GH-3201: Implement a Variant builder to create Variant values (#3202)

Author: gene-db

parquet-variant/src/main/java/org/apache/parquet/variant/VariantArrayBuilder.java

@@ -0,0 +1,68 @@
+/*
+ * 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.variant;
+
+import java.util.ArrayList;
+
+/**
+ * Builder for creating Variant arrays, used by VariantBuilder.
+ */
+public class VariantArrayBuilder extends VariantBuilder {
+  /** The parent VariantBuilder. */
+  private final VariantBuilder parent;
+  /** The offsets of the elements in this array. */
+  private final ArrayList<Integer> offsets;
+  /** The number of values appended to this array. */
+  protected long numValues = 0;
+
+  VariantArrayBuilder(VariantBuilder parent) {
+    this.parent = parent;
+    this.offsets = new ArrayList<>();
+  }
+
+  /**
+   * @return the list of element offsets in this array
+   */
+  ArrayList<Integer> validateAndGetOffsets() {
+    if (offsets.size() != numValues) {
+      throw new IllegalStateException(String.format(
+          "Number of offsets (%d) do not match the number of values (%d).", offsets.size(), numValues));
+    }
+    checkMultipleNested("Cannot call endArray() while a nested object/array is still open.");
+    return offsets;
+  }
+
+  @Override
+  protected void onAppend() {
+    checkAppendWhileNested();
+    offsets.add(writePos);
+    numValues++;
+  }
+
+  @Override
+  protected void onStartNested() {
+    checkMultipleNested("Cannot call startObject()/startArray() without calling endObject()/endArray() first.");
+    offsets.add(writePos);
+    numValues++;
+  }
+
+  @Override
+  int addDictionaryKey(String key) {
+    // Add to the parent dictionary.
+    return parent.addDictionaryKey(key);
+  }
+}