package edu.northwestern.at.utils.corpuslinguistics.ngram;
/* Please see the license information at the end of this file. */
import java.io.*;
import java.util.*;
import java.net.*;
import edu.northwestern.at.utils.*;
/** Extract ngrams from text.
*/
public class NGramExtractor
{
/** Number of words forming an ngram. */
int nGramSize = 2;
/** Window size within which to search for ngrams. */
int windowSize = 2;
/** The list of ngrams and associated counts.
*
* <p>
* Key=ngram string<br />
* Value=Integer(count)
* </p>
*
* <p>
* The ngram string is two or more words with
* a tab character ("\t") separating the words.
* </p>
*/
protected Map<String,Integer> nGramCounts =
MapFactory.createNewMap();
/** Total number of ngrams. */
protected int numberOfNGrams = 0;
/** Create NGrams.
*
* @param nGramSize The number of words forming an ngram.
* @param windowSize The window size (number of words)
* within which to construct ngrams.
*
* <ul>
* <li>windowSize must be greater than or equal to windowSize.</li>
* <li>if windowSize is the same as nGramSize,
* all ngrams are comprised of adjacent words.
* <li>if windowSize is greater than nGramSize, all non-adjacent
* word sets of length nGramSize are extracted from each
* set of windowSize words.
* </ul>
*
* <p>
* Example: nGramSize=2, windowSize=3, text="a quick brown fox".
* </p>
*
* <p>
* The first window is "a quick brown".
* The ngrams are "a quick", "a brown", and "quick brown".
* </p>
*
* <p>
* The second window is "quick brown fox."
* The ngrams are "quick brown", "quick fox", and "brown fox".
* </p>
*/
public NGramExtractor
(
int nGramSize ,
int windowSize
)
{
this.nGramSize = nGramSize;
this.windowSize = windowSize;
}
/** Add words from string array of words.
*
* @param words The string array with the words.
*/
public void addWords( String[] words )
{
// Generate ngrams.
addWords( Arrays.asList( words ) );
}
/** Add words from list words.
*
* @param wordList The list with the words.
*/
public void addWords( List<String> wordList )
{
// Generate the ngrams and
// compute the count of each.
for ( int i = nGramSize - 1 ;
i < wordList.size() ;
i++
)
{
StringBuffer sb = new StringBuffer();
for ( int j = ( nGramSize - 1 ) ; j >= 0 ; j-- )
{
if ( sb.length() > 0 )
{
sb = sb.append( "\t" );
}
sb = sb.append( (String)wordList.get( i - j ) );
}
String nGramString = sb.toString();
if ( nGramCounts.containsKey( nGramString ) )
{
int freq =
((Integer)nGramCounts.get( nGramString )).intValue();
freq++;
nGramCounts.put( nGramString , new Integer( freq ) );
}
else
{
nGramCounts.put( nGramString , new Integer( 1 ) );
}
}
}
/** Merge ngrams from another NGramExtractor.
*
* @param extractor Merge ngrams from another extractor.
*/
public void mergeNGramExtractor( NGramExtractor extractor )
{
Map<String, Integer> otherMap = extractor.getNGramMap();
for ( String nGramString : otherMap.keySet() )
{
if ( nGramCounts.containsKey( nGramString ) )
{
int freq =
((Integer)nGramCounts.get( nGramString )).intValue();
freq++;
nGramCounts.put( nGramString , new Integer( freq ) );
}
else
{
nGramCounts.put( nGramString , new Integer( 1 ) );
}
}
// Compute total ngram count.
numberOfNGrams = 0;
for ( String nGramString : otherMap.keySet() )
{
Integer count = nGramCounts.get( nGramString );
numberOfNGrams += count.intValue();
}
}
/** Return count for a specific ngram.
*
* @param ngram The ngram whose count is desired.
*
* @return The count of the ngram in the text.
*/
public int getNGramCount( String ngram )
{
int result = 0;
if ( nGramCounts.containsKey( ngram ) )
{
Integer count = nGramCounts.get( ngram );
result = count.intValue();
}
return result;
}
/** Return NGrams.
*
* @return String array of ngrams.
*/
public String[] getNGrams()
{
int nNGrams = nGramCounts.size();
String[] nGrams = new String[ nNGrams ];
Set<String> keyset = nGramCounts.keySet();
Iterator<String> iterator = keyset.iterator();
for ( int i = 0 ; i < nNGrams ; i++ )
{
nGrams[ i ] = iterator.next();
}
return nGrams;
}
/** Return NGram map.
*
* @return NGram map.
*/
public Map<String, Integer> getNGramMap()
{
return nGramCounts;
}
/** Returns the total number of ngrams.
*
* @return The total number of ngrams.
*/
public int getNumberOfNGrams()
{
// Compute total ngram count.
numberOfNGrams = 0;
for ( String nGramString : nGramCounts.keySet() )
{
Integer count = nGramCounts.get( nGramString );
numberOfNGrams += count.intValue();
}
return numberOfNGrams;
}
/** Returns the number of unique ngrams.
*
* @return The number of unique ngrams.
*/
public int getNumberOfUniqueNGrams()
{
return nGramCounts.size();
}
/** Returns the individual words comprising an ngram.
*
* @param ngram The ngram to parse.
*
* @return String array of the individual words
* (in order) comprising the ngram.
*/
public static String[] splitNGramIntoWords( String ngram )
{
return StringUtils.makeTokenArray( ngram , "\t" );
}
}
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