string - Understanding Knuth-Morris-Pratt Algorithm

Question

Can someone explain this to me? I've been reading about it and it still is hard to follow.

text : ababdbaababa
pattern: ababa

table for ababa is -1 0 0 1 2.

I think I understand how the table is constructed but, I dont understand how to shift once mismatch has occurred. Seems like we dont even use the table when shifting?

when do we use the table?

Answer 1

Here I have briefly described computing the prefix function and shifting through the text here.

For further information: Knuth–Morris–Pratt string search algorithm

Shifting through the text :

Text:     ABC ABCDAB ABCDABCDABDE
Pattern : ABCDABD

Scenario 1 - There is/are some matching character/s in Pattern and Text.
e.g 1: In here there are 3 matching characters.

Get the value from table for 3 characters. (index 2, ABC) i.e 0 Therefore shift = 3 - 0 i.e 3

e.g 2: In here there are 6 matching characters.

Get the value from table for 6 characters. (index 5, ABCDAB) i.e 2 Therefore shift = 6 - 2 i.e 4

Scenario 2 - If there is no matching characters then shift by one.