# [racket-dev] Implementation of bit vectors

Hi All,
I have implemented an alternative version of bit-vectors using bignums
to represent the bits.
As is the bignum implementation is much slower, than the vector-of-fixnum one.
The main reason as far as I can tell is due to bit-vector-set! .
Since bignums aren't mutable I can not simply flip a bit and need to compute
a new bignum. Unless bignums are sharing limbs this will be slow for large
bit-vectors.
Another possibility is that I have missed something obvious.
The functions bit-vector-set! is here:
(define (bit-vector-set! bv n b)
; bv is a bit-vector
; n is the bit number
; b is #f or #t
(define bits (bit-vector-bits bv))
(define mask (arithmetic-shift 1 n))
(cond
[b
(set-bit-vector-bits! bv (bitwise-ior bits mask))]
[(bitwise-bit-set? bits n)
(set-bit-vector-bits! bv (bitwise-xor bits mask))]
[else (void)]))
The entire implementation is here:
https://github.com/soegaard/racket/blob/4b299ea66a77100538940794cd799cb88929b7e3/collects/data/bit-vector-bignum.rkt
The benchmark is here:
https://github.com/soegaard/racket/blob/4b299ea66a77100538940794cd799cb88929b7e3/collects/data/benchmark-bit-vector-representations.rkt
--
Jens Axel Søgaard