package org.apache.lucene.search.spell;

/*
 * 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.
 */

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.
 *  
 *  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, its only used for merging results from multiple shards 
    // in "distributed spellcheck", and its 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;
  }
}
