KMP

/**
 * Author        : Dipu Kumar Mohanto Dip
 *                 CSE, BRUR.
 *                 Batch - 6
 *
 * Problem       : KMP Algorithm Implementation
 * Algorithm     : Knuth Morris Pratt Algorithm
 * Complexity    : O(m+n)
 * Category      : String, Search
 *
 * Difficulty    : Easy
 *
 * Source        : GeeksForGeeks
 *
 * Veridict      : OK
 *
 * Date          :
 * E-mail        : dipukumarmohanto1@gmail.com
**/

#include<bits/stdc++.h>

using namespace std;

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);

    // 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 (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;
        }
    }
}

// Fills lps[] for given patttern 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++;
            }
        }
    }
}


int main()
{
    char *txt = "ABABDABACDABABCABAB";
    char *pat = "ABABCABAB";
    KMPSearch(pat, txt);
    return 0;
}

/*****************************************************

>>> Output :
Found pattern at index 10

******************************************************/