Rabin Karp

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The Rabin–Karp algorithm or Karp–Rabin algorithm is a string searching algorithm that uses hashing to find any one of a set of pattern strings in a text.Its average and best case running time is O(n+m) in space O(p), but its worst-case time is O(nm) where n is the length of the text and m is the length of the pattern.

Algorithm implementation in java for string matching

void RabinfindPattern(String text,String pattern){
    /*
    q a prime number
    p hash value for pattern
    t hash value for text
    d is the number of unique characters in input alphabet
    */
    int d=128;
    int q=100;
    int n=text.length();
    int m=pattern.length();
    int t=0,p=0;
    int h=1;
    int i,j;
//hash value calculating function
    for (i=0;i<m-1;i++)
            h = (h*d)%q;
        for (i=0;i<m;i++){
        p = (d*p + pattern.charAt(i))%q;
        t = (d*t + text.charAt(i))%q;
        }
//search for the pattern 
    for(i=0;i<end-m;i++){
        if(p==t){
//if the hash value matches match them character by character
            for(j=0;j<m;j++)
                if(text.charAt(j+i)!=pattern.charAt(j))
                break;
            if(j==m && i>=start)
                System.out.println("Pattern match found at index "+i);            
        }
        if(i<end-m){
            t =(d*(t - text.charAt(i)*h) + text.charAt(i+m))%q;
            if(t<0)
                t=t+q;
        }    
    }                                
}

While calculating hash value we are dividing it by a prime number in order to avoid collision.After dividing by prime number the chances of collision will be less, but still ther is a chance that the hash value can be same for two strings,so when we get a match we have to check it character by character to make sure that we got a proper match.

t =(d*(t - text.charAt(i)*h) + text.charAt(i+m))%q;

This is to recalculate the hash value for pattern,first by removing the left most character and then adding the new character from the text.

Found a mistake? Have a question or improvement idea? Let me know.

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