programming-for-dummies

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Sequential Text Search
The Rabin-Karp algorithm
Although much faster than a brute force search, the Boyer-Moore algorithm
still searches one character at a time. If you’re searching for a text string,
you can speed up the search by examining blocks of text rather than individual
characters.
For example, if you’re searching for the string GAG, you could examine three
characters at a time rather than examining a single character three times. To
make searching blocks of characters faster, two computer scientists (Michael
O. Rabin and Richard M. Karp) created the Rabin-Karp algorithm.
This algorithm uses a hash function to convert a block of characters into a
numeric value. Instead of examining individual characters, the Rabin-Karp
algorithm uses its hash function to convert the original search string into a
numeric value. So a hash function might convert the three-character string
to search (GAG) into a numeric value of 3957.
After converting the search string into a numeric value, the Rabin-Karp algorithm
repetitively searches for blocks of characters that are the same length
of the search string (such as three-characters) and uses its hash function to
convert those blocks of text into a numeric value. Now instead of searching
for matching characters, the Rabin-Karp algorithm searches just for matching
hash values, as shown in Figure 3-3.
Figure 3-3:
The Rabin-
Karp
algorithm
searches for
hash values.
Searching for the string GAG
G A T A G
G 0 1 1 1 0 1 0 1 1
A
G
A
T
The key to the Rabin-Karp algorithm is the speed and method of its hash
function. If the hash function can create values quickly and insure that different
strings never create the same hash value, this algorithm can run quickly.
If the hash function calculates hash values slower than the computer can
examine characters individually, this algorithm may run slower than another
algorithm, such as the Boyer-Moore algorithm. Also if the hash function calculates
identical hash values for two different strings, this algorithm won’t
be accurate enough because it finds the wrong data.