# KMP Algorithm in C

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Given a text txt and a pattern pat, the objective of this program will be to print all the occurance of pat in txt.

Examples:

Input:

```txt[] =  "THIS IS A TEST TEXT"
pat[] = "TEST"```

output:

`Pattern found at index 10`

Input:

```txt[] =  "AABAACAADAABAAABAA"
pat[] = "AABA"```

output:

```Pattern found at index 0
Pattern found at index 9
Pattern found at index 13```

C Language Implementation:

```// C program for implementation of KMP pattern searching
// algorithm
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

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

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 = (int *)malloc(sizeof(int)*M);
int j  = 0;  // index for pat[]

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

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

if (j == M)
{
printf("Found pattern at index %d \n", 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;
}
}
free(lps); // to avoid memory leak
}

void computeLPSArray(char *pat, int M, int *lps)
{
int len = 0;  // length of the previous longest prefix suffix
int i;

lps[0] = 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++;
lps[i] = len;
i++;
}
else // (pat[i] != pat[len])
{
if (len != 0)
{
// This is tricky. Consider the example
// AAACAAAA and i = 7.
len = lps[len-1];

// Also, note that we do not increment i here
}
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;
}```

Output:

`Found pattern at index 10`

Reference:

http://www.geeksforgeeks.org/searching-for-patterns-set-2-kmp-algorithm/