EditDistance.java

package edu.stanford.nlp.util; 
import edu.stanford.nlp.util.logging.Redwood;

import java.util.Arrays;

/** Find the (Levenshtein) edit distance between two Strings or Character
 *  arrays.
 * By default it allows transposition. 
 *  <br>
 *  This is an object so that you can save on the cost of allocating /
 *  deallocating the large array when possible
 *  @author Dan Klein
 *  @author John Bauer - rewrote using DP instead of memorization
 */
public class EditDistance  {

  /** A logger for this class */
  private static Redwood.RedwoodChannels log = Redwood.channels(EditDistance.class);
  final boolean allowTranspose;

  protected double[][] score = null;

  public EditDistance() {
    allowTranspose = true;
  }

  public EditDistance(boolean allowTranspose) {
    this.allowTranspose = allowTranspose;
  }

  protected void clear(int sourceLength, int targetLength) {
    if (score == null || score.length < sourceLength + 1 || score[0].length < targetLength + 1) {
      score = new double[sourceLength + 1][targetLength + 1];
    }
    for (double[] aScore : score) {
      Arrays.fill(aScore, worst());
    }
  }

  // CONSTRAINT SEMIRING START

  protected double best() {
    return 0.0;
  }

  protected double worst() {
    return Double.POSITIVE_INFINITY;
  }

  protected double unit() {
    return 1.0;
  }

  protected double better(double x, double y) {
    if (x < y) {
      return x;
    }
    return y;
  }

  protected double combine(double x, double y) {
    return x + y;
  }

  // CONSTRAINT SEMIRING END

  // COST FUNCTION BEGIN

  protected double insertCost(Object o) {
    return unit();
  }

  protected double deleteCost(Object o) {
    return unit();
  }

  protected double substituteCost(Object source, Object target) {
    if (source.equals(target)) {
      return best();
    }
    return unit();
  }

  double transposeCost(Object s1, Object s2, Object t1, Object t2) {
    if (s1.equals(t2) && s2.equals(t1)) {
      if (allowTranspose) {
        return unit();
      } else {
        return 2*unit();
      }
    }
    return worst();
  }

  // COST FUNCTION END

  double score(Object[] source, int sPos, Object[] target, int tPos) {
    for (int i = 0; i <= sPos; ++i) {
      for (int j = 0; j <= tPos; ++j) {
        double bscore = score[i][j];
        if (bscore != worst())
          continue;
        if (i == 0 && j == 0) {
          bscore = best();
        } else {
          if (i > 0) {
            bscore = better(bscore, 
                            (combine(score[i - 1][j], 
                                     deleteCost(source[i - 1]))));
          }
          if (j > 0) {
            bscore = better(bscore, 
                            (combine(score[i][j - 1], 
                                     insertCost(target[j - 1]))));
          }
          if (i > 0 && j > 0) {
            bscore = better(bscore, 
                            (combine(score[i - 1][j - 1], 
                                     substituteCost(source[i - 1], 
                                                    target[j - 1]))));
          }
          if (i > 1 && j > 1) {
            bscore = better(bscore, 
                            (combine(score[i - 2][j - 2], 
                                     transposeCost(source[i - 2], source[i - 1], 
                                                   target[j - 2], target[j - 1]))));
          }
        }
        score[i][j] = bscore;
      }
    }
    return score[sPos][tPos];
  }

  public double score(Object[] source, Object[] target) {
    clear(source.length, target.length);
    return score(source, source.length, target, target.length);
  }

  public double score(String sourceStr, String targetStr) {
    if(sourceStr.equals(targetStr))
      return 0;
    Object[] source = Characters.asCharacterArray(sourceStr);
    Object[] target = Characters.asCharacterArray(targetStr);
    clear(source.length, target.length);
    return score(source, source.length, target, target.length);
  }

  public static void main(String[] args) {
    if (args.length >= 2) {
      EditDistance d = new EditDistance();
      System.out.println(d.score(args[0], args[1]));
    } else {
      log.info("usage: java EditDistance str1 str2");
    }
  }

}