kmpAlgorithm.java

package kmp;

public class kmpAlgorithm {
	
	void computeLPSArray(String pat, int []lps)
	{
	    int j = 0;  // length of the previous longest prefix suffix
	    int i;
	    lps[0] = 0; // lps[0] is always 0
	    i = 1;
	 
	    // the loop calculates lps[i] for i = 1 to M-1
	    while (i < pat.length())
	    {
	       if (pat.charAt(j) == pat.charAt(i))
	       {
	         lps[i] = j+1;
	         i +=1; j+=1;
	       }
	       else // (pat[i] != pat[j])
	       {
	         if (j != 0)
	         {
	           j = lps[j-1];
	         }
	         else // if (j == 0)
	         {
	           lps[i] = 0;
	           i++;
	         }
	       }
	    }
	}
	
	public void patternSearch(String pattern, String text, int[] lps)
	{
		int pat_index = 0, text_index = 0;
		
		if(pattern.length() == 0)
		{
			return;
		}
		
		while(text_index < text.length())
		{
			// if characters match, look for next character match
			if(pattern.charAt(pat_index) == text.charAt(text_index))
			{
				pat_index++; text_index++;
				
				// indicates that complete pattern has matched
				if(pat_index == pattern.length())
				{
					System.out.println("Match");
					pat_index = lps[pat_index-1];
				}
			}
			
			// if the characters do not match, don't go back in the text. Just adjust the pattern_index
			else 
			{
				if(pat_index != 0)
				{
					pat_index = lps[pat_index-1];
				}
				else
				{
					text_index++;
				}
			}
		}
	 }
	
	
	public static void main(String []args)
	{
		kmpAlgorithm solution = new kmpAlgorithm();
		
		String pattern = "ananab#banana";
		int[] lps = new int[pattern.length()];
		solution.computeLPSArray(pattern, lps);
		
		for(int i = 0; i < lps.length; i++)
		{
			System.out.println(lps[i]);
		}
		
		
		String text = "ananab#banancananab#banana";
		
		solution.patternSearch(pattern, text, lps);
	}
}