feat: implement DataWriter for Iceberg data files

Implements DataWriter class for writing Iceberg data files as part of
issue #441 (task 2).

Implementation:
- Static factory method DataWriter::Make() for creating writer instances
- Support for Parquet and Avro file formats via WriterFactoryRegistry
- Complete DataFile metadata generation including partition info,
  column statistics, serialized bounds, and sort order ID
- Proper lifecycle management with Write/Close/Metadata methods
- Idempotent Close() - multiple calls succeed (no-op after first)
- PIMPL idiom for ABI stability
- Not thread-safe (documented)

Tests:
- 13 comprehensive unit tests including parameterized format tests
- Coverage: creation, write/close lifecycle, metadata generation,
  error handling, feature validation, and data size verification
- All tests passing (13/13)

Related to #441

Author: shangxinli

src/iceberg/data/data_writer.cc

@@ -19,20 +19,127 @@
 
 #include "iceberg/data/data_writer.h"
 
+#include "iceberg/file_writer.h"
+#include "iceberg/manifest/manifest_entry.h"
+#include "iceberg/util/macros.h"
+
 namespace iceberg {
 
 class DataWriter::Impl {
  public:
+  static Result<std::unique_ptr<Impl>> Make(DataWriterOptions options) {
+    WriterOptions writer_options;
+    writer_options.path = options.path;
+    writer_options.schema = options.schema;
+    writer_options.io = options.io;
+    writer_options.properties = WriterProperties::FromMap(options.properties);
+
+    ICEBERG_ASSIGN_OR_RAISE(
+        auto writer, WriterFactoryRegistry::Open(options.format, writer_options));
+
+    return std::unique_ptr<Impl>(
+        new Impl(std::move(options), std::move(writer)));
+  }
+
+  Status Write(ArrowArray* data) {
+    if (!writer_) {
+      return InvalidArgument("Writer not initialized");
+    }
+    return writer_->Write(data);
+  }
+
+  Result<int64_t> Length() const {
+    if (!writer_) {
+      return InvalidArgument("Writer not initialized");
+    }
+    return writer_->length();
+  }
+
+  Status Close() {
+    if (!writer_) {
+      return InvalidArgument("Writer not initialized");
+    }
+    if (closed_) {
+      // Idempotent: no-op if already closed
+      return {};
+    }
+    ICEBERG_RETURN_UNEXPECTED(writer_->Close());
+    closed_ = true;
+    return {};
+  }
+
+  Result<FileWriter::WriteResult> Metadata() {
+    if (!closed_) {
+      return InvalidArgument("Cannot get metadata before closing the writer");
+    }
+
+    ICEBERG_ASSIGN_OR_RAISE(auto metrics, writer_->metrics());
+    ICEBERG_ASSIGN_OR_RAISE(auto length, writer_->length());
+    auto split_offsets = writer_->split_offsets();
+
+    auto data_file = std::make_shared<DataFile>();
+    data_file->content = DataFile::Content::kData;
+    data_file->file_path = options_.path;
+    data_file->file_format = options_.format;
+    data_file->partition = options_.partition;
+    data_file->record_count = metrics.row_count.value_or(0);
+    data_file->file_size_in_bytes = length;
+    data_file->sort_order_id = options_.sort_order_id;
+    data_file->split_offsets = std::move(split_offsets);
+
+    // Convert metrics maps from unordered_map to map
+    for (const auto& [col_id, size] : metrics.column_sizes) {
+      data_file->column_sizes[col_id] = size;
+    }
+    for (const auto& [col_id, count] : metrics.value_counts) {
+      data_file->value_counts[col_id] = count;
+    }
+    for (const auto& [col_id, count] : metrics.null_value_counts) {
+      data_file->null_value_counts[col_id] = count;
+    }
+    for (const auto& [col_id, count] : metrics.nan_value_counts) {
+      data_file->nan_value_counts[col_id] = count;
+    }
+
+    // Serialize literal bounds to binary format
+    for (const auto& [col_id, literal] : metrics.lower_bounds) {
+      ICEBERG_ASSIGN_OR_RAISE(auto serialized, literal.Serialize());
+      data_file->lower_bounds[col_id] = std::move(serialized);
+    }
+    for (const auto& [col_id, literal] : metrics.upper_bounds) {
+      ICEBERG_ASSIGN_OR_RAISE(auto serialized, literal.Serialize());
+      data_file->upper_bounds[col_id] = std::move(serialized);
+    }
+
+    FileWriter::WriteResult result;
+    result.data_files.push_back(std::move(data_file));
+    return result;
+  }
+
+ private:
+  Impl(DataWriterOptions options, std::unique_ptr<Writer> writer)
+      : options_(std::move(options)), writer_(std::move(writer)) {}
+
+  DataWriterOptions options_;
+  std::unique_ptr<Writer> writer_;
+  bool closed_ = false;
 };
 
+DataWriter::DataWriter(std::unique_ptr<Impl> impl) : impl_(std::move(impl)) {}
+
 DataWriter::~DataWriter() = default;
 
-Status DataWriter::Write(ArrowArray* data) { return NotImplemented(""); }
+Result<std::unique_ptr<DataWriter>> DataWriter::Make(const DataWriterOptions& options) {
+  ICEBERG_ASSIGN_OR_RAISE(auto impl, Impl::Make(options));
+  return std::unique_ptr<DataWriter>(new DataWriter(std::move(impl)));
+}
+
+Status DataWriter::Write(ArrowArray* data) { return impl_->Write(data); }
 
-Result<int64_t> DataWriter::Length() const { return NotImplemented(""); }
+Result<int64_t> DataWriter::Length() const { return impl_->Length(); }
 
-Status DataWriter::Close() { return NotImplemented(""); }
+Status DataWriter::Close() { return impl_->Close(); }
 
-Result<FileWriter::WriteResult> DataWriter::Metadata() { return NotImplemented(""); }
+Result<FileWriter::WriteResult> DataWriter::Metadata() { return impl_->Metadata(); }
 
 }  // namespace iceberg

src/iceberg/data/data_writer.h

@@ -51,10 +51,16 @@ struct ICEBERG_EXPORT DataWriterOptions {
 };
 
 /// \brief Writer for Iceberg data files.
+///
+/// This class is not thread-safe. Concurrent calls to Write(), Close(), or Metadata()
+/// from multiple threads may result in undefined behavior.
 class ICEBERG_EXPORT DataWriter : public FileWriter {
  public:
   ~DataWriter() override;
 
+  /// \brief Create a new DataWriter instance.
+  static Result<std::unique_ptr<DataWriter>> Make(const DataWriterOptions& options);
+
   Status Write(ArrowArray* data) override;
   Result<int64_t> Length() const override;
   Status Close() override;
@@ -63,6 +69,8 @@ class ICEBERG_EXPORT DataWriter : public FileWriter {
  private:
   class Impl;
   std::unique_ptr<Impl> impl_;
+
+  explicit DataWriter(std::unique_ptr<Impl> impl);
 };
 
 }  // namespace iceberg