/*
 * 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;

import org.apache.lucene.util.IntsRef;

/**
 * Damerau-Levenshtein (optimal string alignment) implemented in a consistent way as Lucene's
 * FuzzyTermsEnum with the transpositions option enabled.
 *
 * <p>Notes:
 *
 * <ul>
 *   <li>This metric treats full unicode codepoints as characters
 *   <li>This metric scales raw edit distances into a floating point score based upon the shortest
 *       of the two terms
 *   <li>Transpositions of two adjacent codepoints are treated as primitive edits.
 *   <li>Edits are applied in parallel: for example, "ab" and "bca" have distance 3.
 * </ul>
 *
 * NOTE: this class is not particularly efficient. It is only intended for merging results from
 * multiple DirectSpellCheckers.
 */
public final class LuceneLevenshteinDistance implements StringDistance {

  /** Creates a new comparator, mimicing the behavior of Lucene's internal edit distance. */
  public LuceneLevenshteinDistance() {}

  @Override
  public float getDistance(String target, String other) {
    IntsRef targetPoints;
    IntsRef otherPoints;
    int n;
    int[][] d; // cost array

    // NOTE: if we cared, we could 3*m space instead of m*n space, similar to
    // what LevenshteinDistance does, except cycling thru a ring of three
    // horizontal cost arrays... but this comparator is never actually used by
    // DirectSpellChecker, it's only used for merging results from multiple shards
    // in "distributed spellcheck", and it's inefficient in other ways too...

    // cheaper to do this up front once
    targetPoints = toIntsRef(target);
    otherPoints = toIntsRef(other);
    n = targetPoints.length;
    final int m = otherPoints.length;
    d = new int[n + 1][m + 1];

    if (n == 0 || m == 0) {
      if (n == m) {
        return 0;
      } else {
        return Math.max(n, m);
      }
    }

    // indexes into strings s and t
    int i; // iterates through s
    int j; // iterates through t

    int t_j; // jth character of t

    int cost; // cost

    for (i = 0; i <= n; i++) {
      d[i][0] = i;
    }

    for (j = 0; j <= m; j++) {
      d[0][j] = j;
    }

    for (j = 1; j <= m; j++) {
      t_j = otherPoints.ints[j - 1];

      for (i = 1; i <= n; i++) {
        cost = targetPoints.ints[i - 1] == t_j ? 0 : 1;
        // minimum of cell to the left+1, to the top+1, diagonally left and up +cost
        d[i][j] = Math.min(Math.min(d[i - 1][j] + 1, d[i][j - 1] + 1), d[i - 1][j - 1] + cost);
        // transposition
        if (i > 1
            && j > 1
            && targetPoints.ints[i - 1] == otherPoints.ints[j - 2]
            && targetPoints.ints[i - 2] == otherPoints.ints[j - 1]) {
          d[i][j] = Math.min(d[i][j], d[i - 2][j - 2] + cost);
        }
      }
    }

    return 1.0f - ((float) d[n][m] / Math.min(m, n));
  }

  private static IntsRef toIntsRef(String s) {
    IntsRef ref = new IntsRef(s.length()); // worst case
    int utf16Len = s.length();
    for (int i = 0, cp = 0; i < utf16Len; i += Character.charCount(cp)) {
      cp = ref.ints[ref.length++] = Character.codePointAt(s, i);
    }
    return ref;
  }

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

  @Override
  public int hashCode() {
    // constant hashCode since all instances of this class are equal()
    return 6;
  }
}
