Add UseZipToWrapArrayContents

Performance Cop for the more efficient way to generate an Array of Arrays.

Performs 40-70% faster than mapping to iteratively wrap array contents.
Performs 5 - 55% faster on ranges, depending on size.

changelog/new_merge_pull_request_459_from.md

@@ -0,0 +1 @@
+* [#462](https://github.com/rubocop/rubocop-performance/pull/462): Add new `Performance/UseZipToWrapArrayContents` cop that checks patterns like `.map { |id| [id] }` and `.map { [_1] }`can replace with `.zip`. ([@corsonknowles][])

config/default.yml

@@ -372,3 +372,8 @@ Performance/UriDefaultParser:
   Description: 'Use `URI::DEFAULT_PARSER` instead of `URI::Parser.new`.'
   Enabled: true
   VersionAdded: '0.50'
+
+Performance/UseZipToWrapArrayContents:
+  Description: 'Checks for `.map { |id| [id] }` and suggests replacing it with `.zip`.'
+  Enabled: pending
+  VersionAdded: <<next>>

lib/rubocop/cop/performance/use_zip_to_wrap_array_contents.rb

@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # Checks for `.map { |id| [id] }` and suggests replacing it with `.zip`.
+      #
+      # @example
+      #   # bad
+      #   [1, 2, 3].map { |id| [id] }
+      #
+      #   # good
+      #   [1, 2, 3].zip
+      #
+      # @example
+      #   # bad
+      #   [1, 2, 3].map { [_1] }
+      #
+      #   # good
+      #   [1, 2, 3].zip
+      #
+      # @example
+      #   # good (no offense)
+      #   [1, 2, 3].map { |id| id }
+      #
+      # @example
+      #   # good (no offense)
+      #   [1, 2, 3].map { |id| [id, id] }
+      class UseZipToWrapArrayContents < Base
+        extend AutoCorrector
+
+        RESTRICT_ON_SEND = %i[map].freeze
+        MSG = 'Use `.zip` instead of `.map { |id| [id] }`.'
+
+        # Matches regular block form `.map { |e| [e] }`
+        # @!method map_with_array?(node)
+        def_node_matcher :map_with_array?, <<~PATTERN
+          (block
+            (send _ :map)
+            (args (arg _id))
+            (array (lvar _id)))
+        PATTERN
+
+        # Matches numblock form `.map { [_1] }`
+        # @!method map_with_array_numblock?(node)
+        def_node_matcher :map_with_array_numblock?, <<~PATTERN
+          (numblock
+            (send _ :map)
+            1
+            (array (lvar _))
+          )
+        PATTERN
+
+        def on_send(node)
+          return unless node.method?(:map)
+
+          parent = node.parent
+          return unless parent
+
+          register_offense(parent) if parent.block_type? && map_with_array?(parent)
+        end
+
+        def on_numblock(node)
+          return unless node.method?(:map)
+          return unless map_with_array_numblock?(node)
+
+          register_offense(node)
+        end
+
+        private
+
+        def register_offense(node)
+          map_node = node.children.first # The send node for `.map`
+          add_offense(map_node.loc.selector) do |corrector|
+            autocorrect(node, corrector)
+          end
+        end
+
+        def autocorrect(node, corrector)
+          map_call = node.children.first
+          receiver = map_call.receiver.source
+          corrector.replace(node, "#{receiver}.zip")
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -53,3 +53,4 @@
 require_relative 'performance/unfreeze_string'
 require_relative 'performance/uri_default_parser'
 require_relative 'performance/chain_array_allocation'
+require_relative 'performance/use_zip_to_wrap_array_contents'

spec/rubocop/cop/performance/use_zip_to_wrap_array_contents_spec.rb

@@ -0,0 +1,132 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::UseZipToWrapArrayContents, :config do
+  context 'when using map with array literal' do
+    it 'registers an offense and corrects to use zip with no arguments' do
+      expect_offense(<<~RUBY)
+        [1, 2, 3].map { |id| [id] }
+                  ^^^ Use `.zip` instead of `.map { |id| [id] }`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        [1, 2, 3].zip
+      RUBY
+    end
+  end
+
+  context 'when using map with a short iterator name' do
+    it 'registers an offense and corrects to use zip with no arguments' do
+      expect_offense(<<~RUBY)
+        [1, 2, 3].map { |e| [e] }
+                  ^^^ Use `.zip` instead of `.map { |id| [id] }`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        [1, 2, 3].zip
+      RUBY
+    end
+  end
+
+  context 'when using map on a range with another iterator name' do
+    it 'registers an offense and corrects the code' do
+      expect_offense(<<~RUBY)
+        (1..3).map { |x| [x] }
+               ^^^ Use `.zip` instead of `.map { |id| [id] }`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        (1..3).zip
+      RUBY
+    end
+  end
+
+  context 'when using map in a do end block' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        (1..2).map do |m| [m] end
+               ^^^ Use `.zip` instead of `.map { |id| [id] }`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        (1..2).zip
+      RUBY
+    end
+  end
+
+  context 'when using map with a numerical argment reference' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        [1, 2, 3].map { [_1] }
+                  ^^^ Use `.zip` instead of `.map { |id| [id] }`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        [1, 2, 3].zip
+      RUBY
+    end
+  end
+
+  context 'when the map block does not contain an array literal' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| id }
+      RUBY
+    end
+  end
+
+  context 'when using select with an array literal' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].select { |id| [id] }
+      RUBY
+    end
+  end
+
+  context 'when using map with an array literal containing multiple elements' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| [id, id] }
+      RUBY
+    end
+  end
+
+  context 'when using map with doubly wrapped arrays' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| [[id]] }
+      RUBY
+    end
+  end
+
+  context 'when using map with addition' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| id + 1 }
+      RUBY
+    end
+  end
+
+  context 'when using map with array addition' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| [id] + [id] }
+      RUBY
+    end
+  end
+
+  context 'when using map with indexing into an array' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| [id][id] }
+      RUBY
+    end
+  end
+
+  context 'when using Array.wrap' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        [1, 2, 3].map { |id| Array.wrap(id) }
+      RUBY
+    end
+  end
+end