chore: add license

Author: zhjwpku

LICENSE

@@ -253,3 +253,17 @@ https://github.com/apache/arrow/blob/main/cpp/src/arrow/visit_type_inline.h
 Copyright: 2016-2025 The Apache Software Foundation.
 Home page: https://arrow.apache.org/
 License: https://www.apache.org/licenses/LICENSE-2.0
+
+--------------------------------------------------------------------------------
+
+The file src/iceberg/expression/decimal.h contains code adapted from
+
+https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/decimal.h
+
+The file src/iceberg/expression/decimal.cc contains code adapted from
+
+https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/decimal.cc
+
+Copyright: 2016-2025 The Apache Software Foundation.
+Home page: https://arrow.apache.org/
+License: https://www.apache.org/licenses/LICENSE-2.0

src/iceberg/expression/decimal.cc

@@ -17,6 +17,11 @@
  * under the License.
  */
 
+/// \file iceberg/expression/decimal.cc
+/// \brief 128-bit fixed-point decimal numbers.
+/// Adapted from Apache Arrow with only Decimal128 support.
+/// https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/decimal.cc
+
 #include "iceberg/expression/decimal.h"
 
 #include <algorithm>

src/iceberg/expression/decimal.h

@@ -19,6 +19,11 @@
 
 #pragma once
 
+/// \file iceberg/expression/decimal.h
+/// \brief 128-bit fixed-point decimal numbers.
+/// Adapted from Apache Arrow with only Decimal128 support.
+/// https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/decimal.h
+
 #include <array>
 #include <cstdint>
 #include <iosfwd>