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Lucene example source code file (NGramDistance.java)

This example Lucene source code file (NGramDistance.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

ngramdistance, ngramdistance, string, string, stringdistance, stringdistance

The Lucene NGramDistance.java source code

package org.apache.lucene.search.spell;

/**
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements.  See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License.  You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

/**
 * N-Gram version of edit distance based on paper by Grzegorz Kondrak, 
 * "N-gram similarity and distance". Proceedings of the Twelfth International 
 * Conference on String Processing and Information Retrieval (SPIRE 2005), pp. 115-126, 
 * Buenos Aires, Argentina, November 2005. 
 * http://www.cs.ualberta.ca/~kondrak/papers/spire05.pdf
 * 
 * This implementation uses the position-based optimization to compute partial
 * matches of n-gram sub-strings and adds a null-character prefix of size n-1 
 * so that the first character is contained in the same number of n-grams as 
 * a middle character.  Null-character prefix matches are discounted so that 
 * strings with no matching characters will return a distance of 0.
 * 
 */
public class NGramDistance implements StringDistance {

  private int n;
  
  /**
   * Creates an N-Gram distance measure using n-grams of the specified size.
   * @param size The size of the n-gram to be used to compute the string distance.
   */
  public NGramDistance(int size) {
    this.n = size;
  }
  
  /**
   * Creates an N-Gram distance measure using n-grams of size 2.
   */
  public NGramDistance() {
    this(2);
  }
  
  public float getDistance(String source, String target) {
    final int sl = source.length();
    final int tl = target.length();
    
    if (sl == 0 || tl == 0) {
      if (sl == tl) {
        return 1;
      }
      else {
        return 0;
      }
    }

    int cost = 0;
    if (sl < n || tl < n) {
      for (int i=0,ni=Math.min(sl,tl);i<ni;i++) {
        if (source.charAt(i) == target.charAt(i)) {
          cost++;
        }
      }
      return (float) cost/Math.max(sl, tl);
    }

    char[] sa = new char[sl+n-1];
    float p[]; //'previous' cost array, horizontally
    float d[]; // cost array, horizontally
    float _d[]; //placeholder to assist in swapping p and d
    
    //construct sa with prefix
    for (int i=0;i<sa.length;i++) {
      if (i < n-1) {
        sa[i]=0; //add prefix
      }
      else {
        sa[i] = source.charAt(i-n+1);
      }
    }
    p = new float[sl+1]; 
    d = new float[sl+1]; 
  
    // indexes into strings s and t
    int i; // iterates through source
    int j; // iterates through target

    char[] t_j = new char[n]; // jth n-gram of t

    for (i = 0; i<=sl; i++) {
        p[i] = i;
    }

    for (j = 1; j<=tl; j++) {
        //construct t_j n-gram 
        if (j < n) {
          for (int ti=0;ti<n-j;ti++) {
            t_j[ti]=0; //add prefix
          }
          for (int ti=n-j;ti<n;ti++) {
            t_j[ti]=target.charAt(ti-(n-j));
          }
        }
        else {
          t_j = target.substring(j-n, j).toCharArray();
        }
        d[0] = j;
        for (i=1; i<=sl; i++) {
            cost = 0;
            int tn=n;
            //compare sa to t_j
            for (int ni=0;ni<n;ni++) {
              if (sa[i-1+ni] != t_j[ni]) {
                cost++;
              }
              else if (sa[i-1+ni] == 0) { //discount matches on prefix
                tn--;
              }
            }
            float ec = (float) cost/tn;
            // minimum of cell to the left+1, to the top+1, diagonally left and up +cost
            d[i] = Math.min(Math.min(d[i-1]+1, p[i]+1),  p[i-1]+ec);
        }
        // copy current distance counts to 'previous row' distance counts
        _d = p;
        p = d;
        d = _d;
    }

    // our last action in the above loop was to switch d and p, so p now
    // actually has the most recent cost counts
    return 1.0f - (p[sl] / Math.max(tl, sl));
  }

}

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