Re: A coding challenge: was: luaD_precall() question

[Dibyendu Majumdar <mobile@...>]

Hi,

On 2 July 2017 at 01:50, Coda Highland <chighland@gmail.com> wrote:
> Well, let's see here.
>
> If you support four integer types, two floating-point types, void, and
> nothing else, then you can handle no parameters or one parameter and
> one return type easily enough, but you can't fit two parameters (that
> would need 343 distinct values and you only have 255).
>
> If you require all of the parameters to be of the same type (this
> would let you use most math functions) and explicitly encode the
> function's arity, then you only need six input data types instead of
> seven; you could support up to five parameters this way using a total
> of 217 values. This leaves 38 more slots available, which you can use
> to encode some common other signatures, such as (void*), (void*,
> size_t), and so on. (Since this is C and not C++ you don't have to
> worry about encoding the specific type of the pointer.)
>
> You could also support up to three parameters of the same type, or a
> void* and up to two parameters of the same type, and that would use
> 224 values, leaving 31 slots for special-case parameter lists.
>
> If you put some restrictions on available return types (e.g. restrict
> the choices to "void", "same as the parameter list", and "pointer")
> then you can squeeze out even more options but I don't feel like doing
> the math for how many that is, and you might need to use some special
> cases for 0-ary functions that return a value anyway.
>

Indeed - using a single byte allows me to encode 255 signatures. The
challenge is more how to map the signature and the actual types to a
function pointer without gong through a lot of branchy type checks. So
I want a hashing function that is as follows:

map(function_signature_code, actual_parameter_types[]) -> function pointer.

The idea is that good values map to appropriate function pointer that
can be invoked. Bad values (i.e. wrong parameters on Lua stack) - map
to a special error function.

I want to do this mapping almost as a lookup table ... but the problem
with a lookup table is that it would have to handle all possible
values and have to be multi-dimensional. But conceptually this is what
I would like to do.

Thanks and Regards
Dibyendu