Thanks for the clarification, Sol. You're right: it depends on the
checksum algorithm: http://en.wikipedia.org/wiki/Checksum. I'm not sure
what algorithm Perl uses as part of unpack('%32C*'), but you're right
that POSIX cksum uses a CRC algorithm
(http://en.wikipedia.org/wiki/Cyclic_redundancy_check) that is
position-dependent.
Sol Lederman wrote:
Alex,
Permuting the characters in a string does not produce the same checksum. If
it did, that would make checksums really weak. I don't know of any checksum
algorithm that produces the same checksum when you merely permute the
characters.
Here's an example on my iMac.
echo "The cat chases the dog" > foo1
echo "The dog chases the cat" > foo2
cksum foo1
414128224 23 foo1
cksum foo2
2453586855 23 foo2
Sol
On Fri, May 28, 2010 at 10:26 AM, Alex Bronstein
<[email protected]>wrote:
Hi Eric,
That's not ideal. checksums generate the same number if the letters in the
string are moved. For example "The cat chases the dog" and "The dog chases
the cat" would result in the same checksum.
You'd be better off using md5(): http://perldoc.perl.org/Digest/MD5.html
Something like:
# If you want a short integer (2 bytes: 0 - 65535)
my ($integer) = unpack('S', md5($author . $title));
# If you want a long integer (4 bytes: 0 - 4 billion)
my ($integer) = unpack('L', md5($author . $title));
That would give you uniqueness to within the capability of a short or long
int. If you have few enough items in the list that you're willing to
increase the odds of non-uniqueness in exchange for a smaller maximum
number, you can use the % operator as in:
# If you want an integer between 0 and 9999
my ($integer) = unpack('S', md5($author . $title));
$integer = $integer % 10000;
Alex.
Eric Lease Morgan wrote:
Using Perl, how can I convert the author/title combination into some sort
of integer, checksum, or unique value that is the same every time I run my
script? I don't want to have to remember what was used before because I
don't want to maintain a list of previously used keys. Should I use some
form of the pack function? Should I sum the ASCII values of each character
in the author/title combination?
Thank you for the prompt replies, and invariably I resolved my own
question. Using Perl's unpack function I can generate a checksum based on
the concatenation of the authors and titles:
my $integer = unpack( "%32C*", "$author$title" ) % 65535;
The result is a unique four-digit number that will be consistently
generated as my list of author/title combinations grows. At the same time,
my solution looks much like an incantation -- with magic. Perl-specific and
at a level of computing that is beyond my day-to-day understanding.
TGIF
