package com.interview.string;

/**
 * Date 09/25/2014
 * @author Tushar Roy
 *
 * Rabin Karp algorith for substring matching.
 *
 * Time complexity in worst case O(n^2)(depends on hash function)
 * Space complexity O(1)
 *
 * References
 * https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm
 */
public class RabinKarpSearch {

    private int prime = 101;
    
    public int patternSearch(char[] text, char[] pattern){
        int m = pattern.length;
        int n = text.length;
        long patternHash = createHash(pattern, m - 1);
        long textHash = createHash(text, m - 1);
        for (int i = 1; i <= n - m + 1; i++) {
            if(patternHash == textHash && checkEqual(text, i - 1, i + m - 2, pattern, 0, m - 1)) {
                return i - 1;
            }
            if(i < n - m + 1) {
                textHash = recalculateHash(text, i - 1, i + m - 1, textHash, m);
            }
        }
        return -1;
    }
    
    private long recalculateHash(char[] str,int oldIndex, int newIndex,long oldHash, int patternLen) {
        long newHash = oldHash - str[oldIndex];
        newHash = newHash/prime;
        newHash += str[newIndex]*Math.pow(prime, patternLen - 1);
        return newHash;
    }
    
    private long createHash(char[] str, int end){
        long hash = 0;
        for (int i = 0 ; i <= end; i++) {
            hash += str[i]*Math.pow(prime,i);
        }
        return hash;
    }
    
    private boolean checkEqual(char str1[],int start1,int end1, char str2[],int start2,int end2){
        if(end1 - start1 != end2 - start2) {
            return false;
        }
        while(start1 <= end1 && start2 <= end2){
            if(str1[start1] != str2[start2]){
                return false;
            }
            start1++;
            start2++;
        }
        return true;
    }
    
    public static void main(String args[]){
        RabinKarpSearch rks = new RabinKarpSearch();
        System.out.println(rks.patternSearch("TusharRoy".toCharArray(), "sharRoy".toCharArray()));
        System.out.println(rks.patternSearch("TusharRoy".toCharArray(), "Roy".toCharArray()));
        System.out.println(rks.patternSearch("TusharRoy".toCharArray(), "shas".toCharArray()));
        System.out.println(rks.patternSearch("TusharRoy".toCharArray(), "usha".toCharArray()));
        System.out.println(rks.patternSearch("TusharRoy".toCharArray(), "Tus".toCharArray()));
    }
}