ARROW-5917: [Java] Redesign the dictionary encoder

The current dictionary encoder implementation (org.apache.arrow.vector.dictionary.DictionaryEncoder) has heavy performance overhead, which prevents it from being useful in practice:

* There are repeated conversions between Java objects and bytes (e.g. vector.getObject).
* Unnecessary memory copy (the vector data must be copied to the hash table).
* The hash table cannot be reused for encoding multiple vectors (other data structure & results cannot be reused either).
* The output vector should not be created/managed by the encoder (just like in the out-of-place sorter)
* The hash table requires that the hashCode & equals methods be implemented appropriately, but this is not guaranteed.
We plan to implement a new one in the algorithm module, and gradually deprecate the current one.

Closes #4994 from liyafan82/fly_0712_encode and squashes the following commits:

8b699a8 <liyafan82>  Redesign the dictionary encoder

Authored-by: liyafan82 <[email protected]>
Signed-off-by: Micah Kornfield <[email protected]>

java/algorithm/src/main/java/org/apache/arrow/algorithm/dictionary/SearchDictionaryEncoder.java

@@ -0,0 +1,105 @@
+/*
+ * 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 org.apache.arrow.algorithm.search.VectorSearcher;
+import org.apache.arrow.algorithm.sort.VectorValueComparator;
+import org.apache.arrow.vector.BaseIntVector;
+import org.apache.arrow.vector.ValueVector;
+
+/**
+ * Dictionary encoder based on searching.
+ * @param <E> encoded vector type.
+ * @param <D> decoded vector type, which is also the dictionary type.
+ */
+public class SearchDictionaryEncoder<E extends BaseIntVector, D extends ValueVector> {
+
+  /**
+   * The dictionary for encoding/decoding.
+   * It must be sorted.
+   */
+  private final D dictionary;
+
+  /**
+   * The criteria by which the dictionary is sorted.
+   */
+  private final VectorValueComparator<D> comparator;
+
+  /**
+   * A flag indicating if null should be encoded.
+   */
+  private final boolean encodeNull;
+
+  /**
+   * Constructs a dictionary encoder.
+   * @param dictionary the dictionary. It must be in sorted order.
+   * @param comparator the criteria for sorting.
+   */
+  public SearchDictionaryEncoder(D dictionary, VectorValueComparator<D> comparator) {
+    this(dictionary, comparator, false);
+  }
+
+  /**
+   * Constructs a dictionary encoder.
+   * @param dictionary the dictionary. It must be in sorted order.
+   * @param comparator the criteria for sorting.
+   * @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>
+   *     <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.
+   *     </li>
+   */
+  public SearchDictionaryEncoder(D dictionary, VectorValueComparator<D> comparator, boolean encodeNull) {
+    this.dictionary = dictionary;
+    this.comparator = comparator;
+    this.encodeNull = encodeNull;
+  }
+
+  /**
+   * Encodes an input vector by binary search.
+   * So the algorithm takes O(n * log(m)) time, where n is the length of the input vector,
+   * and m is the length of the dictionary.
+   * @param input the input vector.
+   * @param output the output vector. Note that it must be in a fresh state. At least,
+   *     all its validity bits should be clear.
+   */
+  public void encode(D input, E output) {
+    for (int i = 0; i < input.getValueCount(); i++) {
+      if (!encodeNull && input.isNull(i)) {
+        // for this case, we should simply output a null in the output.
+        // by assuming the output vector is fresh, we do nothing here.
+        continue;
+      }
+
+      int index = VectorSearcher.binarySearch(dictionary, comparator, input, i);
+      if (index == -1) {
+        throw new IllegalArgumentException("The data element is not found in the dictionary: " + i);
+      }
+      output.setWithPossibleTruncate(i, index);
+    }
+    output.setValueCount(input.getValueCount());
+  }
+}