Rcpp sugar indexHash (unique, sort_unique, etc.) distinguish signed and unsigned zeros

[SebKrantz]

Hi Dirk, sorry if this is not exactly as much effort as you expect for an issue, I just wanted to flag something reported to collapse (#648), which is present in both my hash functions written in C and your hash functions, and that is the following:

library(collapse)
#> Warning: package 'collapse' was built under R version 4.3.3
#> collapse 2.0.17, see ?`collapse-package` or ?`collapse-documentation`

x = round(rnorm(100))
unique(x)               # R
#> [1]  1  0  2 -1 -2
funique(x)              # My hash function in C
#> [1]  1  0  0  2 -1 -2
funique(x, sort = TRUE) # Rcpp::sugar::sort_unique()
#> [1] -2 -1  0  0  1  2
# More explicit proof
collapse:::sortuniqueCpp(x)
#> [1] -2 -1  0  0  1  2

# The solution
y = x + 0L

funique(y)              
#> [1]  1  0  2 -1 -2
collapse:::sortuniqueCpp(y)
#> [1] -2 -1  0  1  2

^{Created on 2024-10-31 with reprex v2.0.2}

In words: R functions like round() create signed and unsigned zeros, whose hashes differ. A quite efficient remedy is to add an integer zero (gives like a 3% slower execution on my very efficient C hash). I'm considering to roll this out, but of course cannot control your code. So just pushing it to you as food for thought.

[eddelbuettel]

Thanks for the note. We will ponder this, Making a change may risk upsetting existing code, but documenting your workaround of adding an explicit zero seems like a good idea.

[SebKrantz]

I'd be surprised if adding a zero breaks any code - except for this particular issue which is clearly not meant to be as far as R is concerned. But yeah, haven't checked yet what base R does to avoid sign bits with zeros - probably something smarter but slower...