GH-49055: [Ruby] Add support for writing decimal128/256 arrays (#49056)

kou · web-flow · commit aae58402049d · 2026-01-30T09:25:46.000+09:00
### Rationale for this change Decimal128/256 arrays are only supported. ### What changes are included in this PR? Add `ArrowFormat::DecimalType#to_flatbuffers`. ### Are these changes tested? Yes. ### Are there any user-facing changes? Yes. * GitHub Issue: #49055 Authored-by: Sutou Kouhei <kou@clear-code.com> Signed-off-by: Sutou Kouhei <kou@clear-code.com>
diff --git a/ruby/red-arrow-format/lib/arrow-format/type.rb b/ruby/red-arrow-format/lib/arrow-format/type.rb
@@ -627,6 +627,14 @@ def initialize(byte_width, precision, scale)
       @precision = precision
       @scale = scale
     end
+
+    def to_flatbuffers
+      fb_type = FB::Decimal::Data.new
+      fb_type.bit_width = @byte_width * 8
+      fb_type.precision = @precision
+      fb_type.scale = @scale
+      fb_type
+    end
   end
 
   class Decimal128Type < DecimalType
diff --git a/ruby/red-arrow-format/test/test-writer.rb b/ruby/red-arrow-format/test/test-writer.rb
@@ -68,6 +68,12 @@ def convert_type(red_arrow_type)
       ArrowFormat::UTF8Type.singleton
     when Arrow::LargeStringDataType
       ArrowFormat::LargeUTF8Type.singleton
+    when Arrow::Decimal128DataType
+      ArrowFormat::Decimal128Type.new(red_arrow_type.precision,
+                                      red_arrow_type.scale)
+    when Arrow::Decimal256DataType
+      ArrowFormat::Decimal256Type.new(red_arrow_type.precision,
+                                      red_arrow_type.scale)
     when Arrow::FixedSizeBinaryDataType
       ArrowFormat::FixedSizeBinaryType.new(red_arrow_type.byte_width)
     else
@@ -444,6 +450,62 @@ def test_write
                          @values)
           end
         end
+
+        sub_test_case("Decimal128") do
+          def build_array
+            @positive_small = "1.200"
+            @positive_large = ("1234567890" * 3) + "12345.678"
+            @negative_small = "-1.200"
+            @negative_large = "-" + ("1234567890" * 3) + "12345.678"
+            Arrow::Decimal128Array.new({precision: 38, scale: 3},
+                                       [
+                                         @positive_large,
+                                         @positive_small,
+                                         nil,
+                                         @negative_small,
+                                         @negative_large,
+                                       ])
+          end
+
+          def test_write
+            assert_equal([
+                           BigDecimal(@positive_large),
+                           BigDecimal(@positive_small),
+                           nil,
+                           BigDecimal(@negative_small),
+                           BigDecimal(@negative_large),
+                         ],
+                         @values)
+          end
+        end
+
+        sub_test_case("Decimal256") do
+          def build_array
+            @positive_small = "1.200"
+            @positive_large = ("1234567890" * 7) + "123.456"
+            @negative_small = "-1.200"
+            @negative_large = "-" + ("1234567890" * 7) + "123.456"
+            Arrow::Decimal256Array.new({precision: 76, scale: 3},
+                                       [
+                                         @positive_large,
+                                         @positive_small,
+                                         nil,
+                                         @negative_small,
+                                         @negative_large,
+                                       ])
+          end
+
+          def test_write
+            assert_equal([
+                           BigDecimal(@positive_large),
+                           BigDecimal(@positive_small),
+                           nil,
+                           BigDecimal(@negative_small),
+                           BigDecimal(@negative_large),
+                         ],
+                         @values)
+          end
+        end
       end
     end
   end
