mirror of
https://github.com/TheAlgorithms/Python.git
synced 2024-11-23 21:11:08 +00:00
90db98304e
* fixed: spelling nonegative -> non-negative * fixed: spelling transpostiion -> transposition * fixed: spelling topolical -> topological * fixed: spelling sufix -> suffix
66 lines
1.6 KiB
Python
66 lines
1.6 KiB
Python
"""
|
||
https://cp-algorithms.com/string/prefix-function.html
|
||
|
||
Prefix function Knuth–Morris–Pratt algorithm
|
||
|
||
Different algorithm than Knuth-Morris-Pratt pattern finding
|
||
|
||
E.x. Finding longest prefix which is also suffix
|
||
|
||
Time Complexity: O(n) - where n is the length of the string
|
||
"""
|
||
|
||
|
||
def prefix_function(input_string: str) -> list:
|
||
"""
|
||
For the given string this function computes value for each index(i),
|
||
which represents the longest coincidence of prefix and suffix
|
||
for given substring (input_str[0...i])
|
||
|
||
For the value of the first element the algorithm always returns 0
|
||
|
||
>>> prefix_function("aabcdaabc")
|
||
[0, 1, 0, 0, 0, 1, 2, 3, 4]
|
||
>>> prefix_function("asdasdad")
|
||
[0, 0, 0, 1, 2, 3, 4, 0]
|
||
"""
|
||
|
||
# list for the result values
|
||
prefix_result = [0] * len(input_string)
|
||
|
||
for i in range(1, len(input_string)):
|
||
|
||
# use last results for better performance - dynamic programming
|
||
j = prefix_result[i - 1]
|
||
while j > 0 and input_string[i] != input_string[j]:
|
||
j = prefix_result[j - 1]
|
||
|
||
if input_string[i] == input_string[j]:
|
||
j += 1
|
||
prefix_result[i] = j
|
||
|
||
return prefix_result
|
||
|
||
|
||
def longest_prefix(input_str: str) -> int:
|
||
"""
|
||
Prefix-function use case
|
||
Finding longest prefix which is suffix as well
|
||
|
||
>>> longest_prefix("aabcdaabc")
|
||
4
|
||
>>> longest_prefix("asdasdad")
|
||
4
|
||
>>> longest_prefix("abcab")
|
||
2
|
||
"""
|
||
|
||
# just returning maximum value of the array gives us answer
|
||
return max(prefix_function(input_str))
|
||
|
||
|
||
if __name__ == "__main__":
|
||
import doctest
|
||
|
||
doctest.testmod()
|