On Mon, 30 Jul 2001, nj bloodline wrote:
> What is "random scene generation" when all the scenes are in a certain
> order?! There's full maps out on the net to Pifall!, so I'm assuming the
> screens are always in the same order.
A Linear Feedback Shift Register (LFSR) is a type of pseudo-random number
generator. Most pseudo-random number generators take a starting value,
called a "seed" and will produce a sequence of numbers based on an algorithm
and that number.
For instance, if I had a 3-bit register that always shifted to the right
and loaded the XOR of bit 1 and bit 0 into bit 2, the following sequence
results:
OLD -> NEW
--- ---
0 0
1 4
2 5
3 1
4 2
5 6
6 7
7 3
If I preload the register with a given seed number the following sequences
result:
SEED SEQUENCE
---- --------
1 1, 4, 2, 5, 6, 7, 3, 1 (repeats)
6 6, 7, 3, 1, 4, 2, 5, 6 (repeats)
3 3, 1, 4, 2, 5, 6, 7, 3 (repeats)
The sequence is always the same and it always repeats. It just starts in
a different position each time. The one special case is zero. If the
LFSR is loaded with zero, its output will be zero forever.
This is a pretty simple example. It isn't reversible (try it and see
what happens) but the sequence looks random (until it starts repeating).
Hope that helps!
-Chris
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