RE: API to get length of array cdata
- From: William Adams <william_a_adams@xxxxxxx>
- To: <luajit@xxxxxxxxxxxxx>
- Date: Sun, 13 May 2012 20:18:02 +0000
I think you're going to have to help out one way or another.
think of this, how would you make this work in C:
int array_size(void *)
Without some amount of type information, it's pretty impossible.
In your hot path case, you're going to have to figure out what 'type' the
elements are somewhere. You either already know it, or you're going to have to
do something like I did in the example.
Perhaps it's just a matter of figuring out where is the most judicious place to
create the least amount of garbage.
What kind of arrays are you dealing with?
-- William
===============================
- Shaping clay is easier than digging it out of the ground.
http://williamaadams.wordpress.com
http://www.thingiverse.com/WilliamAAdams
https://github.com/Wiladams
> From: cosmin.apreutesei@xxxxxxxxx
> Date: Sun, 13 May 2012 21:05:11 +0300
> Subject: Re: API to get length of array cdata
> To: luajit@xxxxxxxxxxxxx
>
> Nice trick. I'm still looking for a garbageless solution for hot code
> though. Not sure how much this obsession with not generating garbage
> in hot loops is justified with the new generational gc and/or LJ2, but
> since this discussion is not really accessible to my therapist.. :)
>
> On Sun, May 13, 2012 at 5:52 PM, William Adams <william_a_adams@xxxxxxx>
> wrote:
> > The hacked up way I'm doing it is this:
> >
> > function array_size(buff)
> > local typestr = tostring(ffi.typeof(buff))
> > local elemtype, nelem = string.match(typestr,
> > "ctype<(%w+)%s+%[(%d+)%]>")
> >
> > return nelem;
> > end
> >
> > It's fragile because it depends on the string representation returned by the
> > "tostring", but it seems to work.
> > I'm sure if the string representation changes, this definition could be made
> > a bit more adaptable.
> > Since there's only a single numeric value in the string representation, you
> > could just return the
> > numeric part: (%d+) and ignore all the rest.
> >
> > local nelem = string.match(typestr, "(%d+)")
> >
> > of course, if you've done a double, triple, or more array, then the string
> > will look like:
> > local arr2 = ffi.new("int[256][10]")
> > ctype<int [256][10]>
> >
> > In that case, you'll have to use a more complex pattern, and use gmatch to
> > capture them all.
> >
> >
> >
> > -- William
> >
> > ===============================
> > - Shaping clay is easier than digging it out of the ground.
> >
> > http://williamaadams.wordpress.com
> > http://www.thingiverse.com/WilliamAAdams
> > https://github.com/Wiladams
> >
> >
> >
> >> From: cosmin.apreutesei@xxxxxxxxx
> >> Date: Sat, 12 May 2012 18:24:06 +0300
> >> Subject: API to get length of array cdata
> >> To: luajit@xxxxxxxxxxxxx
> >
> >>
> >> Hi,
> >>
> >> Is it possible to have a function to get the length of an array cdata
> >> (VLA or not)? Right now I have a function that returns sizeof(cdata) /
> >> sizeof(elem_ctype) but I have to pass it the element's ctype. #cdata
> >> would be swell.
> >>
> >> Cosmin.
> >>
>
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