/* Title:     Boyer-Moore string matching algorithm
 * Author:    H.W. Lang
 *            Fachhochschule Flensburg, University of Applied Sciences
 *            Flensburg, Germany
 * Date:      2007
 * Mail:      lang@fh-flensburg.de
 * Web:       http://www.inf.fh-flensburg.de/lang/algorithmen/pattern/bmen.htm
 * Reference: R.S. Boyer, J.S. Moore: A Fast String Searching Algorithm.
 *            Communications of the ACM, 20, 10, 762-772 (1977)
 */

public class BmStringMatcher
{
    private char[] p, t;       // pattern, text
    private int m, n;          // pattern length, text length
    private static int alphabetsize=256;
    private int[] occ;         // occurence function
    private String matches;    // string of match positions
    private char[] showmatches;// char array that shows matches
    private int[] f;
    private int[] s;

    private static String name="Boyer-Moore";
   
    public BmStringMatcher()
    {
       occ=new int[alphabetsize];
    }

    public void search(String tt, String pp)
    {
        setText(tt);
        setPattern(pp);
        bmSearch();
    }
    
    /** sets the text
     */ 
    private void setText(String tt)
    {
        n=tt.length();
        t=tt.toCharArray();
        initmatches();
    }
    
    /** sets the pattern
     */ 
    public void setPattern(String pp)
    {
        m=pp.length();
        p=pp.toCharArray();
        f=new int[m+1];
        s=new int[m+1];
        bmPreprocess();
    }

    /** computation of the occurrence function
     */ 
   private void bmInitocc()
   {
       char a;
      int j;

       for (a=0; a<alphabetsize; a++)
           occ[a]=-1;

       for (j=0; j<m; j++)
       {
           a=p[j];
           occ[a]=j;
       }
   }
   
    /** preprocessing according to the pattern (part 1)
     */ 
   private void bmPreprocess1()
   {
      int i=m, j=m+1;
      f[i]=j;
      while (i>0)
      {
         while (j<=m && p[i-1]!=p[j-1])
         {
            if (s[j]==0) s[j]=j-i;
            j=f[j];
         }
         i--; j--;
         f[i]=j;
      }
   }
   
    /** preprocessing according to the pattern (part 2)
     */ 
   private void bmPreprocess2()
   {
      int i, j;
      j=f[0];
      for (i=0; i<=m; i++)
      {
         if (s[i]==0) s[i]=j;
         if (i==j) j=f[j];
      }
   }
   
    /** preprocessing according to the pattern
     */ 
   private void bmPreprocess()
   {
      bmInitocc();
      bmPreprocess1();
      bmPreprocess2();
   }

    /** initializes match positions and the array showmatches
     */ 
    private void initmatches()
    {
        matches="";
        showmatches=new char[n];
        for (int i=0; i<n; i++)
            showmatches[i]=' ';
    }
    
    /** searches the text for all occurences of the pattern
     */ 
   private void bmSearch()
   {
       int i=0, j;
       while (i<=n-m)
       {
           j=m-1;
           while (j>=0 && p[j]==t[i+j]) j--;
           if (j<0)
         {
            report(i);
            i+=s[0];
         }
         else
            i+=Math.max(s[j+1], j-occ[t[i+j]]);
       }
   }
   
    /** reports a match
     */
    private void report(int i)
    {
        matches+=i+" ";
        showmatches[i]='^';
    }

    /** returns the match positions after the search
     */ 
    public String getMatches()
    {
        return matches;
    }

    // only for test purposes
    public static void main(String[] args)
    {
        BmStringMatcher stm=new BmStringMatcher();

        System.out.println(name);

        String tt, pp;
        tt="abcdabcd";
        pp="abc";
        stm.search(tt, pp);
        System.out.println(pp);
        System.out.println(tt);
        System.out.println(stm.showmatches);
        System.out.println(stm.getMatches());

        tt="abababaa";
        pp="aba";
        stm.search(tt, pp);
        System.out.println(pp);
        System.out.println(tt);
        System.out.println(stm.showmatches);
        System.out.println(stm.getMatches());
    }
       
   
}   // end class BmStringMatcher