[R] Count matches of a sequence in a vector?

[Jeff Brown]

Phil's algorithm is a good one, unless you're worried about optimizing for
speed.  It makes N * M comparisons, where N is the length of the first
vector and M is the length of the second.  Explicitly iterating through the
longer vector, you could reduce the number of comparisons to M.  As is often
the case, here there's a tradeoff between human effort and computer effort.

OTOH, because explicit iteration incurs higher interpretation costs in R, it
could conceivably be that Phil's algorithm performs faster.  I don't know.
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