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A library for spelling-correction based on Levenshtein Automata.

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package com.github.liblevenshtein.distance;

import com.github.liblevenshtein.collection.SymmetricImmutablePair;

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

  private static final long serialVersionUID = 1L;

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

    int distance = memo.getOrDefault(key, DEFAULT_RETURN_VALUE);
    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 (w.length() > 1) {
      distance = memoizedDistance(x, f(w, 1));

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

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

    if (v.length() > 1) {
      distance = memoizedDistance(f(v, 1), y);

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

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

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