content: /*
   * 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.arrow.algorithm.dictionary;
  
  import java.util.HashMap;
  import org.apache.arrow.memory.util.ArrowBufPointer;
  import org.apache.arrow.memory.util.hash.ArrowBufHasher;
  import org.apache.arrow.memory.util.hash.SimpleHasher;
  import org.apache.arrow.vector.BaseIntVector;
  import org.apache.arrow.vector.ElementAddressableVector;
  
  /**
   * Dictionary encoder based on hash table.
   *
   * @param <E> encoded vector type.
   * @param <D> decoded vector type, which is also the dictionary type.
   */
  public class HashTableDictionaryEncoder<E extends BaseIntVector, D extends ElementAddressableVector>
      implements DictionaryEncoder<E, D> {
  
    /** The dictionary for encoding/decoding. It must be sorted. */
    private final D dictionary;
  
    /** The hasher used to compute the hash code. */
    private final ArrowBufHasher hasher;
  
    /** A flag indicating if null should be encoded. */
    private final boolean encodeNull;
  
    /**
     * The hash map for distinct dictionary entries. The key is the pointer to the dictionary element,
     * whereas the value is the index in the dictionary.
     */
    private HashMap<ArrowBufPointer, Integer> hashMap = new HashMap<>();
  
    /** The pointer used to probe each element to encode. */
    private ArrowBufPointer reusablePointer;
  
    /**
     * Constructs a dictionary encoder.
     *
     * @param dictionary the dictionary.
     */
    public HashTableDictionaryEncoder(D dictionary) {
      this(dictionary, false);
    }
  
    /**
     * Constructs a dictionary encoder.
     *
     * @param dictionary the dictionary.
     * @param encodeNull a flag indicating if null should be encoded. It determines the behaviors for
     *     processing null values in the input during encoding/decoding.
     *     <li>For encoding, when a null is encountered in the input, 1) If the flag is set to true,
     *         the encoder searches for the value in the dictionary, and outputs the index in the
     *         dictionary. 2) If the flag is set to false, the encoder simply produces a null in the
     *         output.
     *     <li>For decoding, when a null is encountered in the input, 1) If the flag is set to true,
     *         the decoder should never expect a null in the input. 2) If set to false, the decoder
     *         simply produces a null in the output.
     */
    public HashTableDictionaryEncoder(D dictionary, boolean encodeNull) {
      this(dictionary, encodeNull, SimpleHasher.INSTANCE);
    }
  
    /**
     * Constructs a dictionary encoder.
     *
     * @param dictionary the dictionary.
     * @param encodeNull a flag indicating if null should be encoded. It determines the behaviors for
     *     processing null values in the input during encoding. When a null is encountered in the
     *     input, 1) If the flag is set to true, the encoder searches for the value in the dictionary,
     *     and outputs the index in the dictionary. 2) If the flag is set to false, the encoder simply
     *     produces a null in the output.
     * @param hasher the hasher used to calculate the hash code.
     */
    public HashTableDictionaryEncoder(D dictionary, boolean encodeNull, ArrowBufHasher hasher) {
      this.dictionary = dictionary;
      this.hasher = hasher;
      this.encodeNull = encodeNull;
  
      reusablePointer = new ArrowBufPointer(hasher);
  
      buildHashMap();
    }
  
    private void buildHashMap() {
      for (int i = 0; i < dictionary.getValueCount(); i++) {
        ArrowBufPointer pointer = new ArrowBufPointer(hasher);
        dictionary.getDataPointer(i, pointer);
        hashMap.put(pointer, i);
      }
    }
  
    /**
     * Encodes an input vector by a hash table. So the algorithm takes O(n) time, where n is the
     * length of the input vector.
     *
     * @param input the input vector.
     * @param output the output vector.
     */
    @Override
    public void encode(D input, E output) {
      for (int i = 0; i < input.getValueCount(); i++) {
        if (!encodeNull && input.isNull(i)) {
          continue;
        }
  
        input.getDataPointer(i, reusablePointer);
        Integer index = hashMap.get(reusablePointer);
  
        if (index == null) {
          throw new IllegalArgumentException("The data element is not found in the dictionary");
        }
        output.setWithPossibleTruncate(i, index);
      }
      output.setValueCount(input.getValueCount());
    }
  }
  