Z algorithm


There are some algorithms of exact substring searching (e.g. Knuth-Morris-Pratt,Boyer-Moore etc.) I want to explain one of them which is called Z algorithm in some sources.

Z-boxes and Z-values

Let’s consider the concept of Z-box. Take the string S = "abcxxxabyyy". We have an internal part "ab" in the string which repeats its prefix. The internal"ab" is a Z-box. It has the beginning at the position with index 6 and the end in 7 (0-based). Z-boxes are substrings which match string prefixes with the same length. For the Z-box, let’s call the corresponding prefix the prototype (it’s my term but I think it’s OK to use it.)

Z algorithm

In fact, the string "abcxxxabyyy" has three non-empty (i.e. with length greater than zero) Z-boxes:
1) the whole strings matches it’s prefix length 11;
2) 'a' character at the position 6;
3) just considered substring "ab" (6-7).

Let Z-value, Zi(S) or just Zi be the length of a Z-box in string S which starts at the position i. Obviously, Z-value can’t be bigger than the length of the rest of the string…