Finding the Number of States for the Naive Dfa is Easy
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.
Examples:
Input: txt[] = "THIS IS A TEST TEXT" pat[] = "TEST" Output: Pattern found at index 10 Input: txt[] = "AABAACAADAABAABA" pat[] = "AABA" Output: Pattern found at index 0 Pattern found at index 9 Pattern found at index 12
Pattern searching is an important problem in computer science. When we do search for a string in notepad/word file or browser or database, pattern searching algorithms are used to show the search results.
We have discussed the following algorithms in the previous posts:
Naive Algorithm
KMP Algorithm
Rabin Karp Algorithm
In this post, we will discuss Finite Automata (FA) based pattern searching algorithm. In FA based algorithm, we preprocess the pattern and build a 2D array that represents a Finite Automata. Construction of the FA is the main tricky part of this algorithm. Once the FA is built, the searching is simple. In search, we simply need to start from the first state of the automata and the first character of the text. At every step, we consider next character of text, look for the next state in the built FA and move to a new state. If we reach the final state, then the pattern is found in the text. The time complexity of the search process is O(n).
Before we discuss FA construction, let us take a look at the following FA for pattern ACACAGA.
The above diagrams represent graphical and tabular representations of pattern ACACAGA.
Number of states in FA will be M+1 where M is length of the pattern. The main thing to construct FA is to get the next state from the current state for every possible character. Given a character x and a state k, we can get the next state by considering the string "pat[0..k-1]x" which is basically concatenation of pattern characters pat[0], pat[1] … pat[k-1] and the character x. The idea is to get length of the longest prefix of the given pattern such that the prefix is also suffix of "pat[0..k-1]x". The value of length gives us the next state. For example, let us see how to get the next state from current state 5 and character 'C' in the above diagram. We need to consider the string, "pat[0..4]C" which is "ACACAC". The length of the longest prefix of the pattern such that the prefix is suffix of "ACACAC"is 4 ("ACAC"). So the next state (from state 5) is 4 for character 'C'.
In the following code, computeTF() constructs the FA. The time complexity of the computeTF() is O(m^3*NO_OF_CHARS) where m is length of the pattern and NO_OF_CHARS is size of alphabet (total number of possible characters in pattern and text). The implementation tries all possible prefixes starting from the longest possible that can be a suffix of "pat[0..k-1]x". There are better implementations to construct FA in O(m*NO_OF_CHARS) (Hint: we can use something like lps array construction in KMP algorithm). We have covered the better implementation in our next post on pattern searching.
C
#include<stdio.h>
#include<string.h>
#define NO_OF_CHARS 256
int getNextState( char *pat, int M, int state, int x)
{
if (state < M && x == pat[state])
return state+1;
int ns, i;
for (ns = state; ns > 0; ns--)
{
if (pat[ns-1] == x)
{
for (i = 0; i < ns-1; i++)
if (pat[i] != pat[state-ns+1+i])
break ;
if (i == ns-1)
return ns;
}
}
return 0;
}
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);
}
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);
int i, state=0;
for (i = 0; i < N; i++)
{
state = TF[state][txt[i]];
if (state == M)
printf ( "\n Pattern found at index %d" ,
i-M+1);
}
}
int main()
{
char *txt = "AABAACAADAABAAABAA" ;
char *pat = "AABA" ;
search(pat, txt);
return 0;
}
CPP
#include <bits/stdc++.h>
using namespace std;
#define NO_OF_CHARS 256
int getNextState(string pat, int M, int state, int x)
{
if (state < M && x == pat[state])
return state+1;
int ns, i;
for (ns = state; ns > 0; ns--)
{
if (pat[ns-1] == x)
{
for (i = 0; i < ns-1; i++)
if (pat[i] != pat[state-ns+1+i])
break ;
if (i == ns-1)
return ns;
}
}
return 0;
}
void computeTF(string 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);
}
void search(string pat, string txt)
{
int M = pat.size();
int N = txt.size();
int TF[M+1][NO_OF_CHARS];
computeTF(pat, M, TF);
int i, state=0;
for (i = 0; i < N; i++)
{
state = TF[state][txt[i]];
if (state == M)
cout<< " Pattern found at index " << i-M+1<<endl;
}
}
int main()
{
string txt = "AABAACAADAABAAABAA" ;
string pat = "AABA" ;
search(pat, txt);
return 0;
}
Java
class GFG {
static int NO_OF_CHARS = 256 ;
