GH-51262: [Ruby] Add ListArray values constructor

ruby/red-arrow-format/lib/arrow-format/array.rb

@@ -1037,6 +1037,53 @@ def slice!(offset, size)
   end
 
   class ListArray < VariableSizeListArray
+    include BufferAlignable
+
+    def initialize(type, *args)
+      if args.size == 1
+        args = build_data(type, args.first)
+      elsif args.size != 4
+        raise ArgumentError,
+              "wrong number of arguments (given #{args.size + 1}, expected 2 or 5)"
+      end
+
+      super(type, *args)
+    end
+
+    private
+    def build_data(type, data)
+      n = 0
+      validity_buffer_builder = nil
+
+      child_values = []
+      offsets = [0]
+      data.each_with_index do |value, i|
+        if value.nil?
+          validity_buffer_builder ||= SparseBitmapBuilder.new
+          validity_buffer_builder.unset(i)
+        else
+          child_values.concat(value)
+        end
+        offsets << child_values.size
+        n += 1
+      end
+
+      validity_buffer = validity_buffer_builder&.finish(n)
+
+      offsets_data = offsets.pack("#{type.offset_pack_template}*")
+      pad!(offsets_data, buffer_padding_size(offsets_data))
+      offsets_data.freeze
+      offsets_buffer = IO::Buffer.for(offsets_data)
+
+      child = type.child.type.build_array(child_values)
+
+      [
+        n,
+        validity_buffer,
+        offsets_buffer,
+        child,
+      ]
+    end
   end
 
   class LargeListArray < VariableSizeListArray

ruby/red-arrow-format/lib/arrow-format/type.rb

@@ -1021,6 +1021,10 @@ def offset_buffer_type
       :s32 # TODO: big endian support
     end
 
+    def offset_pack_template
+      "l"
+    end
+
     def build_array(...)
       ListArray.new(self, ...)
     end

ruby/red-arrow-format/test/test-list-array.rb

@@ -0,0 +1,54 @@
+# 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 TestListArray < Test::Unit::TestCase
+  def setup
+    child = ArrowFormat::Field.new("item", ArrowFormat::Int32Type.singleton)
+    @type = ArrowFormat::ListType.new(child)
+  end
+
+  sub_test_case("#initialize") do
+    def test_no_null
+      values = [[-1, 0], [], [1, 2, 3]]
+      array = ArrowFormat::ListArray.new(@type, values)
+      assert_same(@type, array.type)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_null_list
+      values = [[1, 2], nil, [], [3]]
+      array = ArrowFormat::ListArray.new(@type, values)
+      assert_equal([0, 2, 2, 2, 3], array.offsets)
+      assert_equal([1, 2, 3], array.child.to_a)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_null_child
+      values = [[1, nil], [], [nil, 2]]
+      array = ArrowFormat::ListArray.new(@type, values)
+      assert_equal([1, nil, nil, 2], array.child.to_a)
+      assert_equal(values, array.to_a)
+    end
+
+    def test_empty
+      array = ArrowFormat::ListArray.new(@type, [])
+      assert_equal([0], array.offsets)
+      assert_equal([], array.child.to_a)
+      assert_equal([], array.to_a)
+    end
+  end
+end