R: In-memory Compression and Decompression

memCompress {base} | R Documentation

## In-memory Compression and Decompression

### Description

In-memory compression or decompression for raw vectors.

### Usage

    memCompress(from, type = c("gzip", "bzip2", "xz", "none"))

    memDecompress(from,
                  type = c("unknown", "gzip", "bzip2", "xz", "none"),
                  asChar = FALSE)

### Arguments

`from` | A raw vector. For `memCompress` a character vector will be converted to a raw vector with character strings separated by `"\n"`.
`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?

### Details

`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 `"bzip"` 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.

### Value

A raw vector or a character string (if `asChar = TRUE`).

### See Also

connections.

http://en.wikipedia.org/wiki/Data_compression for background on data compression, http://zlib.net/, http://en.wikipedia.org/wiki/Gzip, http://www.bzip.org/, http://en.wikipedia.org/wiki/Bzip2, http://tukaani.org/xz/ and http://en.wikipedia.org/wiki/Xz for references about the particular schemes used.

### Examples

    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))

[Package base version 3.1.2 Index]