cpp/src/arrow/array/builder_union.h

// Licensed to the Apache Software Foundation (ASF) under one
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// 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
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// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

#pragma once

#include <memory>
#include <string>
#include <vector>

#include "arrow/array.h"
#include "arrow/array/builder_base.h"
#include "arrow/buffer-builder.h"

namespace arrow {

/// \class DenseUnionBuilder
///
/// You need to call AppendChild for each of the children builders you want
/// to use. The function will return an int8_t, which is the type tag
/// associated with that child. You can then call Append with that tag
/// (followed by an append on the child builder) to add elements to
/// the union array.
///
/// You can either specify the type when the UnionBuilder is constructed
/// or let the UnionBuilder infer the type at runtime (by omitting the
/// type argument from the constructor).
///
/// This API is EXPERIMENTAL.
class ARROW_EXPORT DenseUnionBuilder : public ArrayBuilder {
 public:
  /// Use this constructor to incrementally build the union array along
  /// with types, offsets, and null bitmap.
  explicit DenseUnionBuilder(MemoryPool* pool,
                             const std::shared_ptr<DataType>& type = NULLPTR);

  Status AppendNull() final {
    ARROW_RETURN_NOT_OK(types_builder_.Append(0));
    ARROW_RETURN_NOT_OK(offsets_builder_.Append(0));
    return AppendToBitmap(false);
  }

  Status AppendNulls(int64_t length) final {
    ARROW_RETURN_NOT_OK(types_builder_.Reserve(length));
    ARROW_RETURN_NOT_OK(offsets_builder_.Reserve(length));
    ARROW_RETURN_NOT_OK(Reserve(length));
    for (int64_t i = 0; i < length; ++i) {
      types_builder_.UnsafeAppend(0);
      offsets_builder_.UnsafeAppend(0);
    }
    return AppendToBitmap(length, false);
  }

  /// \brief Append an element to the UnionArray. This must be followed
  ///        by an append to the appropriate child builder.
  /// \param[in] type index of the child the value will be appended
  /// \param[in] offset offset of the value in that child
  Status Append(int8_t type, int32_t offset) {
    ARROW_RETURN_NOT_OK(types_builder_.Append(type));
    ARROW_RETURN_NOT_OK(offsets_builder_.Append(offset));
    return AppendToBitmap(true);
  }

  Status FinishInternal(std::shared_ptr<ArrayData>* out) override;

  /// \cond FALSE
  using ArrayBuilder::Finish;
  /// \endcond

  Status Finish(std::shared_ptr<UnionArray>* out) { return FinishTyped(out); }

  /// \brief Make a new child builder available to the UnionArray
  ///
  /// \param[in] child the child builder
  /// \param[in] field_name the name of the field in the union array type
  /// if type inference is used
  /// \return child index, which is the "type" argument that needs
  /// to be passed to the "Append" method to add a new element to
  /// the union array.
  int8_t AppendChild(const std::shared_ptr<ArrayBuilder>& child,
                     const std::string& field_name = "") {
    children_.push_back(child);
    field_names_.push_back(field_name);
    return static_cast<int8_t>(children_.size() - 1);
  }

 private:
  TypedBufferBuilder<int8_t> types_builder_;
  TypedBufferBuilder<int32_t> offsets_builder_;
  std::vector<std::string> field_names_;
};

}  // namespace arrow