GH-46860: [C++] Making HalfFloatBuilder accept Float16 as well as uint16_t (#46981)

### Rationale for this change

#46860 Adding convenience methods for appending and retrieving Float16 to HalfFloatBuilder.

### What changes are included in this PR?

HalfFloatBuilder has functions overloaded to accept Float16, tests, and documentation.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #46860

Lead-authored-by: Eric Dinse <293818+dinse@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: dinse

cpp/src/arrow/array/array_test.cc

@@ -38,6 +38,7 @@
 #include "arrow/array/builder_binary.h"
 #include "arrow/array/builder_decimal.h"
 #include "arrow/array/builder_dict.h"
+#include "arrow/array/builder_primitive.h"
 #include "arrow/array/builder_run_end.h"
 #include "arrow/array/builder_time.h"
 #include "arrow/array/data.h"
@@ -60,6 +61,7 @@
 #include "arrow/util/bitmap_builders.h"
 #include "arrow/util/checked_cast.h"
 #include "arrow/util/decimal.h"
+#include "arrow/util/float16.h"
 #include "arrow/util/key_value_metadata.h"
 #include "arrow/util/macros.h"
 #include "arrow/util/range.h"
@@ -72,6 +74,7 @@ namespace arrow {
 
 using internal::checked_cast;
 using internal::checked_pointer_cast;
+using util::Float16;
 
 class TestArray : public ::testing::Test {
  public:
@@ -4099,4 +4102,73 @@ TYPED_TEST(TestPrimitiveArray, IndexOperator) {
   }
 }
 
+class TestHalfFloatBuilder : public ::testing::Test {
+ public:
+  void VerifyValue(const HalfFloatBuilder& builder, int64_t index, float expected) {
+    ASSERT_EQ(builder.GetValue(index), Float16(expected).bits());
+    ASSERT_EQ(builder.GetValue<Float16>(index), Float16(expected));
+    ASSERT_EQ(builder.GetValue<uint16_t>(index), Float16(expected).bits());
+    ASSERT_EQ(builder[index], Float16(expected).bits());
+  }
+};
+
+TEST_F(TestHalfFloatBuilder, TestAppend) {
+  HalfFloatBuilder builder;
+  ASSERT_OK(builder.Append(Float16(0.0f)));
+  ASSERT_OK(builder.Append(Float16(1.0f).bits()));
+  ASSERT_OK(builder.AppendNull());
+  ASSERT_OK(builder.Reserve(3));
+  builder.UnsafeAppend(Float16(3.0f));
+  builder.UnsafeAppend(Float16(4.0f).bits());
+  builder.UnsafeAppend(uint16_t{15872});  // 1.5f
+
+  VerifyValue(builder, 0, 0.0f);
+  VerifyValue(builder, 1, 1.0f);
+  VerifyValue(builder, 3, 3.0f);
+  VerifyValue(builder, 4, 4.0f);
+  VerifyValue(builder, 5, 1.5f);
+}
+
+TEST_F(TestHalfFloatBuilder, TestBulkAppend) {
+  HalfFloatBuilder builder;
+
+  ASSERT_OK(builder.AppendValues(5, Float16(1.5)));
+  uint16_t val = Float16(2.0f).bits();
+  ASSERT_OK(builder.AppendValues({val, val, val, val}, {0, 1, 0, 1}));
+  ASSERT_EQ(builder.length(), 9);
+  for (int i = 0; i < 5; i++) {
+    VerifyValue(builder, i, 1.5f);
+  }
+
+  {
+    ASSERT_OK_AND_ASSIGN(auto array, builder.Finish());
+    ASSERT_OK(array->ValidateFull());
+    ASSERT_EQ(array->null_count(), 2);
+    ASSERT_EQ(array->length(), 9);
+    auto comp = ArrayFromJSON(float16(), "[1.5,1.5,1.5,1.5,1.5,null,2,null,2]");
+    AssertArraysEqual(*array, *comp);
+  }
+
+  std::vector<Float16> vals = {Float16(1.0f), Float16(2.0f), Float16(3.0f)};
+  std::vector<bool> is_valid = {true, false, true};
+  std::vector<uint8_t> valid_bytes = {1, 0, 1};
+  std::vector<uint8_t> bitmap = {0b00000101};
+  ASSERT_OK(builder.AppendValues(vals));
+  ASSERT_OK(builder.AppendValues(vals, is_valid));
+  ASSERT_OK(builder.AppendValues(vals.data(), vals.size(), is_valid));
+  ASSERT_OK(builder.AppendValues(vals.data(), vals.size()));
+  ASSERT_OK(builder.AppendValues(vals.data(), vals.size(), valid_bytes.data()));
+  ASSERT_OK(builder.AppendValues(vals.data(), vals.size(), bitmap.data(), 0));
+
+  {
+    ASSERT_OK_AND_ASSIGN(auto array, builder.Finish());
+    ASSERT_OK(array->ValidateFull());
+    ASSERT_EQ(array->null_count(), 4);
+    ASSERT_EQ(array->length(), 18);
+    auto comp =
+        ArrayFromJSON(float16(), "[1,2,3,1,null,3,1,null,3,1,2,3,1,null,3,1,null,3]");
+    AssertArraysEqual(*array, *comp);
+  }
+}
+
 }  // namespace arrow

