failure in permutation generator

[shevek]

https://github.com/robertdavidgraham/masscan/blob/master/src/rand-blackrock.c

FYI the error is that % rounds towards zero, and does not generate nonnegative equivalence classes.

If you rewrite using e.g. LongMath.mod() as per http://code.google.com/p/guava-libraries/wiki/MathExplained then the bidirectional verification code you have works.

If you don't, then you're basically doing many extra rounds sometimes because you generate a negative number which is masked because you compute with unsigned.

I have ported your code to Java, and it now passes the bidirectional verifier.