Implementing Rabin Karp Algorithm Using Rolling Hash in Java

• Last Updated : 06 Aug, 2021

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 indicies   // 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 indicies         // ai where if the pattern exists         System.out.print(             "Pattern is found at the indicies : ");           // Calling the above search() method         // in the main() method         search(s, pattern);     } }

Output

Pattern is found at the indicies : 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

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

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