RE: Just how predictable is Lua's random function

["Jerome Vuarand" <jerome.vuarand@...>]

Rici Lake wrote:
> On 21-Feb-07, at 12:25 PM, Jeremy Darling wrote:
> 
>> Just as the subject says, I'm curious just exactly how predictable
>> the random function is given a known seed.  If I use the same seed
>> on all platforms (Windows, Linux, and Mac) can I expect the next N
>> calls to random to always generate the same values?  This seems to
>> be the case on Windows, but I can't validate it on the other
>> platforms (yet). 
> 
> Lua uses the standard C library random function, which is
> deterministic. However, it might not be the same implementation on
> all platforms; i.e. you might get a different sequence starting from
> the same seed on a 64-bit platform than on a 32-bit platform (for
> example), or with different C standard library implementations.    

If portability of the pseudo-random sequence is important to you you can simply roll your own random function. Some pseudo-random generators have very basic implementations:
http://en.wikipedia.org/wiki/List_of_pseudorandom_number_generators

For example here is an example implementation (took me 5 minutes) of the first one on the list above (Blum Blum Shub) in Lua:

do
local p = 7907 -- 11 in wikipedia example, but has a too short cycle
local q = 7919 -- 19 in wikipedia example
local s = 3
local xn = s

assert(p%4==3)
assert(q%4==3)

function randombit()
    xn = xn^2 % p*q
    return xn % 2
end

function randomseed(seed)
    xn = seed
end

function random()
    local n = 0
    for i=0,31 do
        n = n*2 + randombit()
    end
    return n
end
end

randomseed(37)
for i=1,10 do
    print(string.format("%08X", random()))
end

print("------------------------")

randomseed(37)
for i=1,10 do
    print(string.format("%08X", random()))
end