[Python-Dev] Re: [Distutils] Questions about distutils strategy

[Tim Peters]

[Tim]
> If WinZip unzips Jim's files without griping, the odds that he's
> got the wrong CRC are about 1 in 2**36 <wink>.

[JimA]
> You mean 2**32, right?

Nope!  For each of the 2**32 polynomials you may have pulled out of thin
air, there are about a dozen common variations in the details of CRC
algorithms.  For example, a CRC used for hashing usually initializes "the
register" to 0, but a CRC used to protect against transmission errors
usually initializes to a block of 1 bits (since leading zeroes don't affect
the result, and a common transmission error is dropping a prefix of the
msg).  Similarly, algorithms vary in the order they scan the data; in
whether they use the raw data or its complement; and in whether they return
the actual remainder, the complement of the remainder, or a checksum
cleverly computed so that "the other end" always sees a fixed remainder
other than 0 (or ~0).

> Oh, sorry, you must be using a DEC-10  <wink again>.

I used a Univac 1108 in college, back when ASCII was in its infancy.  They
couldn't decide on the natural size for a character, so the 36-bit 1108
could be configured to treat each word as either 6 6-bit bytes or 4 9-bit
ones.  If they had been thinking ahead, they would have defined it as two
Unicode characters plus a 4-bit tag field for the Python implementation to
play with <wink>.

now-they-make-their-living-suing-.gif-bandits-ly y'rs  - tim