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

[Sebastian Martin Krantz]

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

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