ARROW-6350: [Ruby] Remove Arrow::Struct and use Hash instead

This is backward incompatible change. But this simple API will be
useful for users.

Closes apache#5190 from kou/ruby-remove-struct and squashes the following commits:

e565c3b <Sutou Kouhei>  Remove Arrow::Struct and use Hash instead

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Yosuke Shiro <[email protected]>

ruby/red-arrow/lib/arrow/struct-array-builder.rb

@@ -73,20 +73,14 @@ def append_value(*args)
           value.each_with_index do |sub_value, i|
             self[i].append(sub_value)
           end
-        when Arrow::Struct
-          append_value_raw
-          value.values.each_with_index do |sub_value, i|
-            self[i].append(sub_value)
-          end
         when Hash
           append_value_raw
           value.each do |name, sub_value|
             self[name].append(sub_value)
           end
         else
           message =
-            "struct value must be nil, Array, " +
-            "Arrow::Struct or Hash: #{value.inspect}"
+            "struct value must be nil, Array or Hash: #{value.inspect}"
           raise ArgumentError, message
         end
       else

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

@@ -15,18 +15,20 @@
 # specific language governing permissions and limitations
 # under the License.
 
-require "arrow/struct"
-
 module Arrow
   class StructArray
     # @param i [Integer]
     #   The index of the value to be gotten. You must specify the value index.
     #
     #   You can use {Arrow::Array#[]} for convenient value access.
     #
-    # @return [Arrow::Struct] The `i`-th value.
+    # @return [Hash] The `i`-th struct.
     def get_value(i)
-      Struct.new(self, i)
+      value = {}
+      value_data_type.fields.zip(fields) do |field, field_array|
+        value[field.name] = field_array[i]
+      end
+      value
     end
 
     # @overload find_field(index)
@@ -45,20 +47,20 @@ def find_field(index_or_name)
         (@name_to_field ||= build_name_to_field)[name.to_s]
       else
         index = index_or_name
-        cached_fields[index]
+        fields[index]
       end
     end
 
-    private
-    def cached_fields
-      @fields ||= fields
+    alias_method :fields_raw, :fields
+    def fields
+      @fields ||= fields_raw
     end
 
+    private
     def build_name_to_field
       name_to_field = {}
-      field_arrays = cached_fields
-      value_data_type.fields.each_with_index do |field, i|
-        name_to_field[field.name] = field_arrays[i]
+      value_data_type.fields.zip(fields) do |field, field_array|
+        name_to_field[field.name] = field_array
       end
       name_to_field
     end

ruby/red-arrow/test/test-struct-array-builder.rb

@@ -168,12 +168,12 @@ def setup
       @builder.get_field_builder(1).append(2)
       array = @builder.finish
       assert_equal([
-                     [true, 1],
-                     [false, 2],
+                     {"visible" => true,  "count" => 1},
+                     {"visible" => false, "count" => 2},
                    ],
                    [
-                     array.get_value(0).values,
-                     array.get_value(1).values,
+                     array.get_value(0),
+                     array.get_value(1),
                    ])
     end
   end

ruby/red-arrow/test/test-struct-array.rb

@@ -50,16 +50,16 @@ def setup
 
     test("#[]") do
       assert_equal([
-                     Arrow::Struct.new(@array, 0),
-                     Arrow::Struct.new(@array, 1),
+                     {"visible" => true,  "count" => 1},
+                     {"visible" => false, "count" => 2},
                    ],
                    @array.to_a)
     end
 
     test("#get_value") do
       assert_equal([
-                     Arrow::Struct.new(@array, 0),
-                     Arrow::Struct.new(@array, 1),
+                     {"visible" => true,  "count" => 1},
+                     {"visible" => false, "count" => 2},
                    ],
                    [
                      @array.get_value(0),