| memCompress {base} | R Documentation |
In-memory compression or decompression for raw vectors.
memCompress(from, type = c("gzip", "bzip2", "xz", "none"))
memDecompress(from,
type = c("unknown", "gzip", "bzip2", "xz", "none"),
asChar = FALSE)
from |
A raw vector. For |
type |
character string, the type of compression. May be abbreviated to a single letter, defaults to the first of the alternatives. |
asChar |
logical: should the result be converted to a character string? |
type = "none" passes the input through unchanged, but may be
useful if type is a variable.
type = "unknown" attempts to detect the type of compression
applied (if any): this will always succeed for bzip2
compression, and will succeed for other forms if there is a suitable
header. It will auto-detect the ‘magic’ header
("\x1f\x8b") added to files by the gzip program (and
to files written by gzfile), but memCompress does
not add such a header.
bzip2 compression always adds a header ("BZh").
Compressing with type = "xz" is equivalent to compressing a
file with xz -9e (including adding the ‘magic’
header): decompression should cope with the contents of any file
compressed with xz version 4.999 and some versions of
lzma. There are other versions, in particular ‘raw’
streams, that are not currently handled.
All the types of compression can expand the input: for "gzip"
and "bzip2" the maximum expansion is known and so
memCompress can always allocate sufficient space. For
"xz" it is possible (but extremely unlikely) that compression
will fail if the output would have been too large.
A raw vector or a character string (if asChar = TRUE).
extSoftVersion for the versions of the zlib,
bzip2 and xz libraries in use.
https://en.wikipedia.org/wiki/Data_compression for background on data compression, http://zlib.net/, https://en.wikipedia.org/wiki/Gzip, http://www.bzip.org/, https://en.wikipedia.org/wiki/Bzip2, http://tukaani.org/xz/ and https://en.wikipedia.org/wiki/Xz for references about the particular schemes used.
txt <- readLines(file.path(R.home("doc"), "COPYING"))
sum(nchar(txt))
txt.gz <- memCompress(txt, "g")
length(txt.gz)
txt2 <- strsplit(memDecompress(txt.gz, "g", asChar = TRUE), "\n")[[1]]
stopifnot(identical(txt, txt2))
txt.bz2 <- memCompress(txt, "b")
length(txt.bz2)
## can auto-detect bzip2:
txt3 <- strsplit(memDecompress(txt.bz2, asChar = TRUE), "\n")[[1]]
stopifnot(identical(txt, txt3))
## xz compression is only worthwhile for large objects
txt.xz <- memCompress(txt, "x")
length(txt.xz)
txt3 <- strsplit(memDecompress(txt.xz, asChar = TRUE), "\n")[[1]]
stopifnot(identical(txt, txt3))