Implement the Boyer Moore string matching algorithm.

The Boyer-Moore algorithm was developed by R.S.Boyerand J.C.Moore in 1977.

The Boyer-Moore algorithm scans the characters of the pattern from right to left beginning with the rightmost one and performs the comparisons from right to left.

In case of a mismatch or a complete match of the whole pattern it uses two pre-computed functions to shift the window to the right.

lets see java program that implement the Boyer Moore string matching algorithm.

 

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
 
public class BoyerMoore
{
    public void findPattern(String t, String p)
    {
        char[] text = t.toCharArray();
        char[] pattern = p.toCharArray();
        int pos = indexOf(text, pattern);
        if (pos == -1)
            System.out.println("\nNo Match\n");
        else
            System.out.println("Pattern found at position : "+ pos);
    }
    public int indexOf(char[] text, char[] pattern) 
    {
        if (pattern.length == 0) 
            return 0;
        int charTable[] = makeCharTable(pattern);
        int offsetTable[] = makeOffsetTable(pattern);
        for (int i = pattern.length - 1, j; i < text.length;) 
        {
            for (j = pattern.length - 1; pattern[j] == text[i]; --i, --j) 
                     if (j == 0) 
                    return i;
 
              i += Math.max(offsetTable[pattern.length - 1 - j], charTable[text[i]]);
        }
        return -1;
      }
      private int[] makeCharTable(char[] pattern) 
      {
        final int ALPHABET_SIZE = 256;
        int[] table = new int[ALPHABET_SIZE];
        for (int i = 0; i < table.length; ++i) 
               table[i] = pattern.length;
        for (int i = 0; i < pattern.length - 1; ++i) table[pattern[i]] = pattern.length - 1 - i; return table; } private static int[] makeOffsetTable(char[] pattern) { int[] table = new int[pattern.length]; int lastPrefixPosition = pattern.length; for (int i = pattern.length - 1; i >= 0; --i) 
        {
            if (isPrefix(pattern, i + 1)) 
                   lastPrefixPosition = i + 1;
              table[pattern.length - 1 - i] = lastPrefixPosition - i + pattern.length - 1;
        }
        for (int i = 0; i < pattern.length - 1; ++i) 
        {
              int slen = suffixLength(pattern, i);
              table[slen] = pattern.length - 1 - i + slen;
        }
        return table;
    }
    private static boolean isPrefix(char[] pattern, int p) 
    {
        for (int i = p, j = 0; i < pattern.length; ++i, ++j) if (pattern[i] != pattern[j]) return false; return true; } private static int suffixLength(char[] pattern, int p) { int len = 0; for (int i = p, j = pattern.length - 1; i >= 0 && pattern[i] == pattern[j]; --i, --j) 
               len += 1;
        return len;
    }
    public static void main(String[] args) throws IOException
    {    
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Boyer Moore Algorithm Test\n");
        System.out.println("\nEnter Text\n");
        String text = br.readLine();
        System.out.println("\nEnter Pattern\n");
        String pattern = br.readLine();
        BoyerMoore bm = new BoyerMoore(); 
        bm.findPattern(text, pattern);     
    }
}

OUTPUT:
booyer

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