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MALLET is a Java-based package for statistical natural language processing, document classification, clustering, topic modeling, information extraction, and other machine learning applications to text.

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/* Copyright (C) 2002 Univ. of Massachusetts Amherst, Computer Science Dept.
   This file is part of "MALLET" (MAchine Learning for LanguagE Toolkit).
   http://www.cs.umass.edu/~mccallum/mallet
   This software is provided under the terms of the Common Public License,
   version 1.0, as published by http://www.opensource.org.  For further
   information, see the file `LICENSE' included with this distribution. */




/** 
   @author Andrew McCallum [email protected]
 */

package cc.mallet.pipe;

import java.io.*;

import cc.mallet.types.Instance;
import cc.mallet.types.TokenSequence;
/**
 * Transform a character sequence into a token sequence of character N grams.
 *    @author Andrew McCallum [email protected]
 */

public class CharSequence2CharNGrams extends Pipe implements Serializable
{
	int n;
	boolean distinguishBorders = false;

	static char startBorderChar = '>';
	static char endBorderChar = '<';

	public CharSequence2CharNGrams (int n, boolean distinguishBorders)
	{
		this.n = n;
		this.distinguishBorders = distinguishBorders;
	}

	protected String[] ngramify (CharSequence s)
	{
		if (distinguishBorders)
			s = new StringBuffer().append(startBorderChar).append(s).append(endBorderChar).toString();
		int count = s.length() - n;
		String[] ngrams = new String[count];
		for (int i = 0; i < count; i++)
			ngrams[i] = s.subSequence (i, i+n).toString();
		return ngrams;
	}

	public Instance pipe (Instance carrier)
	{
		if (carrier.getData() instanceof CharSequence)
			carrier.setData(new TokenSequence (ngramify ((CharSequence)carrier.getData())));
		else if (carrier.getData() instanceof TokenSequence) {
			TokenSequence ts = (TokenSequence) carrier.getData();
			TokenSequence ret = new TokenSequence ();
			for (int i = 0; i < ts.size(); i++)
				ret.add (ngramify (ts.get(i).getText()).toString());
			carrier.setData(ret);
		} else
			throw new IllegalArgumentException ("Unhandled type "+carrier.getData().getClass());
		return carrier;
	}

	// Serialization 
	
	private static final long serialVersionUID = 1;
	private static final int CURRENT_SERIAL_VERSION = 0;
	
	private void writeObject (ObjectOutputStream out) throws IOException {
		out.writeInt (CURRENT_SERIAL_VERSION);
		out.writeInt(n);
		out.writeBoolean(distinguishBorders);
	}
	
	private void readObject (ObjectInputStream in) throws IOException, ClassNotFoundException {
		int version = in.readInt ();
		n = in.readInt();
		distinguishBorders = in.readBoolean();
	}

}