Merge pull request #2359 from herwinw/range_sum

Optimize Enumerable#sum for integer ranges

src/enumerable.rb

@@ -871,6 +871,10 @@ def sort_by(&block)
 
   def sum(init = 0)
     block_given = block_given?
+    if !block_given && init == 0 && is_a?(Range) && first.is_a?(Integer) && self.last.is_a?(Integer)
+      last = exclude_end? ? self.last - 1 : self.last
+      return (first + last) * (last - first + 1) / 2
+    end
     each do |item|
       if block_given
         init += yield item

test/natalie/enumerable_test.rb

@@ -57,4 +57,17 @@ def each
       -> { [1].zip [], Object.new, [] }.should raise_error(TypeError)
     end
   end
+
+  describe '#sum' do
+    it 'should return the correct result with optimized Range operations' do
+      (1..1000).sum.should == 500500
+      (1...1000).sum.should == 499500
+      (10..20).sum.should == 165
+    end
+
+    it 'falls back to the default implementation when range is not numeric or a start is given' do
+      ('a'..'c').sum('').should == 'abc'
+      (1..3).sum(10).should == 16
+    end
+  end
 end