[R-pkg-devel] fortran integer(kind=)

[Ivan Krylov]

On Thu, 31 Aug 2023 06:52:47 +0000
Berry Boessenkool <berryboessenkool using hotmail.com> wrote:

> my R package dwdradar uses Fortran code with the input parameter
> integer(KIND=2):
> https://github.com/brry/dwdradar/blob/master/src/binary_to_num.f90#L20
> https://github.com/brry/dwdradar/blob/master/src/binary_to_num.f90#L55

The "raw" argument of the Fortran function originates from the
"dat" argument to the R function "bin2num", which, in turn, originates
from readBin(what = raw()):
https://github.com/brry/dwdradar/blob/master/R/bin2num.R#L21-L25
https://github.com/brry/dwdradar/blob/master/R/readRadarFile.R#L51

"Raw" vectors correspond to the C type unsigned char, which you seem to
be interpreting as 16-bit integers instead. (This might be a violation
of aliasing rules, but the compiler is not in a position to see it, and
it's been working for years.) The 16-bit integer type in Fortran can be
declared using the iso_c_binding module:

use, intrinsic :: iso_c_binding, only: c_int16_t
integer(KIND=c_int16_t),DIMENSION(Flength),INTENT(in)::raw

There's probably a safer and only slightly less performant way to
implement binary_to_num in R:

dat <- as.integer(dat)
# interpret the bytes as little-endian double-byte integers
dat <- dat[seq(1, length(dat), 2)] + 256 * dat[seq(2, length(dat), 2)]
# extract the 12-bit data
out <- bitwAnd(dat, 4095)
# set the missing flag
is.na(out) <- as.logical(bitwAnd(dat, bitwShiftL(1, 13)))
# set the negative flag
out <- -1 * as.logical(bitwAnd(dat, bitwShiftL(1, 14)))
# set the clutter flag
out[as.logical(bitwAnd(dat, bitwShiftL(1, 15)))] <- clutter

-- 
Best regards,
Ivan