1 files changed, 50 insertions, 0 deletions
diff --git a/spec/ruby/core/enumerable/sum_spec.rb b/spec/ruby/core/enumerable/sum_spec.rb
new file mode 100644
index 0000000000..2eb74db6ac
--- /dev/null
+++ b/spec/ruby/core/enumerable/sum_spec.rb
@@ -0,0 +1,50 @@
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+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 'gives 0 as a default argument' do
+    @enum.sum.should == 5/3r
+  end
+
+  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