Single String

Knuth-Morris-Pratt Algorithm

Define an array $\pi_S$ of size $|S|$ such that $\pi_S[i]$ is equal to the length of the longest nontrivial suffix of the prefix ending at position $i$ the coincides with a prefix of the entire string. Formally,

In other words, for a given index $i$, we would like to compute the length of the longest substring that ends at $i$, such that this string also happens to be a prefix of the entire string. One such string that satisfies this criteria is the prefix ending at $i$; we will be disregarding this solution for obvious reasons.

For instance, for $S = \text{``abcabcd"}$, $\pi_S = [0, 0, 0, 1, 2, 3, 0]$, and the prefix function of $S = \text{``aabaaab"}$ is $\pi_S = [0, 1, 0, 1, 2, 2, 3]$. In the second example, $\pi_S[4] = 2$ because the prefix of length $2$ ($\text{``ab"})$ is equivalent to the substring of length $2$ that ends at index $4$. In the same way, $\pi_S[6] = 3$ because the prefix of length $3$ ($\text{``abb"}$) is equal to the substring of length $3$ that ends at index $6$. For both of these samples, there is no longer substring that satisfies these criterias.

The purpose of the KMP algorithm is to efficiently compute the $\pi_S$ array in linear time. Suppose we have already computed the $\pi_S$ array for indices $0\dots i$, and need to compute the value for index $i + 1$.

Firstly, note that between $\pi_S[i]$ and $\pi_S[i + 1]$, $\pi_S[i + 1]$ can be at most one greater. This occurs when $S[\pi_S[i]] = S[i + 1]$.

In the example above, $\pi_S[i] = 5$, meaning that a the suffix of length $5$ is equivalent to a prefix of length $5$ of the entire string. It follows that if the character at position $5$ of the string is equal to the character at position $i + 1$, then the match is simply extended by a single character. Thus, $\pi_S[i + 1] = \pi_S[i] + 1 = 6$.

In the general case, however, this is not necessarily true. That is to say, $S[\pi_S[i]] \neq S[i + 1]$. Thus, we need to find the largest index $j < \pi_S[i]$ such that the prefix property holds (ie $S[:j - 1] \equiv S[i - j + 1:i]$). For such a length $j$, we repeat the procedure in the first example by comparing characters at indicies $j$ and $i + 1$: if the two are equal, then we can conclude our search and assign $\pi_S[i + 1] = j + 1$, and otherwise, we find the next smallest $j$ and repeat. Indeed, notice that the first example is simply the case where $j$ begins as $\pi_S[i]$.

In the second example above, we let $j = 2$.

The only thing that remains is to be able to efficiently find all the $j$ that we might possibly need. To recap, if the position we're currently at is $j$, to handle transitions we need to find the largest index $k$ that satisfies the prefix property $S[:k - 1] \equiv S[j - k + 1 : j]$. Since $j < i$, this value is simply $\pi_S[j - 1]$, a value that has already been computed. All that remains is to handle the case where $j = 0$. If $S[0] = S[i + 1]$, $\pi_S[i + 1] = 1$, otherwise $\pi_S[i + 1] = 0$.

Copy
vector<int> pi(const string &s) {
	int n = (int)s.size();
	vector<int> pi_s(n);
	for (int i = 1, j = 0; i < n; i++) {
		while (j > 0 && s[j] != s[i]) {
			j = pi_s[j - 1]
		}
		if (s[i] == s[j]) {
			j++;
		}
		pi_s[i] = j;
	}
	return pi_s;
}

Claim: The KMP algorithm runs in $\mathcal{O}(n)$ for computing the $\pi_S$ array on a string $S$ of length $n$.

Proof: Note that $j$ doesn't actually change through multiple iterations. This is because on iteration $i$, we assign $j = \pi_S[i - 1]$. However, in the previous iteration, we assign $\pi_S[i - 1]$ to be $j$. Furthermore, note that $j$ is always non-negative. In each iteration of $i$, $j$ is only increased by at most $1$ in the if statement. Since $j$ remains non-negative and is only increased a constant amount per iteration, it follows that $j$ can only decrease by at most $n$ times through all iterations of $i$. Since the inner loop is completely governed by $j$, the overall complexity amortizes to $\mathcal{O}(n)$. $\blacksquare$

Problems

Z Algorithm

The Z-Algorithm is another linear time string comparison algorithm like KMP, but instead finds the longest common prefix of a string and all of its suffixes.

Palindromes

Manacher

Manacher's Algorithm functions similarly to the Z-Algorithm. It determines the longest palindrome centered at each character.

Palindromic Tree

A Palindromic Tree is a tree-like data structure that behaves similarly to KMP. Unlike KMP, in which the only empty state is $0$, the Palindromic Tree has two empty states: length $0$, and length $-1$. This is because appending a character to a palindrome increases the length by $2$, meaning a single character palindrome must have been created from a palindrome of length $-1$

Multiple Strings

Tries

A trie is a tree-like data structure that stores strings. Each node is a string, and each edge is a character. The root is the empty string, and every node is represented by the characters along the path from the root to that node. This means that every prefix of a string is an ancestor of that string's node.

Aho-Corasick

Aho-Corasick is the combination of trie and KMP. It is essentially a trie with KMP's "fail" array.