.5} , { 0 , 0.5 } , {0.3Q, 1.1Q} }; std::vector<Complex> out{{0, 0}, { 0 , 0 }, { 0 , 0 } , { 0 , 0 } , { 0 , 0 } }; size_t N=in.size(); fftwq_plan p; p = fftwq_plan_dft_1d(N , (Real (*)[2])(in.data()) , (Real (*)[2])(out.data()) , FFTW_FORWARD, FFTW_ESTIMATE); fftwq_execute(p); fftwq_destroy_plan(p); int digs = std::numeric_limits<Real>::digits10; std::cout << "precision digits=" << digs << "\n"; std::cout << std::setprecision(digs+1); for(size_t i = 0 ; i < N ; ++i) { std::cout << in [i] << "\n"; } std::cout << "---\n"; for(size_t i = 0 ; i < N ; ++i) { std::cout << (out[i]/std::sqrt(Real(N))) << "\n"; } std::cout << "---\n"; std::cout << std::sqrt(Real(3.Q)) << "\n"; } You need to link -lfftw3q. Same with -lfftw3l for long double. Just replace s/fftwq/fftwl/gc in function names. And using Real = long double.

double[2], which lacks of the algebraic semantics that std::complex<double> has;

not true. The C++ standard requires that any std::complex<T> number can be reinterpret_cast down to an array[2] of that type, see: https://en.cppreference.com/w/cpp/numeric/complex and https://isocpp.org/files/papers/n4296.pdf paragraph 26.4 page 926

In astrophysics fft is used to solve the differential field equations

It would be good to make sure that your API design is also compatible with the native boost library for solving ordinary differential equations: https://www.boost.org/doc/libs/1_75_0/libs/numeric/odeint/doc/html/index.htm...

3.2 Future outlook The D-dimensional mpi dft is an issue we look forward to tackle using a scalable (D − 1)-dimensional domain decomposition

Since the main focus will be on API (a good choice), please make sure that your multi-dimensional API is compatible with boost::ublas::tensor. Regular users will want to use other ublas::tensor capabilities. Maybe even some of them, such as dimensional slicing (you discuss it in details in the proposal), will be useful for you.

https://github.com/Lagrang3/P2.16_Parallel_FFT/tree/master/solution

this link does not work.

4 Timeline

It is very nicely detailed. It should be easy to track progress with it. I am still worried that there is a little bit too much in it. But we will see. I'm sure it can be adjusted during run-time :) best regards Janek -- Janek Kozicki, PhD. DSc. Arch. Assoc. Prof. Gdańsk University of Technology Faculty of Applied Physics and Mathematics Department of Theoretical Physics and Quantum Information -- http://yade-dem.org/ http://pg.edu.pl/jkozicki (click English flag on top right)