bit32.band like operation in 64 bits

Question

I want to apply a bitwise AND operation in 64 bits in Lua 5.1. Is there an algorithm for it? (I have no idea on how to do it.)

Note : I only need to operate on 48 bits at total, and I am not having trouble with them.

In the game's Lua I'm scripting there's the bit32 library only.

Answer 1

Lua is using double floating numbers internally by default. It has only 52 bits for mantissa, so you can't safely store 64-bit integers without risking to get invalid floating point values.
With 32 bits it's quite safe. Lua 5.2 manuals describe what happens in bit32 lib with the numbers:

Unless otherwise stated, all functions accept numeric arguments in the range (-2^51,+2^51); each argument is normalized to the remainder of its division by 2^32 and truncated to an integer (in some unspecified way), so that its final value falls in the range [0,2^32 - 1]. Similarly, all results are in the range [0,2^32 - 1].

You'll have to work in 32-bit chunks.
Or maybe introduce your own 64-bits type, probably hosted with userdata, and define 64-bit actions for that type.

Answer 2

local function band48 (x, y)
   local xl = x % 4294967296
   local yl = y % 4294967296
   local xh = (x - xl) / 4294967296
   local yh = (y - yl) / 4294967296
   return bit32.band(xh, yh) * 4294967296 + bit32.band(xl, yl)
end

print(band48(7 * 2^33 + 3, 5*2^33 + 5)) --> 5*2^33+1 = 42949672961