[Rd] ALTREP ALTINTEGER_SUM/MIN/MAX Return Value and Behavior

[Gabriel Becker]

Also, @Luke Tierney <luke-tierney using uiowa.edu>  I can prepare a patch that
has wrappers delegate to payload's ALTREP class methods for things like
sum, min, max, etc once conference season calms down a bit.

Best,
~G

On Tue, Jun 29, 2021 at 11:07 AM Gabriel Becker <gabembecker using gmail.com>
wrote:

> Hi Sebastian,
>
> So the way that it is currently factored, there isn't a good way of
> getting what you want under the constraints of what Luke said (ALTINTEGER_SUM
> is not part of the API).
>
> I don't know what his reason are for saying that per say and would not
> want to speak for him, but of the top of my head, I suspect it is because
> ALTREP sum methods are allowed to return NULL (the C version) to say "I
> don't have a sum method that is applicable here, please continue with the
> normal code". So, just as an example, your exact code is likely to
> segfault, I think, if you hit an ALTREP that chooses not to implement a sum
> method because you'll be running around with a SEXP that has the value NULL
> (the C one, not the R one).
>
> One thing you could do, is check for altrepness and then construct and
> evaluate a call to the R sum function in that case, but that probably isn't
> quite what you want either, as this will hit the code you're trying to
> bypass/speedup  in the case where the ALTREP class doesn't implement a sum
> methods. I see that Luke just mentioned this as well but I'll leave it in
> since I had already typed it.
>
> I hope that helps clarify some things.
>
> Best,
> ~G
>
>
> On Tue, Jun 29, 2021 at 10:13 AM Sebastian Martin Krantz <
> sebastian.krantz using graduateinstitute.ch> wrote:
>
>> Thanks both. Is there a suggested way I can get this speedup in a package?
>> Or just leave it for now?
>>
>> Thanks also for the clarification Bill. The issue I have with that is that
>> in my C code ALTREP(x) evaluates to true even after adding and removing
>> dimensions (otherwise it would be handled by the normal sum method and I’d
>> be fine). Also .Internal(inspect(x)) still shows the compact
>> representation.
>>
>> -Sebastian
>>
>> On Tue 29. Jun 2021 at 19:43, Bill Dunlap <williamwdunlap using gmail.com>
>> wrote:
>>
>> > Adding the dimensions attribute takes away the altrep-ness.  Removing
>> > dimensions
>> > does not make it altrep.  E.g.,
>> >
>> > > a <- 1:10
>> > > am <- a ; dim(am) <- c(2L,5L)
>> > > amn <- am ; dim(amn) <- NULL
>> > > .Call("is_altrep", a)
>> > [1] TRUE
>> > > .Call("is_altrep", am)
>> > [1] FALSE
>> > > .Call("is_altrep", amn)
>> > [1] FALSE
>> >
>> > where is_altrep() is defined by the following C code:
>> >
>> > #include <R.h>
>> > #include <Rinternals.h>
>> >
>> > SEXP is_altrep(SEXP x)
>> > {
>> >     return Rf_ScalarLogical(ALTREP(x));
>> > }
>> >
>> >
>> > -Bill
>> >
>> > On Tue, Jun 29, 2021 at 8:03 AM Sebastian Martin Krantz <
>> > sebastian.krantz using graduateinstitute.ch> wrote:
>> >
>> >> Hello together, I'm working on some custom (grouped, weighted) sum, min
>> >> and
>> >> max functions and I want them to support the special case of plain
>> integer
>> >> sequences using ALTREP. I thereby encountered some behavior I cannot
>> >> explain to myself. The head of my fsum C function looks like this (g is
>> >> optional grouping vector, w is optional weights vector):
>> >>
>> >> SEXP fsumC(SEXP x, SEXP Rng, SEXP g, SEXP w, SEXP Rnarm) {
>> >>   int l = length(x), tx = TYPEOF(x), ng = asInteger(Rng),
>> >>     narm = asLogical(Rnarm), nprotect = 1, nwl = isNull(w);
>> >>   if(ALTREP(x) && ng == 0 && nwl) {
>> >>     switch(tx) {
>> >>     case INTSXP: return ALTINTEGER_SUM(x, (Rboolean)narm);
>> >>     case LGLSXP: return ALTLOGICAL_SUM(x, (Rboolean)narm);
>> >>     case REALSXP: return ALTLOGICAL_SUM(x, (Rboolean)narm);
>> >>     default: error("ALTREP object must be integer or real typed");
>> >>     }
>> >>   }
>> >> // ...
>> >> }
>> >>
>> >> when I let x <- 1:1e8, fsum(x) works fine and returns the correct
>> value.
>> >> If
>> >> I now make this a matrix dim(x) <- c(1e2, 1e6) and subsequently turn
>> this
>> >> into a vector again, dim(x) <- NULL, fsum(x) gives  NULL and a warning
>> >> message 'converting NULL pointer to R NULL'. For functions fmin and
>> fmax
>> >> (similarly defined using ALTINTEGER_MIN/MAX), I get this error right
>> away
>> >> e.g. fmin(1:1e8) gives NULL and warning 'converting NULL pointer to R
>> >> NULL'. So what is going on here? What do these functions return? And
>> how
>> >> do
>> >> I make this a robust implementation?
>> >>
>> >> Best regards,
>> >>
>> >> Sebastian Krantz
>> >>
>> >>         [[alternative HTML version deleted]]
>> >>
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>> >>
>> >
>>
>>         [[alternative HTML version deleted]]
>>
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