Implementing Rabin Karp Algorithm Using Rolling Hash in Java

There are so many pattern searching algorithms for the string. KMP algorithm, Z algorithm Rabin Karp algorithm, etc these algorithms are the optimization of Naive Pattern searching Algorithm.

Naive Pattern Searching Algorithm:

Input   :  "AABACACAACAC"
Pattern :  "CAC"
Output  :  [4,9]

AABACACAACAC

Implementation:

Java

// Java Program to Search for a Pattern String in the Input
// String returning the indices

// Importing input java classes
import java.io.*;

// Main class
public class GFG {

    // Method 1
    // To find pattern in the string
    static void search(String s, String pattern)
    {
        // Iterating over the string in which to be searched
        // for using the length() method
        for (int i = 0; i <= s.length() - pattern.length();
             ++i) {

            int check = 1;

            // Iterating over the string for which is to be
            // searched using the length() method
            for (int j = 0; j < pattern.length(); ++j) {

                // Now, checking the elements of pattern
                // with the given string using the charAt()
                // method
                if (s.charAt(i + j) != pattern.charAt(j)) {

                    // Setting check to zero as pattern is
                    // not detected here
                    check = 0;

                    // Break statement to hault
                    // execution of code
                    break;
                }
            }

            // Now if the check remains same as declared
            // then pattern is detected at least once
            if (check == 1) {

                // Printing the position(index) of the
                // pattern string in the input string
                System.out.print(i + " , ");
            }
        }
    }

    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Given custom input string
        String s = "AABACACAACAC";

        // Pattern to be looked after in
        // the above input string
        String pattern = "CAC";

        // Display message for interpreting the indices
        // ai where if the pattern exists
        System.out.print(
            "Pattern is found at the indices : ");

        // Calling the above search() method
        // in the main() method
        search(s, pattern);
    }
}

Output

Pattern is found at the indices : 4 , 9 , 

Output explanation:

The above algorithm for pattern searching is the basic algorithm the worst as the average time complexity of this algorithm is O(n×m) where n is the pattern and m is the given string. 

How can we reduce the complexity of this algorithm?

It is possible with the help of rolling hash. Rabin Karp algorithm is one of the optimized algorithms of the naive algorithm that performs searching by rolling over the string and search the pattern.

Illustration:

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

Procedure:

• Calculate the hash value of the pattern (By creating your own hash function or equation to determining an individual hash value for every character)
• Iterate over the string check the hash value of every substring generated of the size of the pattern if matched check every character of the pattern as well as String if all matched print the starting index of the string.
• If not matched shift to the next character by skipping the first character and adding the hash value of the next character that we don’t calculate the hash value of the next substring in the string only we slide the window skip the first character and add the last character in the window by calculating its hash value i.e Rolling hash.

Implementation: 

Simple Rolling algorithm assuming the pattern of length 2

1. Initialize temp=4(1+3)
2. Roll the hash value to the next element.
3. Iterate the loop ‘i’ <= Array.length-pattern.length
4. Remove the first element from the temp variable and add next element in the temp variable. temp = temp-Array[i]+Array[i.pattern.length()]

Java

// Java Program to illustrating Simple Rolling Hashing

// Importing input output classes
import java.io.*;

// Main class
class GFG {

    // Method 1
    // To search the pattern
    static void search(String S, String pattern)
    {
        // Declaring and initializing the hash values
        int hash1 = 0;
        int hash2 = 0;

        // Iterating over the pattern string to be matched
        // over
        for (int i = 0; i < pattern.length(); ++i) {

            // Storing the hash value of the pattern
            hash1 += pattern.charAt(i) - 'A';

            // Storing First hash value of the string
            hash2 += S.charAt(i) - 'A';
        }

        // Initially declaring with zero
        int j = 0;

        // Iterating over the pattern string to checkout
        // hash values
        for (int i = 0; i <= S.length() - pattern.length();
             ++i) {

            // Checking the hash value
            if (hash2 == hash1) {

                // Checking the value
                for (j = 0; j < pattern.length(); ++j) {

                    // Checking for detection of pattern in a
                    // pattern
                    if (pattern.charAt(j)
                        != S.charAt(i + j)) {

                        // Break statement to hault the
                        // execution of program as no
                        // pattern found
                        break;
                    }
                }
            }

            // If execution is not stopped means
            // pattern(sub-string) is present

            // So now simply detecting for one or more
            // occurrences inbetween pattern string using the
            // length() method
            if (j == pattern.length()) {

                // Pattern is detected so printing the index
                System.out.println(i);
            }
            // for last case of loop, have to check,
            // otherwise,
            // S.charAt(i + pattern.length()) below will
            // throw error
            if (i == S.length() - pattern.length())
                break;

            // Roll the hash value over the string detected
            hash2 = (int)((hash2) - (S.charAt(i) - 'A'))
                    + S.charAt(i + pattern.length()) - 'A';
        }
    }

    // Method 2
    // Main driver method
    public static void main(String[] args)
    {

        // Input string to be traversed
        String S = "AABAACAADAABAABA";

        // Pattern string to be checked
        String pattern = "AABA";

        // Calling the above search() method(Method1)
        // in the main() method
        search(S, pattern);
    }
}

Output

0
9
12