lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

On 10/14/2010 11:55 AM, David Manura wrote:
On Wed, Oct 13, 2010 at 1:12 AM, KHMan wrote:
On 10/13/2010 10:34 AM, David Manura wrote:
  local bits =, bit.lshift(1, nbits)-1)
...Is there any actual technical problem with using this instead?
    bit.rshift(0xFFFFFFFF, 32 - nbits)

One downside is that the code

   local bits =, rshift(0xffffffff, 32 - nbits))
   buffer = bit.rshift(buffer, nbits)

now requires two special cases: nbits == 0 and nbits == 32.  Both
actually occur in my code.  I don't know if this is a rare case, but
it can affect code containing shifts by both n and 32 - n where n in
[0, 32].

I'm sorry, I am interested in this particular problem, but it's very hard to parse the entire context of the brief description. I'm guessing there's involvement of Lua 5.1.x, Lua 5.2.x and LuaJIT, but I can't tell which is which or what's going on with the special cases.

bit mask for data:
bit.rshift(0xffffffff, 32-0) => 0 (all masked, nothing returned)
bit.rshift(0xffffffff, 32-16) => 65536 (1/2 masked, 1/2 returned)
bit.rshift(0xffffffff, 32-32) => 4294967295 (none masked, all returned)

data left in buffer:
bit.rshift(0x12345678, 0) => 0x12345678 (all left)
bit.rshift(0x12345678, 16) => 0x00001234 (half left)
bit.rshift(0x12345678, 32) = > 0x00000000 (none left)

Well, it seems all right on Lua 5.2.x, at first glance. Do you have some sample output you can share?

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