Using Rabin-Karp to search for multiple patterns in a string

Question

According to the wikipedia entry on Rabin-Karp string matching algorithm, it can be used to look for several different patterns in a string at the same time while still maintaining linear complexity. It is clear that this is easily done when all the patterns are of the same length, but I still don't get how we can preserve O(n) complexity when searching for patterns with differing length simultaneously. Can someone please shed some light on this?

Edit (December 2011):

The wikipedia article has since been updated and no longer claims to match multiple patterns of differing length in O(n).

Answer 1

I'm not sure if this is the correct answer, but anyway:

While constructing the hash value, we can check for a match in the set of string hashes. Aka, the current hash value. The hash function/code is usually implemented as a loop and inside that loop we can insert our quick look up.

Of course, we must pick m to have the maximum string length from the set of strings.

Update: From Wikipedia,

[...]
for i from 1 to n-m+1
         if hs ∈ hsubs
             if s[i..i+m-1] = a substring with hash hs
                 return i
         hs := hash(s[i+1..i+m]) // <---- calculating current hash
[...]

We calculate current hash in m steps. On each step there is a temporary hash value that we can look up ( O(1) complexity ) in the set of hashes. All hashes will have the same size, ie 32 bit.

Update 2: an amortized (average) O(n) time complexity ?

Above I said that m must have the maximum string length. It turns out that we can exploit the opposite.
With hashing for shifting substring search and a fixed m size we can achieve O(n) complexity.

If we have variable length strings we can set m to the minimum string length. Additionally, in the set of hashes we don't associate a hash with the whole string but with the first m-characters of it.
Now, while searching the text we check if the current hash is in the hash set and we examine the associated strings for a match.

This technique will increase the false alarms but on average it has O(n) time complexity.