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

[Gabriel Becker]

Hi Sebastian,

min/max do not materialize the vector, you will see it as compact after
same as before. It *does* however do a pass over the data chunked by
region, which is much more expensive than it need be for compact sequences,
that is true.

I think in some version of code that never made it out of the branch, I had
default min/max methods which took sortedness into account if it was known.
One thing that significantly complicated that cod ewas that you have to
find the edge of the NAs(/NaNs for the real case) if narm is TRUE, which
involves a binary search using ELT (or a linear one using
ITERATE_BY_REGION, I suppose).

That said a newer version of the count nas code did get in from a later
patch  to update, so it is available in r-devel and could be used to
revisit that approach.

That aside, it is true that compact sequences in particular never have NAs
so the min and max altrep methods for those classes would be trivial. I
kind of doubt people are creating compact sequences and then asking for the
min/max/mean of them very often in practice.

Best,
~G

On Tue, Jun 29, 2021 at 11:26 AM Sebastian Martin Krantz <
sebastian.krantz using graduateinstitute.ch> wrote:

> Thanks Gabriel and Luke,
>
> I understand now the functions return NULL if no method is applicable. I
> wonder though why do ALTINTEGER_MIN and MAX return NULL on a plain integer
> sequence? I also see that min() and max() are not optimized i.e. min(1:1e8)
> appears to materialize the vector.
>
> In general I expect my functions to mostly be applied to real data so this
> is not a huge issue for me (I’d rather get rid of it again than calling
> sum() or risking that the macros are removed from the API), but it could be
> nice to have this speedup available to packages. If these macros have
> matured and it can be made explicit that they return NULL if no method is
> applicable, or, better, they internally dispatch to a normal sum method if
> this is the case, they could become very manageable and useful.
>
> Best,
>
> Sebastian
>
>
>
> On Tue 29. Jun 2021 at 21:09, Gabriel Becker <gabembecker using gmail.com>
> wrote:
>
>> 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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>>>> >> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>> >>
>>>> >
>>>>
>>>>         [[alternative HTML version deleted]]
>>>>
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