lucene/suggest/src/java/org/apache/lucene/search/spell/NGramDistance.java

/*
 * 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.
 */
package org.apache.lucene.search.spell;

/**
 * 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
 *
 * <p>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);
  }

  @Override
  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));
  }

  @Override
  public int hashCode() {
    return 1427 * n * getClass().hashCode();
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) return true;
    if (null == obj || getClass() != obj.getClass()) return false;

    NGramDistance o = (NGramDistance) obj;
    return o.n == this.n;
  }

  @Override
  public String toString() {
    return "ngram(" + n + ")";
  }
}