This is an implementation of Rabin and Karp's streaming hash, as described
in "Winnowing: Local Algorithms for Document Fingerprinting" by Schleimer,
Wilkerson, and Aiken. Following the suggestion of Schleimer, I am using
their second equation:

  $H[ $c[2..$k + 1] ] = (( $H[ $c[1..$k] ] - $c[1] ** $k ) + $c[$k+1] ) * $k

The results of this hash encodes information about the next k values in
the stream (hense k-gram.) This means for any given stream of length n
integer values (or characters), you will get back n - k + 1 hash values.

For best results, you will want to create a code generator that filters
your data to remove all unnecessary information. For example, in a large
english document, you should probably remove all white space, as well as
removing all capitalization.

WWW: http://search.cpan.org/dist/Algorithm-RabinKarp/