kmp.cpp

#include <bits/stdc++.h>
using namespace std;

#define vi vector<int>

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

// Prints occurrences of txt[] in pat[]
void KMPSearch(string pat, string txt)
{

    int M = pat.length();
    int N = txt.length();

    // 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 \n", i - j);
            cout << endl;
            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(string pat, int M, int *lps)
{
    int len = 0;

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

    int i = 1;
    while (i < M)
    {
        if (pat[i] == pat[len])
        {
            len++;
            lps[i] = len;
            i++;
        }
        else // (pat[i] != pat[len])
        {

            if (len != 0)
            {
                len = lps[len - 1];
            }
            else
            {
                lps[i] = 0;
                i++;
            }
        }
    }
}

int main()
{
    cout << "Enter String and Pattern : ";
    string s, p;
    cin >> s >> p;

    KMPSearch(p, s);

    return 0;
}