Re: Bit ops (why functions are non optimal)

["Mike Panetta" <panetta.mike@...>]

Ahh!  Thats a function I didn't think of.  I like that idea. Hmmm...

Mike

On 9/22/06, Jerome Vuarand <jerome.vuarand@ubisoft.com> wrote:

Lua:
> function prefix2mask(bits)
>     return htonl(math.not((math.lshift(1, (32 - bits))) - 1))
> end

C:
> u_int32_t prefix2mask(int prefix) {
>     return htonl(~((1 << (32 - prefix)) - 1));
> }

IMHO the Lua version is much clearer. Additionnaly,if you make it higher
level, no need to use C tricks with shifts and bitwise not. That could
be written (assuming bit.rep works like string.rep for bitfields):

> function prefix2mask(bits)
>     return htonl(bit.rep(1, bits)..bit.rep(0, 32 - bits))
> end

...which is even clearer.