$ g++ StringMatchingFiniteAutomata.cpp $ a.out pattern found at index 0 pattern found at index 9 pattern found at index 13 ------------------ (program exited with code: 0) Press return to continue
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C++ Program to Perform Finite State Automaton based Search
C++ 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.
Here is source code of the C++ Program to Perform Finite State Automaton based Search. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.
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#include<stdio.h>
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#include<string.h>
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#define NO_OF_CHARS 256
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int getNextState(char *pat, int M, int state, int x)
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{
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// If the character c is same as next character in pattern,
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// then simply increment state
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if (state < M && x == pat[state])
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return state + 1;
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int ns, i; // ns stores the result which is next state
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// ns finally contains the longest prefix which is also suffix
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// in "pat[0..state-1]c"
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// Start from the largest possible value and stop when you find
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// a prefix which is also suffix
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for (ns = state; ns > 0; ns--)
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{
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if (pat[ns - 1] == x)
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{
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for (i = 0; i < ns - 1; i++)
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{
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if (pat[i] != pat[state - ns + 1 + i])
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break;
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}
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if (i == ns - 1)
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return ns;
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}
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}
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return 0;
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}
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/* This function builds the TF table which represents Finite Automata for a
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given pattern */
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void computeTF(char *pat, int M, int TF[][NO_OF_CHARS])
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{
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int state, x;
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for (state = 0; state <= M; ++state)
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for (x = 0; x < NO_OF_CHARS; ++x)
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TF[state][x] = getNextState(pat, M, state, x);
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}
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/* Prints all occurrences of pat in txt */
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void search(char *pat, char *txt)
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{
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int M = strlen(pat);
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int N = strlen(txt);
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int TF[M + 1][NO_OF_CHARS];
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computeTF(pat, M, TF);
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// Process txt over FA.
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int i, state = 0;
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for (i = 0; i < N; i++)
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{
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state = TF[state][txt[i]];
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if (state == M)
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{
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printf("\n pattern found at index %d", i - M + 1);
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}
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}
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}
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// Driver program to test above function
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int main()
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{
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char *txt = "AABAACAADAABAAABAA";
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char *pat = "AABA";
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search(pat, txt);
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return 0;
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}
Output:
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