Boyer – Moore String Search (Good Suffix Shift)

December 7th, 2008 · 1 Comment · Computer Science, python

The next string search algorithm I am going to cover seems pretty cool since the match is made in a reverse fashion. We check if the last character in the keyword equals some character in the text and decrease from that point on.As a result, a mismatch is detected extremely fast and this is also the reason why this algorithm is performs beautifully when the keyword is longer.

Now, when a mismatch occurs, the position where we look for the next character is determined by a table of values mapping each character in the keyword to an integer. Example, we find an “a” in the text which at a particular state is sitting atop an M in the keyword, we shift the position in the text by the corresponding number in our table and look again.

Here is how we generate the table:

bcs = {} #the table

def goodSuffixShift(key):

for i in xrange(len(key)-1, -1, -1):
if key[i] not in bcs.keys():
bcs[key[i]] = len(key)-i-1

Next, comes the actual search:

def search(text, key):

i = len(key)-1

index = len(key) -1

j = i

while True:

print str(i) + ", " + str(j)

if i == 0:
return j #if every character in the keyword matches, return current position in text

elif j > len(text):
return "not found" #if we run out of the string while searching, there is no chance…

elif text[j] != key[i] and text[j] not in bcs.keys():
j += len(key) #If there isn’t a match and we don’t have a switch value in our table, don’t bother looking for characters in the middle and start matching again
i = index

elif text[j] != key[i] and text[j] in bcs.keys():
j += bcs[text[j]] #move forward as table says and start looking again
i = index

else:
j -= 1 #in case a match is found, check for the character to the left
i -= 1

Here are some sample runs:

key_list = ["POWER", "HOUSE", "COMP", "SCIENCE", "SHRIPHANI", "BRUAH"]

text = "SHRIPHANI IS A COMPUTER SCIENCE POWERHOUSE"

for key in key_list:
goodSuffixShift(key)
print key + " location: " + str(search(text, key))
bcs = {}

The output:

tark-b-120:scripts shriphani$ python boyer_moore.py
POWER location: 32
HOUSE location: 37
COMP location: 15
SCIENCE location: 24
SHRIPHANI location: 0
BRUAH location: not found

You can get the entire script at http://shriphani.com/scripts/boyer_moore.py.

Got to get back to Psychology and GUI stuff in Java. Anyway, GUIs are for losers, CLI FTW !!!

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One Comment so far ↓

meade // Jul 9, 2009 at 3:27 pm

There’s a problem with the code if you search for a single character, ‘A’ for example. I changed the following to make it work:
while True:
if i < 0: #changed
return j + 1 #changed

in the search function

Shriphani ‘PSP’ Palakodety

Weblog of an Aspiring Computer Scientist