ARROW-2582: [GLib] Add negate functions for Decimal128

Add garrow_decimal128_negate().

Author: yosuke shiro <[email protected]>

Closes #2047 from shiro615/ARROW-2576-add-negate-functions and squashes the following commits:

7a63d00 [yosuke shiro] rename absolute_value to positive_value
815f449 [yosuke shiro] Add negate functions

Author: shiro615

c_glib/arrow-glib/decimal.cpp

@@ -187,6 +187,21 @@ garrow_decimal128_abs(GArrowDecimal128 *decimal)
   arrow_decimal->Abs();
 }
 
+/**
+ * garrow_decimal128_negate:
+ * @decimal: A #GArrowDecimal128.
+ *
+ * Negate the current value of the @decimal destructively.
+ *
+ * Since: 0.10.0
+ */
+void
+garrow_decimal128_negate(GArrowDecimal128 *decimal)
+{
+  auto arrow_decimal = garrow_decimal128_get_raw(decimal);
+  arrow_decimal->Negate();
+}
+
 G_END_DECLS
 
 GArrowDecimal128 *

c_glib/arrow-glib/decimal.h

@@ -41,5 +41,6 @@ gchar *garrow_decimal128_to_string_scale(GArrowDecimal128 *decimal,
                                          gint32 scale);
 gchar *garrow_decimal128_to_string(GArrowDecimal128 *decimal);
 void garrow_decimal128_abs(GArrowDecimal128 *decimal);
+void garrow_decimal128_negate(GArrowDecimal128 *decimal);
 
 G_END_DECLS

c_glib/test/test-decimal.rb

@@ -36,4 +36,14 @@ def test_abs
     decimal.abs
     assert_equal(absolute_value, decimal.to_s)
   end
+
+  def test_negate
+    positive_value = "23049223942343532412"
+    negative_value = "-23049223942343532412"
+    decimal = Arrow::Decimal128.new(positive_value)
+    decimal.negate
+    assert_equal(negative_value, decimal.to_s)
+    decimal.negate
+    assert_equal(positive_value, decimal.to_s)
+  end
 end