C# Program for KMP Algorithm for Pattern Searching

// C# program for implementation of KMP pattern
// searching algorithm
using System;

class GFG {

    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 = new 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) {
                Console.Write("Found pattern "
                              + "at index " + (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;
            }
        }
    }

    void computeLPSArray(string pat, int M, int[] lps)
    {
        // length of the previous longest prefix suffix
        int len = 0;
        int i = 1;
        lps[0] = 0; // lps[0] is always 0

        // 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])
            {
                // 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] = len;
                    i++;
                }
            }
        }
    }

    // Driver program to test above function
    public static void Main()
    {
        string txt = "ABABDABACDABABCABAB";
        string pat = "ABABCABAB";
        new GFG().KMPSearch(pat, txt);
    }
}

// This code has been contributed by Amit Khandelwal.