Re: bit.lshift and performance - luabitop v.s. lua-5.2.0-work4

[KHMan <keinhong@...>]

On 10/13/2010 10:34 AM, David Manura wrote:
> [snip]
>   local bits = bit.band(buffer, bit.lshift(1, nbits)-1)

Don't like how this line works at all.

> print(bit.lshift(1,32))
0

So it gets % 2^32, then bit.lshift(1,32)-1 = 0-1 = -1 ... ewww

Is there any actual technical problem with using this instead?

    bit.rshift(0xFFFFFFFF, 32 - nbits)

Assuming we are fixed at 32 bits max, it means that we shift out 
unwanted mask bits -- nothing unclear about it. I think 
"bit.lshift(1, nbits)-1" is simply something one does in C. 
Doesn't mean we must paint in Lua in the same way.

I think compression/decompression routines written in Lua is more 
suitable for prototyping work and hence performance is not a 
critical issue -- it's not for any real "normal user" apps, is it?

Heavens, before long, someone will lobby for tweaks to run FFT or 
DCT in Lua...

--
Cheers,
Kein-Hong Man (esq.)
Kuala Lumpur, Malaysia