KMP algorithm - Python & C++ impl

1. Python Program for KMP Algorithm for Pattern Searching

refer to https://www.geeksforgeeks.org/python-program-for-kmp-algorithm-for-pattern-searching-2/
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.

Input: txt[] = "THIS IS A TEST TEXT" pat[] = "TEST" Output: Pattern found at index 10

Input: txt[] = "AABAACAADAABAABA" pat[] = "AABA" Output: Pattern found at index 0 Pattern found at index 9 Pattern found at index 12

Pattern searching is an important problem in computer science. When we do search for a string in notepad/word file or browser or database, pattern searching algorithms are used to show the search results.

python
# Python program for KMP Algorithm
def KMPSearch(pat, txt):
	M = len(pat)
	N = len(txt)

	# create lps[] that will hold the longest prefix suffix 
	# values for pattern
	lps = [0]*M
	j = 0 # index for pat[]

	# Preprocess the pattern (calculate lps[] array)
	computeLPSArray(pat, M, lps)

	i = 0 # index for txt[]
	while i < N:
		if pat[j] == txt[i]:
			i += 1
			j += 1

		if j == M:
			print ("Found pattern at index", str(i-j))
			j = lps[j-1]

		# mismatch after j matches
		elif i < N and pat[j] != txt[i]:
			# Do not match lps[0..lps[j-1]] characters,
			# they will match anyway
			if j != 0:
				j = lps[j-1]
			else:
				i += 1

def computeLPSArray(pat, M, lps):
	len = 0 # length of the previous longest prefix suffix

	lps[0] # lps[0] is always 0
	i = 1

	# the loop calculates lps[i] for i = 1 to M-1
	while i < M:
		if pat[i]== pat[len]:
			len += 1
			lps[i] = len
			i += 1
		else:
			# This is tricky. Consider the example.
			# AAACAAAA and i = 7. The idea is similar 
			# to search step.
			if len != 0:
				len = lps[len-1]

				# Also, note that we do not increment i here
			else:
				lps[i] = 0
				i += 1

txt = "ABABDABACDABABCABAB"
pat = "ABABCABAB"
KMPSearch(pat, txt)

# This code is contributed by Bhavya Jain
# Output : Found pattern at index 10

# Time Complexity: O(m+n)
# Space Complexity: O(m)

2. C++ program for KMP Algorithm

refer to https://www.geeksforgeeks.org/kmp-algorithm-for-pattern-searching/

cpp
// C++ program for implementation of KMP pattern searching
// algorithm

#include <bits/stdc++.h>

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

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

	// create lps[] that will hold the longest prefix suffix
	// values for pattern
	int lps[M];

	// Preprocess the pattern (calculate lps[] array)
	computeLPSArray(pat, M, lps);

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

		if (j == M) {
			printf("Found pattern at index %d ", i - j);
			j = lps[j - 1];
		}

		// mismatch after j matches
		else if (i < N && pat[j] != txt[i]) {
			// Do not match lps[0..lps[j-1]] characters,
			// they will match anyway
			if (j != 0)
				j = lps[j - 1];
			else
				i = i + 1;
		}
	}
}

// Fills lps[] for given pattern pat[0..M-1]
void computeLPSArray(char* pat, int M, int* lps)
{
	// length of the previous longest prefix suffix
	int len = 0;

	lps[0] = 0; // lps[0] is always 0

	// the loop calculates lps[i] for i = 1 to M-1
	int i = 1;
	while (i < M) {
		if (pat[i] == pat[len]) {
			len++;
			lps[i] = len;
			i++;
		}
		else // (pat[i] != pat[len])
		{
			// This is tricky. Consider the example.
			// AAACAAAA and i = 7. The idea is similar
			// to search step.
			if (len != 0) {
				len = lps[len - 1];

				// Also, note that we do not increment
				// i here
			}
			else // if (len == 0)
			{
				lps[i] = 0;
				i++;
			}
		}
	}
}

// Driver code
int main()
{
	char txt[] = "ABABDABACDABABCABAB";
	char pat[] = "ABABCABAB";
	KMPSearch(pat, txt);
	return 0;
}

目录

1. Python Program for KMP Algorithm for Pattern Searching

2. C++ program for KMP Algorithm

Other references.