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Program to Perform Finite State Automaton based Search

Program to Perform Finite State Automaton based Search

This is a C++ Program to search a string using finite automata. 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.

    #include<stdio.h>

    #include<string.h>

    #define NO_OF_CHARS 256

    int getNextState(char *pat, int M, int state, int x)

    {

        // If the character c is same as next character in pattern,

        // then simply increment state

        if (state < M && x == pat[state])

            return state + 1;

        int ns, j; // ns stores the result which is next state

        // ns finally contains the longest prefix which is also suffix

        // in "pat[0..state-1]c"

        // Start from the largest possible value and stop when you find

        // a prefix which is also suffix

        for (ns = state; ns > 0; ns--)

        {

            if (pat[ns - 1] == x)

            {

                for (j = 0; j < ns - 1; j++)

                {

                    if (pat[j] != pat[state - ns + 1 + j])

                        break;

                }

                if (j == ns - 1)

                    return ns;

            }

        }

        return 0;

    }

    /* This function builds the TF table which represents Finite Automata for a given pattern  */

    void computeTF(char *pat, int M, int TF[][NO_OF_CHARS])

    {

        int state, x;

        for (state = 0; state <= M; ++state)

            for (x = 0; x < NO_OF_CHARS; ++x)

                TF[state][x] = getNextState(pat, M, state, x);

    }

    /* Prints all occurrences of pat in txt */

    void search(char *pat, char *txt)

    {

        int M = strlen(pat);

        int N = strlen(txt);

        int TF[M + 1][NO_OF_CHARS];

        computeTF(pat, M, TF);

        // Process txt over FA.

        int j, state = 0;

        for (j = 0; j < N; j++)

        {

            state = TF[state][txt[j]];

            if (state == M)

            {

                printf("\n pattern found at index %d", j - M + 1);

            }

        }

    }

    // Driver program to test above function

    int main()

    {

        char *txt = "Happy Codings - C++ Programming Language Code Examples";

        char *pat = "Codings";

        search(pat, txt);

        return 0;

    }