1 files changed, 40 insertions, 20 deletions
diff --git a/spec/ruby/core/enumerable/sum_spec.rb b/spec/ruby/core/enumerable/sum_spec.rb
index 4881039a8d..2eb74db6ac 100644
--- a/spec/ruby/core/enumerable/sum_spec.rb
+++ b/spec/ruby/core/enumerable/sum_spec.rb
@@ -1,30 +1,50 @@
-require File.expand_path('../../../spec_helper', __FILE__)
-require File.expand_path('../fixtures/classes', __FILE__)
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
 
-ruby_version_is '2.4' do
-  describe 'Enumerable#sum' do
-    before :each do
-      @enum = Object.new.to_enum
-      class << @enum
-        def each
-          yield 0
-          yield(-1)
-          yield 2
-          yield 2/3r
-        end
+describe 'Enumerable#sum' do
+  before :each do
+    @enum = Object.new.to_enum
+    class << @enum
+      def each
+        yield 0
+        yield(-1)
+        yield 2
+        yield 2/3r
       end
     end
+  end
 
-    it 'returns amount of the elements with taking an argument as the initial value' do
-      @enum.sum(10).should == 35/3r
-    end
+  it 'returns amount of the elements with taking an argument as the initial value' do
+    @enum.sum(10).should == 35/3r
+  end
 
-    it 'gives 0 as a default argument' do
-      @enum.sum.should == 5/3r
-    end
+  it 'gives 0 as a default argument' do
+    @enum.sum.should == 5/3r
+  end
 
-    it 'takes a block to transform the elements' do
+  context 'with a block' do
+    it 'transforms the elements' do
       @enum.sum { |element| element * 2 }.should == 10/3r
     end
+
+    it 'does not destructure array elements' do
+      class << @enum
+        def each
+          yield [1,2]
+          yield [3]
+        end
+      end
+
+      @enum.sum(&:last).should == 5
+    end
+  end
+
+  # https://bugs.ruby-lang.org/issues/12217
+  # https://github.com/ruby/ruby/blob/master/doc/ChangeLog/ChangeLog-2.4.0#L6208-L6214
+  it "uses Kahan's compensated summation algorithm for precise sum of float numbers" do
+    floats = [2.7800000000000002, 5.0, 2.5, 4.44, 3.89, 3.89, 4.44, 7.78, 5.0, 2.7800000000000002, 5.0, 2.5].to_enum
+    naive_sum = floats.reduce { |sum, e| sum + e }
+    naive_sum.should == 50.00000000000001
+    floats.sum.should == 50.0
   end
 end