In-memory compression or decompression for raw vectors.
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A raw vector. For
character string, the type of compression. May be abbreviated to a single letter, defaults to the first of the alternatives.
logical: should the result be converted to a character string? NB: character strings have a limit of 2^31 - 1 bytes, so raw vectors should be used for large inputs.
type = "none" passes the input through unchanged, but may be
type is a variable.
type = "unknown" attempts to detect the type of compression
applied (if any): this will always succeed for
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
not add such a header. (It supports RFC 1950 format, sometimes known
as ‘zlib’ format, for compression and decompression and RFC
1952 for decompression only.)
gzip compression uses whatever is the default compression
level of the underlying library (usually
bzip2 compression always adds a header (
underlying library only supports in-memory (de)compression of up to
2^31 - 1 elements. Compression is equivalent to
bzip2 -9 (the default).
type = "xz" is equivalent to compressing a
xz -9e (including adding the ‘magic’
header): decompression should cope with the contents of any file
xz version 4.999 and later, as well as by some
lzma. There are other versions, in particular
‘raw’ streams, that are not currently handled.
All the types of compression can expand the input: for
"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
xz libraries in use.
https://en.wikipedia.org/wiki/Data_compression for background on data compression, https://zlib.net/, https://en.wikipedia.org/wiki/Gzip, http://www.bzip.org/, https://en.wikipedia.org/wiki/Bzip2, https://tukaani.org/xz/ and https://en.wikipedia.org/wiki/Xz for references about the particular schemes used.
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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")[] stopifnot(identical(txt, txt2)) txt.bz2 <- memCompress(txt, "b") length(txt.bz2) ## can auto-detect bzip2: txt3 <- strsplit(memDecompress(txt.bz2, asChar = TRUE), "\n")[] 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")[] stopifnot(identical(txt, txt3))
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