upkr/README.md

Upkr

Upkr is a simple general purpose lz packer designed to be used in the MicroW8 platform. The compressed format is losely based on Shrinkler with the main difference being that Upkr doesn't differentiate between literals at odd or even addresses (by default) and that I went with rANS/rABS instead of a range coder.

Compression rate is on par with Shrinkler.

The differences compare to Shrinkler also makes it interesting on 8bit platforms. The z80 unpacker included in the release is both about twice as fast and smaller than the Shrinkler unpacker.

Inspirations:

• Ferris' blog about his C64 intro packer
• Shrinkler
• Ryg's sample rANS implementation

Unpackers

The release includes a reference c unpacker, as well as some optimized asm unpackers (arm and riscv). The unpckers in c_unpacker and asm_unpackers unpack the default upkr compressed format. The z80_unpacker is based on some variations to the compressed format. (Use upkr --z80 to select those variations.) The 16 bit dos unpacker also uses some variations. (upkr --x86)

Usage

  upkr [-l level(0-9)] [config options] <infile> [<outfile>]
  upkr -u [config options] <infile> [<outfile>]
  upkr --margin [config options] <infile>

 -l, --level N       compression level 0-9
 -0, ..., -9         short form for setting compression level
 -u, --unpack        unpack infile
 --margin            calculate margin for overlapped unpacking of a packed file

Config presets for specific unpackers:
 --z80               --big-endian-bitstream --invert-bit-encoding --simplified-prob-update -9
 --x86               --bitstream --invert-is-match-bit --invert-continue-value-bit --invert-new-offset-bit
 --x86b              --bitstream --invert-continue-value-bit --no-repeated-offsets -9

Config options (need to match when packing/unpacking):
 -b, --bitstream     bitstream mode
 -p, --parity N      use N (2/4) parity contexts
 -r, --reverse       reverse input & output

Config options to tailor output to specific optimized unpackers:
 --invert-is-match-bit
 --invert-new-offset-bit
 --invert-continue-value-bit
 --invert-bit-encoding
 --simplified-prob-update
 --big-endian-bitstream   (implies --bitstream)
 --no-repeated-offsets
 --eof-in-length
 --max-offset N
 --max-length N