Bit packing

does anyone have a working example of bit packing with negative numbers? I'm trying to pack three numbers, 8 bits each (from -255 to 255), by adding 255 to each number before I pack it, and just subtracting the same thing from it after I unpack. For some reason, I get weird ass errors around 0, like packing 0 but unpacking 256, and packing -1 and unpacking -256. I've tried everything I can think of and I have no idea what else to do.

local range = 8

local full = bit.lshift(1, range + 1) - 1
local half = bit.lshift(1, range) - 1

local a, b, c = -32, -64, -128

local x =
    bit.bor(
        bit.lshift( a + half, range*2 + 2),
        bit.lshift( b + half, range + 1),
        c + half
    )

local f = bit.band( x, full ) - half
local e = bit.band( bit.rshift( x, range + 1), full) - half
local d = bit.rshift( x, range*2 + 2) - half

8 bit number can hold only 256 different values, signed -127..128 unsigned 0..255

x itself caps at 32 bits I think, after I add half to each number I give it range + 1 bits, which in this case is 9 bits which should be 0-511

wtf is this language?

lua

maybe because of little endian, anyway better use C for bit packing

bit.lshift
`
>implying you need a function call for bit ops
My sides.

install BSD/Gentoo

RTFW
http://en.wikipedia.org/wiki/Two's_complement
http://en.wikipedia.org/wiki/Sign_extension

>>9
>le kike bit manipulation

Check 'em

switched over to signed magnitude and got it to work, thanks tho