Add sum filter to standard filters

lib/liquid/standardfilters.rb

@@ -869,6 +869,31 @@ def default(input, default_value = '', options = {})
       false_check || (input.respond_to?(:empty?) && input.empty?) ? default_value : input
     end
 
+    # @liquid_public_docs
+    # @liquid_type filter
+    # @liquid_category array
+    # @liquid_summary
+    #   Returns the sum of all elements in an array.
+    # @liquid_syntax array | sum
+    # @liquid_return [number]
+    def sum(input, property = nil)
+      ary = InputIterator.new(input, context)
+
+      return 0 if ary.empty?
+
+      if property.nil?
+        # If we're doing an ordinary sum, all values must be numeric
+        raise(Liquid::ArgumentError, "cannot sum non-numeric values") unless ary.all? { |el| el.is_a?(Numeric) }
+        ary.sum
+      elsif ary.all? { |el| el.respond_to?(:[]) }
+        begin
+          ary.sum { |el| el[property] }
+        rescue TypeError
+          raise_property_error(property)
+        end
+      end
+    end
+
     private
 
     attr_reader :context

test/integration/filter_test.rb

@@ -165,6 +165,34 @@ def test_liquid_argument_error
     assert_match(/\ALiquid error: wrong number of arguments /, exc.message)
     assert_equal(exc.message, Template.parse(source).render)
   end
+
+  def test_sum
+    src_w_prop = "{{ ary | sum: 'qty' }}"
+    src_wo_prop = "{{ ary | sum }}"
+
+    # [Without property] Test empty array
+    assert_template_result("0", src_wo_prop, { "ary" => [] })
+
+    # [Without property] Test array with only numeric elements
+    assert_template_result("6", src_wo_prop, { "ary" => [1, 2, 3] })
+
+    # [Without property] Test array with some non-numeric elements
+    with_non_num = { "ary" => [1, 2, "a", "b", { "qty" => 2 }] }
+    exc = assert_raises(Liquid::ArgumentError) do
+      Template.parse(src_wo_prop).render!(with_non_num)
+    end
+    assert_equal(exc.message, "Liquid error: cannot sum non-numeric values")
+    assert_equal(exc.message, Template.parse(src_wo_prop).render(with_non_num))
+
+    # [Without property] Test array with hashes, all of which have given property
+    assert_template_result("6", src_w_prop, { "ary" => [{ "qty" => 1 }, { "qty" => 2 }, { "qty" => 3 }] })
+
+    # [Without property] Test array with hashes, some of which do not have given property
+    exc = assert_raises(Liquid::ArgumentError) do
+      Template.parse(src_w_prop).render!("ary" => [{ "qty" => 1 }, { "qty" => 2 }, { "not_qty" => 3 }])
+    end
+    assert_equal(exc.message, "Liquid error: cannot select the property 'qty'")
+  end
 end
 
 class FiltersInTemplate < Minitest::Test