ARROW-4593: [Ruby] Arrow::Array#[out_of_range] returns nil

Author: Kouhei Sutou <kou@clear-code.com>

Closes #3666 from kou/ruby-array-ref-out-of-range and squashes the following commits:

f64df5b <Kouhei Sutou>  Array# returns nil

Author: kou

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

@@ -35,8 +35,18 @@ def new(*args)
       end
     end
 
+    # @param i [Integer]
+    #   The index of the value to be gotten.
+    #
+    #   You can specify negative index like for `::Array#[]`.
+    #
+    # @return [Object, nil]
+    #   The `i`-th value.
+    #
+    #   `nil` for NULL value or out of range `i`.
     def [](i)
       i += length if i < 0
+      return nil if i < 0 or i >= length
       if null?(i)
         nil
       else

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

@@ -24,15 +24,30 @@ class ArrayTest < Test::Unit::TestCase
     end
   end
 
-  test("#each") do
-    array = Arrow::BooleanArray.new([true, false, nil, true])
-    assert_equal([true, false, nil, true],
-                 array.to_a)
-  end
+  sub_test_case("instance methods") do
+    def setup
+      @values = [true, false, nil, true]
+      @array = Arrow::BooleanArray.new(@values)
+    end
+
+    test("#each") do
+      assert_equal(@values, @array.to_a)
+    end
 
-  test("#[]") do
-    array = Arrow::BooleanArray.new([true, false, nil, true])
-    assert_equal([true, false, nil, true],
-                 [array[0], array[1], array[2], array[3]])
+    sub_test_case("#[]") do
+      test("valid range") do
+        assert_equal(@values,
+                     @array.length.times.collect {|i| @array[i]})
+      end
+
+      test("out of range") do
+        assert_nil(@array[@array.length])
+      end
+
+      test("negative index") do
+        assert_equal(@values.last,
+                     @array[-1])
+      end
+    end
   end
 end