/**
 * 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.function;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.FieldCache;

import java.io.IOException;

/**
 * Expert: obtains the ordinal of the field value from the default Lucene 
 * {@link org.apache.lucene.search.FieldCache FieldCache} using getStringIndex()
 * and reverses the order.
 * <p>
 * The native lucene index order is used to assign an ordinal value for each field value.
 * <p>
 * Field values (terms) are lexicographically ordered by unicode value, and numbered starting at 1.
 * <br>
 * Example of reverse ordinal (rord):
 * <br>If there were only three field values: "apple","banana","pear"
 * <br>then rord("apple")=3, rord("banana")=2, ord("pear")=1
 * <p>
 * WARNING: 
 * rord() depends on the position in an index and can thus change 
 * when other documents are inserted or deleted,
 * or if a MultiSearcher is used. 
 * 
 * @lucene.experimental
 *
 * <p><b>NOTE</b>: with the switch in 2.9 to segment-based
 * searching, if {@link #getValues} is invoked with a
 * composite (multi-segment) reader, this can easily cause
 * double RAM usage for the values in the FieldCache.  It's
 * best to switch your application to pass only atomic
 * (single segment) readers to this API.</p>
 */

public class ReverseOrdFieldSource extends ValueSource {
  public String field;

  /** 
   * Contructor for a certain field.
   * @param field field whose values reverse order is used.  
   */
  public ReverseOrdFieldSource(String field) {
    this.field = field;
  }

  /*(non-Javadoc) @see org.apache.lucene.search.function.ValueSource#description() */
  @Override
  public String description() {
    return "rord("+field+')';
  }

  /*(non-Javadoc) @see org.apache.lucene.search.function.ValueSource#getValues(org.apache.lucene.index.IndexReader) */
  @Override
  public DocValues getValues(IndexReader reader) throws IOException {
    final FieldCache.StringIndex sindex = FieldCache.DEFAULT.getStringIndex(reader, field);

    final int arr[] = sindex.order;
    final int end = sindex.lookup.length;

    return new DocValues() {
      /*(non-Javadoc) @see org.apache.lucene.search.function.DocValues#floatVal(int) */
      @Override
      public float floatVal(int doc) {
        return (end - arr[doc]);
      }
      /* (non-Javadoc) @see org.apache.lucene.search.function.DocValues#intVal(int) */
      @Override
      public int intVal(int doc) {
        return end - arr[doc];
      }
      /* (non-Javadoc) @see org.apache.lucene.search.function.DocValues#strVal(int) */
      @Override
      public String strVal(int doc) {
        // the string value of the ordinal, not the string itself
        return Integer.toString(intVal(doc));
      }
      /*(non-Javadoc) @see org.apache.lucene.search.function.DocValues#toString(int) */
      @Override
      public String toString(int doc) {
        return description() + '=' + strVal(doc);
      }
      /*(non-Javadoc) @see org.apache.lucene.search.function.DocValues#getInnerArray() */
      @Override
      Object getInnerArray() {
        return arr;
      }
    };
  }

  /*(non-Javadoc) @see java.lang.Object#equals(java.lang.Object) */
  @Override
  public boolean equals(Object o) {
    if (o == this) return true;
    if (o == null) return false;
    if (o.getClass() != ReverseOrdFieldSource.class) return false;
    ReverseOrdFieldSource other = (ReverseOrdFieldSource)o;
    return this.field.equals(other.field); 
  }

  private static final int hcode = ReverseOrdFieldSource.class.hashCode();
  
  /*(non-Javadoc) @see java.lang.Object#hashCode() */
  @Override
  public int hashCode() {
    return hcode + field.hashCode();
  }
}
