FFT: expose OpenBCSolver's spectral Green's function - #5627
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Add greensFunctionFFT() accessors so a caller can keep its own set of Green's functions and move them into and out of the solver, for instance with std::swap, instead of recomputing them with setGreensFunction(). solve() only reads the spectral Green's function, and prepare_openbc() concerns the R2C plans rather than the Green's function values, so one moved in this way needs no further preparation. Note that in two-d mode this is an alias of the Green's function R2C's spectral data: callers may write through it, but must not swap or otherwise re-seat it, because a later setGreensFunction() writes into the aliased array. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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OpenBCSolverrebuilds its spectral Green's function wheneversetGreensFunction()is called, and there is no way to hand it one that was built earlier. AddgreensFunctionFFT()accessors so a caller can keep its own set and move them into and out of the solver withstd::swaprather than recomputing.solve()only reads the spectral Green's function, andprepare_openbc()concerns the R2C plans rather than the Green's function values, so one moved in this way needs no further preparation.The one caveat, documented at the accessor: in 2D,
m_G_fftis an alias of the Green's function R2C's spectral data, which a latersetGreensFunction()writes into. Callers may write through the reference there, but must not swap or otherwise re-seat it.Additive only, so nothing changes for existing users. Exercised downstream in BLAST-WarpX/warpx#7178 & BLAST-ImpactX/impactx#1621, where reusing Green's functions this way reproduces rebuilding them bit for bit, and where it takes an accelerating beam simulation from 100 Green's function builds to 10.