10.KMP-Algorithm.cpp

Given a text txt[0..n-1] and a pattern pat[0..m-1], write a function search(char pat[], char txt[]) 
that prints all occurrences of pat[] in txt[]. You may assume that n > m.

  
  time complexity: O(n) where n is length of txt.
  space complexity: O(m) where m is length of pattern 'pat'

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#include <bits/stdc++.h>

void computeLPSArray(char* pat, int M, int* lps);

// Prints occurrences of txt[] in pat[]
void KMPSearch(char* pat, char* txt)
{
	int M = strlen(pat);
	int N = strlen(txt);
	int lps[M]; // longest proper prefix
	computeLPSArray(pat, M, lps);

	int i = 0; // index for txt[]
	int j = 0; // index for pat[]
	while (i < N) {
		if (pat[j] == txt[i]) {
			j++;
			i++;
		}

		if (j == M) {
			printf("Found pattern at index %d ", i - j);
			j = lps[j - 1];
		}
		else if (i < N && pat[j] != txt[i]) {
			if (j != 0)
				j = lps[j - 1];
			else
				i = i + 1;
		}
	}
}

// Fills lps[] for given patttern pat[0..M-1]
void computeLPSArray(char* pat, int M, int* lps)
{
	int len = 0;// length of the previous longest prefix suffix
	lps[0] = 0; // lps[0] is always 0
  
	int i = 1;// the loop calculates lps[i] for i = 1 to M-1
	while (i < M) {
		if (pat[i] == pat[len]) {
			len++;
			lps[i] = len;
			i++;
		}
		else 
		{
			if (len != 0) {
				len = lps[len - 1];
			}
			else // if (len == 0)
			{
				lps[i] = 0;
				i++;
			}
		}
	}
}

// Driver program to test above function
int main()
{
	char txt[] = "ABABDABACDABABCABAB";
	char pat[] = "ABABCABAB";
	KMPSearch(pat, txt);
	return 0;
}