MemoizedTransposition.java

package com.github.liblevenshtein.distance;

import lombok.val;

import com.github.liblevenshtein.collection.SymmetricImmutablePair;

/**
 * Memoizes the distance between two terms, where the distance is calculated
 * using the standard Levenshtein distance extended with transposition.
 * @author Dylon Edwards
 * @since 2.1.0
 */
public class MemoizedTransposition extends AbstractMemoized {

  private static final long serialVersionUID = 1L;

  /**
   * {@inheritDoc}
   */
  @Override
  @SuppressWarnings("checkstyle:finalparameters")
  public int memoizedDistance(String v, String w) {
    val key = new SymmetricImmutablePair<String>(v, w);

    int distance = memo.getInt(key);
    if (distance != DEFAULT_RETURN_VALUE) {
      return distance;
    }

    if (v.isEmpty()) {
      distance = w.length();
      memo.put(key, distance);
      return distance;
    }

    if (w.isEmpty()) {
      distance = v.length();
      memo.put(key, distance);
      return distance;
    }

    char a = v.charAt(0); String x = v.substring(1);
    char b = w.charAt(0); String y = w.substring(1);

    // Discard identical characters
    while (a == b && x.length() > 0 && y.length() > 0) {
      a = x.charAt(0); v = x; x = v.substring(1);
      b = y.charAt(0); w = y; y = w.substring(1);
    }

    // x.length() == 0 or y.length() == 0
    if (a == b) {
      distance = x.isEmpty() ? y.length() : x.length();
      memo.put(key, distance);
      return distance;
    }

    distance = memoizedDistance(x, w);
    if (0 == distance) {
      memo.put(key, 1);
      return 1;
    }

    int minDistance = distance;

    distance = memoizedDistance(v, y);
    if (0 == distance) {
      memo.put(key, 1);
      return 1;
    }

    if (distance < minDistance) {
      minDistance = distance;
    }

    distance = memoizedDistance(x, y);
    if (0 == distance) {
      memo.put(key, 1);
      return 1;
    }

    if (distance < minDistance) {
      minDistance = distance;
    }

    if (x.length() > 0 && y.length() > 0) {
      final char a1 = x.charAt(0);  // prefix-character of x
      final char b1 = y.charAt(0);  // prefix-character of y
      if (a == b1 && a1 == b) {
        distance = memoizedDistance(f(v, 1), f(w, 1));
        if (0 == distance) {
          memo.put(key, 1);
          return 1;
        }

        if (distance < minDistance) {
          minDistance = distance;
        }
      }
    }

    distance = 1 + minDistance;
    memo.put(key, distance);
    return distance;
  }
}