

An Undecidable Problem in SIP - petithug
http://blog.marc.petit-huguenin.org/2014/08/an-undecidable-problem-in-sip2.html

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zitterbewegung
Even though it seems that you can create pathological SIP rules that are
undecidable could you reduce a subset of the problem if you are only allowed
to do a set of forwards and then you have a graph where you can detect cycles
with a simple cycle detection algorithm?

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userbinator
I think this article is a bit alarmist...

 _Now, let 's say that we want to implement CTS in SIP. The CPL language is
not powerful enough for that but we can define a very simple extension_

...

 _And because we previously proved that SIP with CPL is Turing-Complete_

...

 _Note that at least some SIP systems are not necessarily Turing-Complete -
here we had to add an extension to our very limited system based on CPL to
make it Turing-Complete._

In other words, he had to create a completely theoretical extension, which
AFAIK doesn't actually exist, just to show that it _could be_ Turing-complete?
If there's a real extension in use or set of extensions that could make it
Turing-complete, why didn't he just choose that?

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tlarkworthy
Its not NP to determine if the max-forward reach zero. Just simulate it until
it reaches zero or not... QED

