From 316c16dd3e30c064c3c285f8859d72febb4b3afc Mon Sep 17 00:00:00 2001
From: Yifan Chen <30335308+emecii@users.noreply.github.com>
Date: Wed, 9 Sep 2026 22:13:51 -0700
Subject: [PATCH] GH-51273: [Ruby] Add FixedSizeListArray values constructor

---
 .../lib/arrow-format/array.rb                 | 40 ++++++++++-
 .../test/test-fixed-size-list-array.rb        | 67 +++++++++++++++++++
 2 files changed, 106 insertions(+), 1 deletion(-)
 create mode 100644 ruby/red-arrow-format/test/test-fixed-size-list-array.rb

diff --git a/ruby/red-arrow-format/lib/arrow-format/array.rb b/ruby/red-arrow-format/lib/arrow-format/array.rb
index 992abe815bd..7abe4e6791d 100644
--- a/ruby/red-arrow-format/lib/arrow-format/array.rb
+++ b/ruby/red-arrow-format/lib/arrow-format/array.rb
@@ -1093,7 +1093,15 @@ class LargeListArray < VariableSizeListArray
 
   class FixedSizeListArray < Array
     attr_reader :child
-    def initialize(type, size, validity_buffer, child)
+    def initialize(type, *args)
+      if args.size == 1
+        args = build_data(type, args.first)
+      elsif args.size != 3
+        raise ArgumentError,
+              "wrong number of arguments (given #{args.size + 1}, expected 2 or 4)"
+      end
+
+      size, validity_buffer, child = args
       super(type, size, validity_buffer)
       @child = child
     end
@@ -1112,6 +1120,36 @@ def to_a
     end
 
     private
+    def build_data(type, data)
+      n = 0
+      validity_buffer_builder = nil
+
+      child_values = []
+      data.each_with_index do |value, i|
+        if value.nil?
+          validity_buffer_builder ||= SparseBitmapBuilder.new
+          validity_buffer_builder.unset(i)
+          child_values.concat([nil] * type.size)
+        else
+          unless value.size == type.size
+            message = "list size must be #{type.size}: #{value.inspect}"
+            raise ArgumentError, message
+          end
+          child_values.concat(value)
+        end
+        n += 1
+      end
+
+      validity_buffer = validity_buffer_builder&.finish(n)
+      child = type.child.type.build_array(child_values)
+
+      [
+        n,
+        validity_buffer,
+        child,
+      ]
+    end
+
     def slice!(offset, size)
       super
       @child = @child.slice(@type.size * @offset,
diff --git a/ruby/red-arrow-format/test/test-fixed-size-list-array.rb b/ruby/red-arrow-format/test/test-fixed-size-list-array.rb
new file mode 100644
index 00000000000..bf4bb5afd84
--- /dev/null
+++ b/ruby/red-arrow-format/test/test-fixed-size-list-array.rb
@@ -0,0 +1,67 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# 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
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+class TestFixedSizeListArray < Test::Unit::TestCase
+  def setup
+    child = ArrowFormat::Field.new("item", ArrowFormat::Int16Type.singleton)
+    @type = ArrowFormat::FixedSizeListType.new(child, 2)
+  end
+
+  sub_test_case("#initialize") do
+    def test_no_null
+      values = [[-1, 0], [1, 2]]
+      array = ArrowFormat::FixedSizeListArray.new(@type, values)
+      assert_same(@type, array.type)
+      assert_same(@type.child.type, array.child.type)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_null_list
+      values = [[1, 2], nil, [3, 4]]
+      array = ArrowFormat::FixedSizeListArray.new(@type, values)
+      assert_equal(1, array.n_nulls)
+      assert_equal([1, 2, nil, nil, 3, 4], array.child.to_a)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_null_child
+      values = [[1, nil], [nil, 2]]
+      array = ArrowFormat::FixedSizeListArray.new(@type, values)
+      assert_equal(2, array.child.n_nulls)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_invalid_list_size
+      message = "list size must be 2: [1]"
+      assert_raise(ArgumentError.new(message)) do
+        ArrowFormat::FixedSizeListArray.new(@type, [[1]])
+      end
+    end
+
+    def test_empty
+      array = ArrowFormat::FixedSizeListArray.new(@type, [])
+      assert_equal([], array.child.to_a)
+      assert_equal([], array.to_a)
+    end
+
+    def test_low_level
+      child = ArrowFormat::Int16Array.new([1, 2])
+      array = ArrowFormat::FixedSizeListArray.new(@type, 1, nil, child)
+      assert_equal([[1, 2]], array.to_a)
+    end
+  end
+end