sparsengram.java

Boosting算法软件包

package jboost.examples.ngram;

/**
 * This is the class for sparse ngrams, i.e., all ngrams up to a given
 * length, including those with wildcards.
 *
 * @author Rob Schapire (rewritten by Aaron Arvey)
 */

public class SparseNgram extends AbstractNgram {

    /** current ngram pattern */
    private int n = 1;    
    
    /** stopping position for constructing ngrams */ 
    private int end;      


    /**
     * Constructor.  ngrams will be constructed from string s and have
     * length up to the given size.  The given size cannot exceed 25
     * for overflow reasons (really its 31, but we use 25 out of
     * paranoia).
     * @param s string from which ngrams are constructed
     * @param size maximum size of all ngrams returned
     */

    public SparseNgram(String s, int size) {
	super(s);
	if (size > 25) {
	    throw new IllegalArgumentException("cannot have window size bigger "
					       + "than 25 in SparseNgram"); 
	}
	enumer = new PatternNgram(words, new boolean[] {true});
	end = 1 << size;
    }

    public boolean hasMoreElements() {
	if (n >= end)
	    return false;
	while (!enumer.hasMoreElements() && (n += 2) < end) {
	    int pos = 0;
	    for (int i = n; i != 0; i >>= 1)
		pos++;
	    boolean[] pat = new boolean[pos];
	    pos = 0;
	    for (int i = n; i != 0; i >>= 1) {
		pat[pos] = ((i & 1) != 0);
		pos++;
	    }
	    enumer = new PatternNgram(words, pat);
	}
	return (n < end);
    }

}