cpp/src/arrow/array/builder_primitive.h

@@ -26,6 +26,7 @@
 #include "arrow/result.h"
 #include "arrow/type.h"
 #include "arrow/type_traits.h"
+#include "arrow/util/float16.h"
 
 namespace arrow {
 
@@ -364,7 +365,6 @@ using Int16Builder = NumericBuilder<Int16Type>;
 using Int32Builder = NumericBuilder<Int32Type>;
 using Int64Builder = NumericBuilder<Int64Type>;
 
-using HalfFloatBuilder = NumericBuilder<HalfFloatType>;
 using FloatBuilder = NumericBuilder<FloatType>;
 using DoubleBuilder = NumericBuilder<DoubleType>;
 
@@ -384,6 +384,107 @@ using DurationBuilder = NumericBuilder<DurationType>;
 
 /// @}
 
+/// \addtogroup numeric-builders
+///
+/// @{
+
+class ARROW_EXPORT HalfFloatBuilder : public NumericBuilder<HalfFloatType> {
+ public:
+  using BaseClass = NumericBuilder<HalfFloatType>;
+  using Float16 = arrow::util::Float16;
+
+  using BaseClass::Append;
+  using BaseClass::AppendValues;
+  using BaseClass::BaseClass;
+  using BaseClass::GetValue;
+  using BaseClass::UnsafeAppend;
+
+  /// Scalar append a arrow::util::Float16
+  Status Append(const Float16 val) { return Append(val.bits()); }
+
+  /// Scalar append a arrow::util::Float16, without checking for capacity
+  void UnsafeAppend(const Float16 val) { UnsafeAppend(val.bits()); }
+
+  /// \brief Append a sequence of elements in one shot
+  /// \param[in] values a contiguous array of arrow::util::Float16
+  /// \param[in] length the number of values to append
+  /// \param[in] valid_bytes an optional sequence of bytes where non-zero
+  /// indicates a valid (non-null) value
+  /// \return Status
+  Status AppendValues(const Float16* values, int64_t length,
+                      const uint8_t* valid_bytes = NULLPTR) {
+    return BaseClass::AppendValues(reinterpret_cast<const uint16_t*>(values), length,
+                                   valid_bytes);
+  }
+
+  /// \brief Append a sequence of elements in one shot
+  /// \param[in] values a contiguous array of arrow::util::Float16
+  /// \param[in] length the number of values to append
+  /// \param[in] bitmap a validity bitmap to copy (may be null)
+  /// \param[in] bitmap_offset an offset into the validity bitmap
+  /// \return Status
+  Status AppendValues(const Float16* values, int64_t length, const uint8_t* bitmap,
+                      int64_t bitmap_offset) {
+    return BaseClass::AppendValues(reinterpret_cast<const uint16_t*>(values), length,
+                                   bitmap, bitmap_offset);
+  }
+
+  /// \brief Append a sequence of elements in one shot
+  /// \param[in] values a contiguous array of arrow::util::Float16
+  /// \param[in] length the number of values to append
+  /// \param[in] is_valid a std::vector<bool> indicating valid (1) or null
+  /// (0). Equal in length to values
+  /// \return Status
+  Status AppendValues(const Float16* values, int64_t length,
