Add new Performance/Sum cop

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New features
 
+* [#137](https://github.com/rubocop-hq/rubocop-performance/pull/137): Add new `Performance/Sum` cop. ([@fatkodima][])
 * [#141](https://github.com/rubocop-hq/rubocop-performance/pull/141): Add new `Performance/RedundantStringChars` cop. ([@fatkodima][])
 * [#127](https://github.com/rubocop-hq/rubocop-performance/pull/127): Add new `Performance/IoReadlines` cop. ([@fatkodima][])
 * [#128](https://github.com/rubocop-hq/rubocop-performance/pull/128): Add new `Performance/ReverseFirst` cop. ([@fatkodima][])

config/default.yml

@@ -258,6 +258,12 @@ Performance/StringReplacement:
   Enabled: true
   VersionAdded: '0.33'
 
+Performance/Sum:
+  Description: 'Use `sum` instead of a custom array summation.'
+  Reference: 'https://blog.bigbinary.com/2016/11/02/ruby-2-4-introduces-enumerable-sum.html'
+  Enabled: 'pending'
+  VersionAdded: '1.7'
+
 Performance/TimesMap:
   Description: 'Checks for .times.map calls.'
   AutoCorrect: false

docs/modules/ROOT/pages/cops.adoc

@@ -36,6 +36,7 @@
 * xref:cops_performance.adoc#performancestartwith[Performance/StartWith]
 * xref:cops_performance.adoc#performancestringinclude[Performance/StringInclude]
 * xref:cops_performance.adoc#performancestringreplacement[Performance/StringReplacement]
+* xref:cops_performance.adoc#performancesum[Performance/Sum]
 * xref:cops_performance.adoc#performancetimesmap[Performance/TimesMap]
 * xref:cops_performance.adoc#performanceunfreezestring[Performance/UnfreezeString]
 * xref:cops_performance.adoc#performanceuridefaultparser[Performance/UriDefaultParser]

docs/modules/ROOT/pages/cops_performance.adoc

@@ -1560,6 +1560,40 @@ This cop identifies places where `gsub` can be replaced by
 
 * https://github.com/JuanitoFatas/fast-ruby#stringgsub-vs-stringtr-code
 
+== Performance/Sum
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| Yes
+| 1.7
+| -
+|===
+
+This cop identifies places where custom code finding the sum of elements
+in some Enumerable object can be replaced by `Enumerable#sum` method.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+[1, 2, 3].inject(:+)
+[1, 2, 3].reduce(10, :+)
+[1, 2, 3].reduce { |acc, elem| acc + elem }
+
+# good
+[1, 2, 3].sum
+[1, 2, 3].sum(10)
+[1, 2, 3].sum
+----
+
+=== References
+
+* https://blog.bigbinary.com/2016/11/02/ruby-2-4-introduces-enumerable-sum.html
+
 == Performance/TimesMap
 
