Zip/Window Registry Key
The DEFLATE algorithm compresses a file by locating combinations of bytes
or characters that repeat. The redundancy is removed and replaced by an
efficient pointer to the original grouping. The most efficient compression
then would remove any and all redundant groups from an entire file. This is
certainly possible for small files. With large files the pointers need to
address data further and further away. As the distance grows so do the
pointers and efficiency suffers. The solution is to limit the distance using
a sliding window through the file.
By default JANOS uses a 16KB (16384 byte) sliding window. This Registry key
may be used to adjust the window from a minimum of 2KB (2048) to a maximum of
32KB (32768). In practice there should be no reason to alter this setting. A
change in this setting affects the very next compression that is performed.
With each successive character in a file the compressor looks back over
the sliding window for groupings where replacement by pointer would
provide the greatest compression. This is a time consuming endeavor. As a
tradeoff the JANOS routine employs a queue of likely targets in the window
for each unique character. This reduces the search effort and the time it
takes to perform the compression.
By default the search queue depth is set at 256 entries. Values may range
from a minimum of 16 to a maximum of 1024. In practice there should be no
reason to alter this setting. A change in this setting affects the very
next compression that is performed.
HELP Topics: ZIP