AdvanceCOMP is a set of cross-platform command line data (re-)compression

Data compression

In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 tools. The utilities allow modifying an already-compressed file, with the intent of reducing the file-size by optimising the compressed representation. The AdvanceCOMP suite is freely

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 available under the terms of the GPL

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

 license.

AdvanceCOMP works with files using the LZ/Huffman

Huffman coding

In computer science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression. The term refers to the use of a variable-length code table for encoding a source symbol where the variable-length code table has been derived in a particular way based on...

-based compression algorithm known as DEFLATE

DEFLATE

Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool and was later specified in RFC 1951....

, the most widely-used compression encoding systems in use. A large number of file-formats include DEFLATE as part of their specification, most notably PNG, gzip

Gzip

Gzip is any of several software applications used for file compression and decompression. The term usually refers to the GNU Project's implementation, "gzip" standing for GNU zip. It is based on the DEFLATE algorithm, which is a combination of Lempel-Ziv and Huffman coding...

 and ZIP

ZIP (file format)

Zip is a file format used for data compression and archiving. A zip file contains one or more files that have been compressed, to reduce file size, or stored as is...

.

The DEFLATE encoding

DEFLATE

DEFLATE

Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool and was later specified in RFC 1951....

 specifies a stream-encoding

Encoder

An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...

 such that any compliant decoder is able to parse any valid stream; the algorithm and program used for the compression stage are not mandated.

For generation of compressed sections of DEFLATE data, an encoder available in the zlib

Zlib

zlib is a software library used for data compression. zlib was written by Jean-Loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. Zlib is also a crucial component of many software platforms including Linux, Mac OS X,...

/gzip

Gzip

Gzip is any of several software applications used for file compression and decompression. The term usually refers to the GNU Project's implementation, "gzip" standing for GNU zip. It is based on the DEFLATE algorithm, which is a combination of Lempel-Ziv and Huffman coding...

 reference implementation has typically been utilised. The zlib/gzip compressor offers the user a sliding scale between CPU usage and the likely amount of reduction in size achieved on a range of -0 (no compression) to -9 (maximum gzip compression).

The 7-Zip

7-Zip

7-Zip is an open source file archiver. 7-Zip operates with the 7z archive format, but can read and write several other archive formats. The program can be used from a command line interface, graphical user interface, or with Microsoft Windows shell integration. 7-Zip began in 1999 and is actively...

 DEFLATE

DEFLATE

Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool and was later specified in RFC 1951....

 encoder, used in the AdvanceCOMP suite, effectively extends the sliding scale further. A much more detailed search of compression possibilities is performed, at the expense of significant further processor time spent searching. Effectively, the 10-point scale used in gzip is extended to include extra settings above -9, the previous maximum search level. There will be no difference in decompression speed, regardless of the level of compressed size achieved or time taken to encode the data.

Included utilities

AdvanceCOMP includes four utilities, all of which allow the extend of DEFLATE searching to be set of one of four levels:

• advpng, removal of ancillary chunks, concatenation of all individual IDAT chunks; then re-encoding of the combined PNG image file using the 7-Zip deflate method.
• advmng, attempt to make use of delta encoding, followed by re-encoding of compressed sections using 7-Zip deflate method.
• advdef, general re-encoding of anything featuring a zlib stream, this includes .gz and .png files.
• advzip, re-encoding of compressed DEFLATE streams within a .zip archive.

The additional searching performed during compression (and therefore additional CPU usage), can often make gains of 5%-10% in compressed size, according to the AdvanceCOMP authors. After using AdvanceCOMP, the output remains a backwards-compatible and compliant DEFLATE stream, capable of being decoded by any existing tools.