 |===

lib/rubocop/cop/performance/sum.rb

@@ -0,0 +1,135 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # This cop identifies places where custom code finding the sum of elements
+      # in some Enumerable object can be replaced by `Enumerable#sum` method.
+      #
+      # @example
+      #   # bad
+      #   [1, 2, 3].inject(:+)
+      #   [1, 2, 3].reduce(10, :+)
+      #   [1, 2, 3].reduce { |acc, elem| acc + elem }
+      #
+      #   # good
+      #   [1, 2, 3].sum
+      #   [1, 2, 3].sum(10)
+      #   [1, 2, 3].sum
+      #
+      class Sum < Cop
+        include RangeHelp
+
+        MSG = 'Use `%<good_method>s` instead of `%<bad_method>s`.'
+
+        def_node_matcher :sum_candidate?, <<~PATTERN
+          (send _ ${:inject :reduce} $_init ? (sym :+))
+        PATTERN
+
+        def_node_matcher :sum_with_block_candidate?, <<~PATTERN
+          (block
+            $(send _ {:inject :reduce} $_init ?)
+            (args (arg $_acc) (arg $_elem))
+            $send)
+        PATTERN
+
+        def_node_matcher :acc_plus_elem?, <<~PATTERN
+          (send (lvar %1) :+ (lvar %2))
+        PATTERN
+        alias elem_plus_acc? acc_plus_elem?
+
+        def on_send(node)
+          sum_candidate?(node) do |method, init|
+            range = sum_method_range(node)
+            message = build_method_message(method, init)
+
+            add_offense(node, location: range, message: message)
+          end
+        end
+
+        def on_block(node)
+          sum_with_block_candidate?(node) do |send, init, var_acc, var_elem, body|
+            if acc_plus_elem?(body, var_acc, var_elem) || elem_plus_acc?(body, var_elem, var_acc)
+              range = sum_block_range(send, node)
+              message = build_block_message(send, init, var_acc, var_elem, body)
+
+              add_offense(node, location: range, message: message)
+            end
+          end
+        end
+
+        def autocorrect(node)
+          if (matches = sum_candidate?(node))
+            _, init = *matches
+            range = sum_method_range(node)
+          elsif (matches = sum_with_block_candidate?(node))
+            send, init, = matches
+            range = sum_block_range(send, node)
+          else
+            return
+          end
+
+          return if init.empty?
+
+          lambda do |corrector|
+            replacement = build_good_method(init)
+            corrector.replace(range, replacement)
+          end
+        end
+
+        private
+
+        def sum_method_range(node)
+          range_between(node.loc.selector.begin_pos, node.loc.end.end_pos)
+        end
+
+        def sum_block_range(send, node)
+          range_between(send.loc.selector.begin_pos, node.loc.end.end_pos)
+        end
+
+        def build_method_message(method, init)
+          good_method = build_good_method(init)
+          bad_method = build_method_bad_method(init, method)
+          format(MSG, good_method: good_method, bad_method: bad_method)
+        end
+
+        def build_block_message(send, init, var_acc, var_elem, body)
+          good_method = build_good_method(init)
+          bad_method = build_block_bad_method(send.method_name, init, var_acc, var_elem, body)
+          format(MSG, good_method: good_method, bad_method: bad_method)
+        end
+
+        def build_good_method(init)
+          good_method = 'sum'
+
+          unless init.empty?
+            init = init.first
+            good_method += "(#{init.source})" if init.source.to_i != 0
+          end
+          good_method
+        end
+
+        def build_method_bad_method(init, method)
+          bad_method = "#{method}("
+          unless init.empty?
+            init = init.first
+            bad_method += "#{init.source}, "
+          end
+          bad_method += ':+)'
+          bad_method
+        end
+
+        def build_block_bad_method(method, init, var_acc, var_elem, body)
+          bad_method = method.to_s
+
+          unless init.empty?
+            init = init.first
+            bad_method += "(#{init.source})"
+          end
+          bad_method += " { |#{var_acc}, #{var_elem}| #{body.source} }"
+          bad_method
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -36,6 +36,7 @@
 require_relative 'performance/start_with'
 require_relative 'performance/string_include'
 require_relative 'performance/string_replacement'
+require_relative 'performance/sum'
 require_relative 'performance/times_map'
 require_relative 'performance/unfreeze_string'
 require_relative 'performance/uri_default_parser'

spec/rubocop/cop/performance/sum_spec.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::Sum do
+  subject(:cop) { described_class.new }
+
+  %i[inject reduce].each do |method|
+    it "registers an offense and corrects when using `array.#{method}(10, :+)`" do
+      source = "array.#{method}(10, :+)"
+      inspect_source(source)
+
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.highlights).to eq(["#{method}(10, :+)"])
+
+      new_source = autocorrect_source(source)
+      expect(new_source).to eq('array.sum(10)')
+    end
+
+    it "registers an offense and corrects when using `array.#{method}(10) { |acc, elem| acc + elem }`" do
+      source = "array.#{method}(10) { |acc, elem| acc + elem }"
+      inspect_source(source)
+
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.highlights).to eq(["#{method}(10) { |acc, elem| acc + elem }"])
+
+      new_source = autocorrect_source(source)
+      expect(new_source).to eq('array.sum(10)')
+    end
+
+    it "registers an offense and corrects when using `array.#{method}(10) { |acc, elem| elem + acc }`" do
+      source = "array.#{method}(10) { |acc, elem| elem + acc }"
+      inspect_source(source)
+
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.highlights).to eq(["#{method}(10) { |acc, elem| elem + acc }"])
+
+      new_source = autocorrect_source(source)
+      expect(new_source).to eq('array.sum(10)')
+    end
+
+    it 'does not autocorrect when initial value is not provided' do
+      source = "array.#{method}(:+)"
+      inspect_source(source)
+
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.highlights).to eq(["#{method}(:+)"])
+
+      new_source = autocorrect_source(source)
+      expect(new_source).to eq(source)
+    end
+
+    it 'does not register an offense when block does not implement summation' do
+      source = "array.#{method} { |acc, elem| elem * 2 }"
+      inspect_source(source)
+      expect(cop.offenses.size).to eq(0)
+    end
+
+    it 'does not register an offense when using `sum`' do
+      expect_no_offenses(<<~RUBY)
+        array.sum
+      RUBY
+    end
+  end
+end