🚀 go-pugleaf

taft@adobe.com (Ed Taft) writes:
> dmlaur@ernie.Princeton.EDU (David M. Laur) writes:
> >Secondly, is there something wonderfully magic about the number 85
> >that makes the encoding algorithm Very Easy(tm)?
> 
> I'm not aware of any tricks that apply specifically to multiplying and
> dividing by 85. The choice of 85 as a base derives from it being the
> smallest integer n such than n^5 >= 2^32.

No, 85 is not particularly easy, it's just the largest "safe" value for this
application.  Base-85 representation is customarily used for representing
arbitrary binary files as ASCII text so that they can be mailed around on
Usenet.  It happens that there are 85 printable ASCII characters that have
exact EBCDIC equivalents.  If you try to use all 95 ASCII characters, you
run the risk of your file getting mangled in an ASCII <=> EBCDIC conversion
performed by some mail gateway.  (And of course, using any ASCII control
characters or characters with the 8th bit set is hopeless...)

At least, that was *my* understanding of the design.  The fact that 5
base-85 characters can represent 32 bits is convenient but it's not really
the driving force.

				regards, tom lane

ps: any further discussion should probably move to comp.compression.