apacheGH-3530: Optimize DICTIONARY encoding/decoding data structures and use ByteBuffer

Encoding improvements:
- Replace LinkedOpenHashMap with OpenHashMap + ArrayList for all
  DictionaryValuesWriter subclasses, eliminating insertion-order
  overhead and enabling O(1) indexed access for dictionary page
  serialization and fallback
- Make IntList.size() O(1) by tracking totalSize incrementally
  instead of summing across slab arrays

Decoding improvements:
- Convert PlainValuesDictionary numeric constructors (INT32, INT64,
  FLOAT, DOUBLE) from InputStream-based per-byte reads to direct
  ByteBuffer.getInt/getLong/getFloat/getDouble (JVM intrinsics)

Binary hashCode caching:
- Cache hashCode() for Binary instances that are not backed by reusable
  byte arrays, avoiding redundant recomputation during dictionary
  lookups (hash map probes)

JMH benchmarks:
- DictionaryEncodingBenchmark: scalar encoding for INT32, INT64, FLOAT,
  DOUBLE, BINARY, and FIXED_LEN_BYTE_ARRAY with LOW/HIGH cardinality
  and variable-length string/FLBA dimensions
- DictionaryDecodingBenchmark: scalar decoding for all types with
  matching parameterization
- TestDataFactory: shared data generation utility for reproducible
  benchmark inputs
- BenchmarkEncodingUtils: helper to drain DictionaryValuesWriter into
  encoded dictionary page + data bytes for decoder setup

Author: iemejia

parquet-benchmarks/src/main/java/org/apache/parquet/benchmarks/BenchmarkEncodingUtils.java

@@ -0,0 +1,70 @@
+/*
+ * 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.parquet.benchmarks;
+
+import java.io.IOException;
+import org.apache.parquet.column.page.DictionaryPage;
+import org.apache.parquet.column.values.dictionary.DictionaryValuesWriter;
+
+/**
+ * Shared helpers for encode/decode micro-benchmarks.
+ */
+final class BenchmarkEncodingUtils {
+
+  private BenchmarkEncodingUtils() {}
+
+  /**
+   * Container for the two artefacts produced by a dictionary-encoded page:
+   * the encoded dictionary indices ({@link #dictData}) and the dictionary
+   * page itself ({@link #dictPage}). The dictionary page may be {@code null}
+   * if the writer fell back to plain encoding (for example, when the
+   * dictionary exceeded its configured maximum size).
+   */
+  static final class EncodedDictionary {
+    final byte[] dictData;
+    final DictionaryPage dictPage;
+
+    EncodedDictionary(byte[] dictData, DictionaryPage dictPage) {
+      this.dictData = dictData;
+      this.dictPage = dictPage;
+    }
+
+    boolean fellBackToPlain() {
+      return dictPage == null;
+    }
+  }
+
+  /**
+   * Drains a {@link DictionaryValuesWriter} into an {@link EncodedDictionary}.
+   *
+   * <p>The writer's data bytes (the RLE-encoded indices) and the dictionary
+   * page are returned separately so both pieces can be measured or fed to a
+   * decoder symmetrically. The dictionary page buffer is copied so it remains
+   * valid after the writer's allocator is released.
+   *
+   * <p>The writer is closed via {@code toDictPageAndClose()}; callers must not
+   * call {@link DictionaryValuesWriter#close()} again afterwards.
+   */
+  static EncodedDictionary drainDictionary(DictionaryValuesWriter writer) throws IOException {
+    byte[] dictData = writer.getBytes().toByteArray();
+    DictionaryPage rawPage = writer.toDictPageAndClose();
+    DictionaryPage dictPage = rawPage == null ? null : rawPage.copy();
+    return new EncodedDictionary(dictData, dictPage);
+  }
+}