Can Metaprogramming Help Here?

[H. S. Teoh]

On Tue, Feb 23, 2021 at 10:24:50PM +0000, Mike Brown via Digitalmars-d-learn wrote:
> Hi all,
> 
> Im porting some C++ code, which has a mess of a section that
> implements prime number type id's. I've had to smother it to death
> with test cases to get it reliable, I think metaprogramming that D
> provides is the better solution - Id rather not reimplement that C++
> mess ideally.

Try something like this:

-----------------------------snip-----------------------------
import std;

int[] firstNPrimes(int n) {
	// FIXME: replace this with actual primes computation
	return [ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31 ];
}

string genEnum(string enumName, idents...)() {
	string code = "enum " ~ enumName ~ " {";
	auto primes = firstNPrimes(idents.length);
	foreach (i, ident; idents) {
		code ~= ident ~ " = " ~ primes[i].to!string ~ ", ";
	}
	code ~= "}";
	return code;
}

template PrimeEnum(idents...) {
	mixin(genEnum!("PrimeEnum", idents));
}

alias MyEnum = PrimeEnum!(
	"unknown", "newline", "identifier", "var", "user_defined",
);

void main() {
	writefln("%(%d\n%)", [
		MyEnum.unknown,
		MyEnum.newline,
		MyEnum.identifier,
		MyEnum.var,
		MyEnum.user_defined
	]);
}
-----------------------------snip-----------------------------


You can substitute the body of firstNPrimes with any standard
prime-generation algorithm. As long as it's not too heavyweight, you
should be able to get it to compile without the compiler soaking up
unreasonable amounts of memory. :-D  If you find the compiler using up
too much memory, try precomputing the list of primes beforehand and
pasting it into firstNPrimes (so that the CTFE engine doesn't have to
recompute it every time you compile).

Note that PrimeEnum can be used to generate any number of enums you wish
to have prime values.  Or if you replace the call to firstNPrimes with
something else, you can generate enums whose identifiers map to any
integer sequence of your choosing.


T

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