static int getNextState( char [] pat, int M,
int state, int x)
{
if (state < M && x == pat[state])
return state + 1 ;
int ns, i;
for (ns = state; ns > 0 ; ns--)
{
if (pat[ns- 1 ] == x)
{
for (i = 0 ; i < ns- 1 ; i++)
if (pat[i] != pat[state-ns+ 1 +i])
break ;
if (i == ns- 1 )
return ns;
}
}
return 0 ;
}
static void computeTF( char [] pat, int M, int TF[][])
{
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);
}
static void search( char [] pat, char [] txt)
{
int M = pat.length;
int N = txt.length;
int [][] TF = new int [M+ 1 ][NO_OF_CHARS];
computeTF(pat, M, TF);
int i, state = 0 ;
for (i = 0 ; i < N; i++)
{
state = TF[state][txt[i]];
if (state == M)
System.out.println( "Pattern found "
+ "at index " + (i-M+ 1 ));
}
}
public static void main(String[] args)
{
char [] pat = "AABAACAADAABAAABAA" .toCharArray();
char [] txt = "AABA" .toCharArray();
search(txt,pat);
}
}
Python3
NO_OF_CHARS = 256
def getNextState(pat, M, state, x):
if state < M and x = = ord (pat[state]):
return state + 1
i = 0
for ns in range (state, 0 , - 1 ):
if ord (pat[ns - 1 ]) = = x:
while (i<ns - 1 ):
if pat[i] ! = pat[state - ns + 1 + i]:
break
i + = 1
if i = = ns - 1 :
return ns
return 0
def computeTF(pat, M):
global NO_OF_CHARS
TF = [[ 0 for i in range (NO_OF_CHARS)]\
for _ in range (M + 1 )]
for state in range (M + 1 ):
for x in range (NO_OF_CHARS):
z = getNextState(pat, M, state, x)
TF[state][x] = z
return TF
def search(pat, txt):
global NO_OF_CHARS
M = len (pat)
N = len (txt)
TF = computeTF(pat, M)
state = 0
for i in range (N):
state = TF[state][ ord (txt[i])]
if state = = M:
print ( "Pattern found at index: {}" .\
format (i - M + 1 ))
def main():
txt = "AABAACAADAABAAABAA"
pat = "AABA"
search(pat, txt)
if __name__ = = '__main__' :
main()
C#
using System;
class GFG
{
public static int NO_OF_CHARS = 256;
public static int getNextState( char [] pat, int M,
int state, int x)
{
if (state < M && ( char )x == pat[state])
{
return state + 1;
}
int ns, i;
for (ns = state; ns > 0; ns--)
{
if (pat[ns - 1] == ( char )x)
{
for (i = 0; i < ns - 1; i++)
{
if (pat[i] != pat[state - ns + 1 + i])
{
break ;
}
}
if (i == ns - 1)
{
return ns;
}
}
}
return 0;
}
public static void computeTF( char [] pat,
int M, int [][] TF)
{
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);
}
}
}
public static void search( char [] pat,
char [] txt)
{
int M = pat.Length;
int N = txt.Length;
int [][] TF = RectangularArrays.ReturnRectangularIntArray(M + 1,
NO_OF_CHARS);
computeTF(pat, M, TF);
int i, state = 0;
for (i = 0; i < N; i++)
{
state = TF[state][txt[i]];
if (state == M)
{
Console.WriteLine( "Pattern found " +
"at index " + (i - M + 1));
}
}
}
public static class RectangularArrays
{
public static int [][] ReturnRectangularIntArray( int size1,
int size2)
{
int [][] newArray = new int [size1][];
for ( int array1 = 0; array1 < size1; array1++)
{
newArray[array1] = new int [size2];
}
return newArray;
}
}
public static void Main( string [] args)
{
char [] pat = "AABAACAADAABAAABAA" .ToCharArray();
char [] txt = "AABA" .ToCharArray();
search(txt,pat);
}
}
Javascript
<script>
let NO_OF_CHARS = 256;
function getNextState(pat,M,state,x)
{
if (state < M && x == pat[state].charCodeAt(0))
return state + 1;
let ns, i;
for (ns = state; ns > 0; ns--)
{
if (pat[ns-1].charCodeAt(0) == x)
{
for (i = 0; i < ns-1; i++)
if (pat[i] != pat[state-ns+1+i])
break ;
if (i == ns-1)
return ns;
}
}
return 0;
}
function computeTF(pat,M,TF)
{
let state, x;
for (state = 0; state <= M; ++state)
for (x = 0; x < NO_OF_CHARS; ++x)
TF[state][x] = getNextState(pat, M, state, x);
}
function search(pat,txt)
{
let M = pat.length;
let N = txt.length;
let TF = new Array(M+1);
for (let i=0;i<M+1;i++)
{
TF[i]= new Array(NO_OF_CHARS);
for (let j=0;j<NO_OF_CHARS;j++)
TF[i][j]=0;
}
computeTF(pat, M, TF);
let i, state = 0;
for (i = 0; i < N; i++)
{
state = TF[state][txt[i].charCodeAt(0)];
if (state == M)
document.write( "Pattern found " + "at index " + (i-M+1)+ "<br>" );
}
}
let pat = "AABAACAADAABAAABAA" .split( "" );
let txt = "AABA" .split( "" );
search(txt,pat);
</script>
Output:
Pattern found at index 0 Pattern found at index 9 Pattern found at index 13
Time Complexity: O(m2)
Auxiliary Space: O(m)
References:
Introduction to Algorithms by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, Clifford Stein
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Source: https://www.geeksforgeeks.org/finite-automata-algorithm-for-pattern-searching/
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