Compress function (additional information)

[Alejandro Tejada]

Hi all,

Who could help me and post this information about
Compress() function in Online Dictionary:
http://docs.runrev.com/Function/compress

You corrections and additions are welcome!
:-)

Thanks in advance!

Alejandro

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The compress function returns a gzip compressed
string of binary data structured in the following format:

1) A 10-byte header, containing:

A magic number, composed by two bytes:
1F 8B (Hexadecimal) or 31 139 (ASCII Numbers
of both characters)

A version number (always 08), that specify compression
compression method used in the file.
In this case Rev write the byte 08.
This single character correspond to DEFLATE compression.

Additional information like timestamp, optional extra headers,
(as the original file name) are not included and in their place
Rev writes six null bytes: 00 00 00 00 00 00

Last char of 10 byte header correspond to the type of
file system on which compression took place.
Rev writes the single byte 03, that correspond to
Unix.

For example: First Ten characters of a gzipped binary string
(Hexadecimal)
1F 8B 08 00 00 00 00 00 00 03
(ASCII Characters numbers)
31 139 08 00 00 00 00 00 00 03

2- a body, containing DEFLATE-compressed data

3- Last 8 characters contains a CRC-32 checksum and
length of the original uncompressed data:
For example: Last 8 characters of gzipped binary string
(Hexadecimal)
8C 72 E5 F5 1A 34 02 00
Last four bytes: 1A 34 02 00 = 0002341A = 144,410
represent the file length (142 k)
Bytes 8C 72 E5 F5 correspond to CRC-32 of
this file: F5E5728C

If you need to use only the binary string of DEFLATE-compressed
data, without the gzip header and footer, use a script like this:

put compress(myData) into myGzippedData
put char 11 to -9 of myGzippedData into myDeflateData

or the short version:
put char 11 to -9 of compress(myData) into myDeflateData

This binary string of DEFLATE-compressed data is useful
if you need to write a Flate Encoded stream in a PDF file
like this:

put "xÚ" & char 11 to -9 of myGzippedData into myFlateEncodedStream