Deflate (String function)

Compress a longstring with deflate (String class)

[Introduced in Sirius Mods 7.4]

This function compresses a longstring using the "deflate" algorithm. The deflate algorithm is described completely in RFC 1951. It is very effective with HTML and XML data.

Syntax

%outString = string:Deflate[( [FixedCode= boolean], [LazyMatch= boolean], - [MaxChain= number], [Software= boolean])]

Syntax terms

%outString	The resulting compressed string.
string	The string to be compressed.
FixedCode	FixedCode is an optional, name required, parameter that is a Boolean value that specifies whether the compression uses fixed codes or is dynamic, based on the contents of the input string. With fixed code compression (fixedCode=true), the tables used for compression (defined as part of RFC 1951) are somewhat optimized for ASCII character data, but they slightly decrease the amount of CPU required to perform compression. Also, since the codes are already defined as part of the specification, they are not included in the compressed data. With dynamic compression (fixedCode=false), the compression code tables are generated based on the input data. Dynamic tables typically provide somewhat better compression on most types of data. There is a very slight CPU overhead in computing the frequencies of byte values in the input data. Also, since the code tables are dynamic, they are included as part of the compressed data. This will increase the size of the compressed longstring, but these tables are small, since they are also stored in a compressed form. The default value for this argument is False (use dynamic compression).
LazyMatch	LazyMatch is an optional, name required, parameter that is a Boolean value that specifies whether to use "lazy match" compression, as specified in RFC 1951. The default value for this argument is False (do not use "lazy match" compression).
MaxChain	MaxChain is an optional, name required, parameter that is a numeric value that specifies the maximum hash chain length, as explained in RFC 1951. The default value for this argument is 0. If specified, it must be between 0 and 99, inclusive.
Software	[Introduced in Model 204 version 8.0] Software is an optional, name required, parameter that is a Boolean value that specifies whether to force the use of software compression instead of hardware compression. When software=true, compression is always performed in software, even if the DFLTCC hardware instruction is available on the processor. This can be useful for diagnostic purposes or when byte-for-byte reproducibility with pre-8.0 behavior is required. When software=false (the default), hardware compression is used automatically when DFLTCC is available (IBM z15 and above), falling back to software compression on older processors. The default value for this argument is False.

Usage notes

• The NCMPBUF parameter must be set to a non-0 value during Model 204 initialization to allow use of the Deflate function; otherwise, invoking Deflate causes request cancellation.
• As with any compression scheme, it is possible that a particular string will become longer after compression. This would happen, for example, if a deflated string were passed to Deflate.
• Short strings (less than 128 bytes) typically compress better with fixedCode=true.

Hardware vs. software compression

[Introduced in Model 204 version 8.0]

On IBM z15 and above processors, the Deflate method automatically exploits the DFLTCC (DEFLate Conversion Call) hardware instruction for significantly improved compression performance. The following differences apply when using hardware compression:

• The LazyMatch and MaxChain parameters have no effect when hardware compression is used. The hardware uses its own internal matching algorithm.
• The FixedCode parameter is honored by both hardware and software compression. When fixedCode=true, the hardware generates compressed data with fixed Huffman codes instead of generating a Dynamic Huffman Table (GDHT).
• Compressing the same input data multiple times may produce different compressed output due to the implementation-dependent hash function for duplicate string matching, as documented in the z/Architecture Principles of Operation. This is a property of the hardware itself, not a software issue.
• Hardware-compressed data can always be decompressed by any standards-compliant inflate implementation (and vice versa).

These differences also apply to the Gzip and Zip methods, which use Deflate compression internally.

Examples

In the following example, %out is set to the compressed version of the given string:

%out = 'How much wood could a woodchuck chuck':deflate(fixedCode=true)

The following example forces software compression:

%out = 'How much wood could a woodchuck chuck':deflate(software=true)

See also

• Inflate is used to decompress the string to its original value.
• Other related methods:
  • Gunzip
  • GunzipDatetimeNS
  • GunzipFilename
  • Gzip
  • Zip

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