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 *     http://www.apache.org/licenses/LICENSE-2.0
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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
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.
/**
 * 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.
Params:
size – The size of the n-gram to be used to compute the string distance./**
   * 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.
/**
   * 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 + ")";
  }
}