UNIX for Advanced & Expert Users

-b maxbits Specify the maximum number of bits the
compress algorithm will use. The default is
16 and the range can be any integer between 9
and 16.

compress uses the modified Lempel-Ziv algorithm popularized in A
Technique for High Performance Data Compression , Terry A. Welch, IEEE
Computer, vol. 17, no. 6 (June 1984), pages 8-19. Common substrings
in the file are first replaced by 9-bit codes 257 and up. When code
512 is reached, the algorithm switches to 10-bit codes and continues
to use more bits until the limit specified by the -b flag is reached
(default 16).

After the maxbits limit is attained, compress periodically checks the
compression ratio. If it is increasing, compress continues to use the
existing code dictionary. However, if the compression ratio is
decreasing, compress discards the table of substrings and rebuilds it
from scratch. This allows the algorithm to adapt to the next "block"
of the file.

Note that the -b flag is omitted for uncompress since the maxbits
parameter specified during compression is encoded within the output,
along with a magic number to ensure that neither decompression of
random data nor recompression of compressed data is attempted.