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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