support runtime choice of precision

[stevengj]

As commented in #1544, it would be good to be able to switch precision at runtime, by making fields<T>, fields_chunk<T>, structure<T>, and structure_chunk<T> template classes parameterized by the realnum type T.

[oskooi]

I have started working on this by first going through meep.hpp and templating any class which currently includes a realnum type or contains a member function which calls/uses a templated class. The problem is the sheer number of changes that are necessary. Apart from templating the classes fields, fields_chunk, structure, and structure_chunk, there are also other classes susceptibility, boundary_region, flux_vol, etc, that are also templated. Next I would need to template the definitions of all the member functions of these classes which are scattered across numerous files. This is not a straightforward task given the dependencies among the various classes/functions. It will take some time to set this up correctly.

A more practical alternative is to keep the current compile-time option and just enable single precision by default. All we would then need to do is modify the unit tests so that everything passes.

[oskooi]

Another complication to adding templates are two existing typedefs in meep.hpp which cannot be templated but which involve parameters that are templated classes susceptibility and fields_chunk:

meep/src/meep.hpp

Lines 1318 to 1323 in 2dcfa9f

	// data for each susceptibility
	typedef struct polarization_state_s {
	void *data; // internal polarization data for the susceptibility
	const susceptibility *s;
	struct polarization_state_s *next; // linked list
	} polarization_state;

meep/src/meep.hpp

Lines 1476 to 1479 in 2dcfa9f

	typedef void (*field_chunkloop)(fields_chunk *fc, int ichunk, component cgrid, ivec is, ivec ie,
	vec s0, vec s1, vec e0, vec e1, double dV0, double dV1, ivec shift,
	std::complex<double> shift_phase, const symmetry &S, int sn,
	void *chunkloop_data);

The issue here is that a fair amount of code reorganization (not directly related to templating) is required to set this all up.

[stevengj]

I think that any competent C++ programmer should be able to do this — it doesn't require any particular knowledge of how Meep works, and is just a (tedious but straightforward) matter of changing a type in one place and then tracing how that change propagates through the library, which is a common software-engineering task.

[stevengj]

As an alternative, when you build with --enable-single, it could just rename the files when it installs from meep to meep_single.

[oskooi]

Replaced by #1544, #1563, #1688, etc.