On Wednesday 31 January 2007 00:31, Carl Love wrote:
> Unfortunately, the only way we know how to
> figure out what the LFSR value that corresponds to the number in the
> sequence that is N before the last value (0xFFFFFF) is to calculate the
> previous value N times. It is like trying to ask what is the pseudo
> random number that is N before this pseudo random number?
Well, you can at least implement the lfsr both ways, and choose the one
that is faster to get at, like
u32 get_lfsr(u32 v)
{
int i;
u32 r = 0xffffff;
if (v < 0x7fffff) {
for (i = 0; i < v; i++)
r = lfsr_forwards(r);
} else {
for (i = 0; i < (0x1000000 - v); i++)
r = lfsr_backwards(r);
}
return r;
}
Also, if the value doesn't have to be really exact, you could have
a small lookup table with precomputed values, like:
u32 get_lfsr(u32 v)
{
static const lookup[256] = {
0xab3492, 0x3e3f34, 0xc47610c, ... /* insert actual values */
};
return lookup[v >> 16];
}
Arnd <><
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