I need to decompress an array of bytes that were compressed using zlib. The
bytes are stored within a binary file. I can seek to the location, read the
length of the compressed byte array, and read the bytes into R. What I need
to do then is decompress the byte array to an array of doubles.
The code looks something like this:
compresslength = readBin(infile, integer(), 1, 4)
data = readBin(infile, integer(), compresslength, 1)
#need something here like..... result = zlibDecompress(data)
I implemented it in Python using the zlib module and the decompress method.
The Python code is:
data = file.read(compresslength) #read x number of bytes
result = struct.unpack("3d", zlib.decompress(data)) #decompress data
and
convert bytes to 3 doubles
I also tried:
data = readBin(gzcon(infile), integer(), compresslength, 1)
But it returns what I asked for....a vector of 1 bytes with a length equal
to compresslength. It seems that the arguments to readBin require
prespecification of the result, which I can't really provide. gzfile and
gzcon wrap a connection, but then in order to readBin from the connection, I
need to specify the data type, the number of items, and the number of bytes
per item. But I don't know this since zlib did the compressing. The gzip
website says to use the zlib library for in-memory compression, so maybe
what I am trying to do cannot be done with what currently exists in R.
The other issue in this is the conversion of the bytes to doubles. Since I
have a lot of compress values - ~ 250000 - I'm guessing that I can't
afford
to write a little R function to operate on a vector of 1 byte integers. So
that is why I was hoping to find a way to do it with existing (compiled C
code) R.
Maybe I am missing something...I don't know :) Thanks for your help.
Ben Stabler
PTV America, Inc.
1128 NE 2nd St, Suite 204
Corvallis, OR 97330
541-754-6836 x205
541-754-6837 fax
www.ptvamerica.com
