algorithm Substring Search Python Implementation of KMP algorithm.
Example
Haystack: The string in which given pattern needs to be searched.
Needle: The pattern to be searched.
Time complexity: Search portion (strstr method) has the complexity O(n) where n is the length of haystack but as needle is also pre parsed for building prefix table O(m) is required for building prefix table where m is the length of the needle.
Therefore, overall time complexity for KMP is O(n+m)
Space complexity: O(m) because of prefix table on needle.
Note: Following implementation returns the start position of match in haystack (if there is a match) else returns -1, for edge cases like if needle/haystack is an empty string or needle is not found in haystack.
def get_prefix_table(needle):
prefix_set = set()
n = len(needle)
prefix_table = [0]*n
delimeter = 1
while(delimeter<n):
prefix_set.add(needle[:delimeter])
j = 1
while(j<delimeter+1):
if needle[j:delimeter+1] in prefix_set:
prefix_table[delimeter] = delimeter - j + 1
break
j += 1
delimeter += 1
return prefix_table
def strstr(haystack, needle):
# m: denoting the position within S where the prospective match for W begins
# i: denoting the index of the currently considered character in W.
haystack_len = len(haystack)
needle_len = len(needle)
if (needle_len > haystack_len) or (not haystack_len) or (not needle_len):
return -1
prefix_table = get_prefix_table(needle)
m = i = 0
while((i<needle_len) and (m<haystack_len)):
if haystack[m] == needle[i]:
i += 1
m += 1
else:
if i != 0:
i = prefix_table[i-1]
else:
m += 1
if i==needle_len and haystack[m-1] == needle[i-1]:
return m - needle_len
else:
return -1
if __name__ == '__main__':
needle = 'abcaby'
haystack = 'abxabcabcaby'
print strstr(haystack, needle)