+                      const std::vector<bool>& is_valid) {
+    return BaseClass::AppendValues(reinterpret_cast<const uint16_t*>(values), length,
+                                   is_valid);
+  }
+
+  /// \brief Append a sequence of elements in one shot
+  /// \param[in] values a std::vector<arrow::util::Float16>
+  /// \param[in] is_valid a std::vector<bool> indicating valid (1) or null
+  /// (0). Equal in length to values
+  /// \return Status
+  Status AppendValues(const std::vector<Float16>& values,
+                      const std::vector<bool>& is_valid) {
+    return AppendValues(values.data(), static_cast<int64_t>(values.size()), is_valid);
+  }
+
+  /// \brief Append a sequence of elements in one shot
+  /// \param[in] values a std::vector<arrow::util::Float16>
+  /// \return Status
+  Status AppendValues(const std::vector<Float16>& values) {
+    return AppendValues(values.data(), static_cast<int64_t>(values.size()));
+  }
+
+  /// \brief Append one value many times in one shot
+  /// \param[in] length the number of values to append
+  /// \param[in] value a arrow::util::Float16
+  Status AppendValues(int64_t length, Float16 value) {
+    RETURN_NOT_OK(Reserve(length));
+    data_builder_.UnsafeAppend(length, value.bits());
+    ArrayBuilder::UnsafeSetNotNull(length);
+    return Status::OK();
+  }
+
+  /// \brief Get the value at a certain index
+  /// \param[in] index the zero-based index
+  /// @tparam T arrow::util::Float16 or value_type (uint16_t)
+  template <typename T = BaseClass::value_type>
+  T GetValue(int64_t index) const {
+    static_assert(std::is_same_v<T, BaseClass::value_type> ||
+                  std::is_same_v<T, arrow::util::Float16>);
+    if constexpr (std::is_same_v<T, BaseClass::value_type>) {
+      return BaseClass::GetValue(index);
+    } else {
+      return Float16::FromBits(BaseClass::GetValue(index));
+    }
+  }
+};
+
+/// @}
+
 class ARROW_EXPORT BooleanBuilder
     : public ArrayBuilder,
       public internal::ArrayBuilderExtraOps<BooleanBuilder, bool> {

cpp/src/arrow/type_fwd.h

@@ -242,12 +242,17 @@ _NUMERIC_TYPE_DECL(UInt8)
 _NUMERIC_TYPE_DECL(UInt16)
 _NUMERIC_TYPE_DECL(UInt32)
 _NUMERIC_TYPE_DECL(UInt64)
-_NUMERIC_TYPE_DECL(HalfFloat)
 _NUMERIC_TYPE_DECL(Float)
 _NUMERIC_TYPE_DECL(Double)
 
 #undef _NUMERIC_TYPE_DECL
 
+class HalfFloatType;
+using HalfFloatArray = NumericArray<HalfFloatType>;
+class HalfFloatBuilder;
+struct HalfFloatScalar;
+using HalfFloatTensor = NumericTensor<HalfFloatType>;
+
 enum class DateUnit : char { DAY = 0, MILLI = 1 };
 
 class DateType;

cpp/src/arrow/util/float16.h

@@ -178,6 +178,7 @@ class ARROW_EXPORT Float16 {
   }
 };
 
+static_assert(std::is_standard_layout_v<Float16>);
 static_assert(std::is_trivial_v<Float16>);
 static_assert(sizeof(Float16) == sizeof(uint16_t));