On 22 Lip, 09:52, Timothy Pratley <timothyprat...@gmail.com> wrote:
> Could you give a more detailed example to illustrate what this means?
>
> > (with-bitfields arr 0 {last 1, term 1, dest 22, char 8}
> > [last term dest char])
Perhaps a good illustration will be what it macroexpands to:
(let [last (+ (bit-and (aget arr (unchecked-add 0 0)) 1))
term (+ (bit-shift-right (bit-and (aget arr (unchecked-add 0 0))
3) 1))
dest (+ (bit-shift-right (bit-and (aget arr (unchecked-add 0 0))
255) 2)
(bit-shift-left (bit-and (aget arr (unchecked-add 0 1))
255) 6)
(bit-shift-left (bit-and (aget arr (unchecked-add 0 2))
255) 14))
char (+ (bit-and (aget arr (unchecked-add 0 3)) 255))]
[last term dest char])
The background is that in C99, you can declare structures like:
struct foo {
uint32_t last:1;
uint32_t term:1;
uint32_t dest:22;
uint32_t char:8;
};
which means that each of the fields is only that many bits long (e.g.
last can only hold values 0 and 1, while dest can range from 0 to
4194303). Such a structure takes up 1+1+22+8 = 32 bits = 4 bytes in
memory. It can also be written to disk, and clj-bitfields gives you a
way to read it back and access the individual fields.
Best regards,
Daniel Janus
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