C++ Programming Code Examples
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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;
}
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