From: already5chosen@yahoo.com
On Mon, 22 Dec 2025 18:41:10 +0100
Janis Papanagnou wrote:
> On 2025-12-22 18:13, James Kuyper wrote:
> > On 2025-12-22 07:18, Janis Papanagnou wrote:
> >> On 2025-12-22 12:44, James Kuyper wrote:
> >>> On 2025-12-22 03:48, Michael Sanders wrote:
> >>>> Is it incorrect to use 0 (zero) to seed srand()?
> >>>>
> >>>> int seed = (argc >= 2 && strlen(argv[1]) == 9)
> >>>> ? atoi(argv[1])
> >>>> : (int)(time(NULL) % 900000000 + 100000000);
> >>>>
> >>>> srand(seed);
> >>>
> >>> No, why whould you think so?
> >>
> >> There's number sequence generators that produce 0 sequences if
> >> seeded with 0. ...
> >
> > The details of how the seed affects the random number sequence are
> > unspecified by the standard. I personally would consider a
> > pseudo-random number generator to be quite defective if there were
> > any seed that produced a constant output.
>
> I wouldn't have mentioned that if there weren't a whole class of
> such functions that expose exactly that behavior by design. Have
> a look for PN-(Pseudo Noise-)generators and LFSR (Linear Feedback
> Shift Registers). These have been defined to produce random noise
> (bit pattern with good statistical distribution). With sophisticated
> generator polynomials they produce also sequences of maximum period;
> say, for N=31 a non-repeating sequence of length 2^N - 1. The one
> element that is missing from the sequence is the 0 (that reproduces
> itself).
>
> Technically you pick some bit-values from fixed positions (depending
> on the generator polynomial) of the register and xor the bits to shift
> the result into the register. Here's ad hoc an example...
>
> #include
> #include
>
> int main ()
> {
> uint32_t init = 0x00000038;
> uint32_t reg = init;
> uint32_t new_bit;
> int count = 0;
> do {
> new_bit = ((reg >> 2) + (reg >> 4) + (reg >> 6) + (reg >>
> 30)) & 0x1;
> reg <<= 1;
> reg |= new_bit;
> reg &= 0x7fffffff;
> count++;
> } while (reg != init);
> printf ("period: %d\n", count);
> }
>
>
> Janis
>
> >> [...]
>
Pay attention that C Standard only requires for the same seed to always
produces the same sequence. There is no requirement that different
seeds have to produce different sequences.
So, for generator in your example, implementation like below would be
fully legal. Personally, I wouldn't even consider it as particularly
poor quality:
void srand(unsigned seed ) { init = seed | 1;}
[O.T.]
In practice, using LFSR for rand() is not particularly bright idea for
different reason: LFSR is a reasonably good PRNG for a single bit, but
not when you want to generate a group of 31 pseudo-random bits. In
order to get 31 new bits, without predictable repetitions from the
previous value, you would have to do 31 steps. That's slow! The process
can be accelerate by generation of several bits at time via look up
tables, but in order to get decent speed the table has to be rater big
and using big tables in standard library is bad sportsmanship.
It seems that overwhelming majority C RTLs use Linear Congruential
Generators, probably because for Stanadard library compactness of both
code and data is considered more important than very high speed (not
that on modern HW LCGs are slow) or superior random properties of
Mersenne Twisters.
[/O.T]
IMHO, decent PRNG with decent thread-safe interface (not POSIX
imbecile rand_r() ) should have been part of the C Standard
library at least since C11. But somehow it did not happen until now.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
