That just looks like a convenience function to me. Rather than passing 1,2,4,8,16 in as the value of N, you pass in 0,1,2,3,4. Has the benefit that it's not possible to pass in a number that's not a power of 2 (which might not be valid, I don't know how scrypt works).
Yes, that looks dodgy - a 64-bit 'N' being renamed 'logN', and its value changed to `1` left shifted by its prior self? I think there's a good bet that's just zeroing it out.
Which ought to be just N - but notice the argument passed in is mysteriously renamed. It may be nothing - I'm not even sure what N is here - I just agree that it looks weird.
