[racket] bytes, bit-vectors and sieving numbers

Folks,

I've just posted an implementation for "Lucid Numbers" on rosettacode.org
for Racket. It sieves numbers using using a "bytes" variable, which, for
this purpose is what I consider to be a "vector of bytes".

The code is different from a "sieve of Eratosthenes", in that (at least my
implementation), each number has to be revisited once for every Lucid
number less than it. Even so, I hardly notice it building the sieve for
25,000 numbers.

However, this representation requires 1 byte per number considered; and
lucidity is a boolean state... representable by nothing more than a bit.
And ooh! Racket has bit-vectors.

I tried transcribing my program, using data/bit-vector - and the same
25,000 entry sieve takes a significant (19 s, or so) to execute. This came
as a shock, since I'd've only expected there to be a layer of shifting and
ANDing to byte (and/or fixnum) vectors.

The GIST of it is here: https://gist.github.com/tim-brown/87b05c16f41aa86a1421


Am I missing something fundamental here?
Can I (require data/bit-vector) in a different way or something like that?
Am I better off hand-rolling this?

I would like to use bit-vectors for things like cellular automata; but the
performance cost seems to be prohibitive (or at least very discouraging).

In the meantime, I have seven spare bits for each entry in my sieve... any
suggestions as to what I might want to flag in there?

Regards,

Tim

-- 
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