Add new Performance/ArraySemiInfiniteRangeSlice cop

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#173](https://github.com/rubocop-hq/rubocop-performance/pull/173): Add new `Performance/BlockGivenWithExplicitBlock` cop. ([@fatkodima][])
 * [#136](https://github.com/rubocop-hq/rubocop-performance/issues/136): Add new `Performance/MethodObjectAsBlock` cop. ([@fatkodima][])
 * [#151](https://github.com/rubocop-hq/rubocop-performance/issues/151): Add new `Performance/ConstantRegexp` cop. ([@fatkodima][])
+* [#175](https://github.com/rubocop-hq/rubocop-performance/pull/175): Add new `Performance/ArraySemiInfiniteRangeSlice` cop. ([@fatkodima][])
 
 ### Changes
 

config/default.yml

@@ -7,6 +7,11 @@ Performance/AncestorsInclude:
   Safe: false
   VersionAdded: '1.7'
 
+Performance/ArraySemiInfiniteRangeSlice:
+  Description: 'Identifies places where slicing arrays with semi-infinite ranges can be replaced by `Array#take` and `Array#drop`.'
+  Enabled: pending
+  VersionAdded: '1.9'
+
 Performance/BigDecimalWithNumericArgument:
   Description: 'Convert numeric argument to string before passing to BigDecimal.'
   Enabled: 'pending'

docs/modules/ROOT/pages/cops.adoc

@@ -13,6 +13,7 @@ Performance cops optimization analysis for your projects.
 === Department xref:cops_performance.adoc[Performance]
 
 * xref:cops_performance.adoc#performanceancestorsinclude[Performance/AncestorsInclude]
+* xref:cops_performance.adoc#performancearraysemiinfiniterangeslice[Performance/ArraySemiInfiniteRangeSlice]
 * xref:cops_performance.adoc#performancebigdecimalwithnumericargument[Performance/BigDecimalWithNumericArgument]
 * xref:cops_performance.adoc#performancebindcall[Performance/BindCall]
 * xref:cops_performance.adoc#performanceblockgivenwithexplicitblock[Performance/BlockGivenWithExplicitBlock]

docs/modules/ROOT/pages/cops_performance.adoc

@@ -30,6 +30,42 @@ A <= B
 
 * https://github.com/JuanitoFatas/fast-ruby#ancestorsinclude-vs--code
 
+== Performance/ArraySemiInfiniteRangeSlice
+
+NOTE: Required Ruby version: 2.7
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| Yes
+| 1.9
+| -
+|===
+
+This cop identifies places where slicing arrays with semi-infinite ranges
+can be replaced by `Array#take` and `Array#drop`.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+# array[..2]
+# array[...2]
+# array[2..]
+# array[2...]
+# array.slice(..2)
+
+# good
+array.take(3)
+array.take(2)
+array.drop(2)
+array.drop(2)
+array.take(3)
+----
+
 == Performance/BigDecimalWithNumericArgument
 
 |===

lib/rubocop/cop/performance/array_semi_infinite_range_slice.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # This cop identifies places where slicing arrays with semi-infinite ranges
+      # can be replaced by `Array#take` and `Array#drop`.
+      #
+      # @example
+      #   # bad
+      #   # array[..2]
+      #   # array[...2]
+      #   # array[2..]
+      #   # array[2...]
+      #   # array.slice(..2)
+      #
+      #   # good
+      #   array.take(3)
+      #   array.take(2)
+      #   array.drop(2)
+      #   array.drop(2)
+      #   array.take(3)
+      #
+      class ArraySemiInfiniteRangeSlice < Base
+        include RangeHelp
+        extend AutoCorrector
+        extend TargetRubyVersion
+
+        minimum_target_ruby_version 2.7
+
+        MSG = 'Use `%<prefer>s` instead of `%<current>s` with semi-infinite range.'
+
+        SLICE_METHODS = Set[:[], :slice].freeze
+        RESTRICT_ON_SEND = SLICE_METHODS
+
+        def_node_matcher :endless_range_slice?, <<~PATTERN
+          (send $_ $%SLICE_METHODS $#endless_range?)
+        PATTERN
+
+        def_node_matcher :endless_range?, <<~PATTERN
+          {
+            ({irange erange} nil? (int positive?))
+            ({irange erange} (int positive?) nil?)
+          }
+        PATTERN
+
+        def on_send(node)
+          endless_range_slice?(node) do |receiver, method_name, range_node|
+            prefer = range_node.begin ? :drop : :take
+            message = format(MSG, prefer: prefer, current: method_name)
+
+            add_offense(node, message: message) do |corrector|
+              corrector.replace(node, correction(receiver, range_node))
+            end
+          end
+        end
+
+        private
+
+        def correction(receiver, range_node)
+          method_call = if range_node.begin
+                          "drop(#{range_node.begin.value})"
+                        elsif range_node.irange_type?
+                          "take(#{range_node.end.value + 1})"
+                        else
+                          "take(#{range_node.end.value})"
+                        end
+
+          "#{receiver.source}.#{method_call}"
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -4,6 +4,7 @@
 require_relative 'mixin/sort_block'
 
 require_relative 'performance/ancestors_include'
+require_relative 'performance/array_semi_infinite_range_slice'
 require_relative 'performance/big_decimal_with_numeric_argument'
 require_relative 'performance/bind_call'
 require_relative 'performance/block_given_with_explicit_block'

spec/rubocop/cop/performance/array_semi_infinite_range_slice_spec.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::ArraySemiInfiniteRangeSlice, :config do
+  subject(:cop) { described_class.new(config) }
+
+  context 'TargetRubyVersion >= 2.7', :ruby27 do
+    it 'registers an offense and corrects when using `[]` with beginless range' do
+      expect_offense(<<~RUBY)
+        array[..2]
+        ^^^^^^^^^^ Use `take` instead of `[]` with semi-infinite range.
+        array[...2]
+        ^^^^^^^^^^^ Use `take` instead of `[]` with semi-infinite range.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        array.take(3)
+        array.take(2)
+      RUBY
+    end
+
+    it 'registers an offense and corrects when using `[]` with endless range' do
+      expect_offense(<<~RUBY)
+        array[2..]
+        ^^^^^^^^^^ Use `drop` instead of `[]` with semi-infinite range.
+        array[2...]
+        ^^^^^^^^^^^ Use `drop` instead of `[]` with semi-infinite range.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        array.drop(2)
+        array.drop(2)
+      RUBY
+    end
+
+    it 'registers an offense and corrects when using `slice` with semi-infinite ranges' do
+      expect_offense(<<~RUBY)
+        array.slice(2..)
+        ^^^^^^^^^^^^^^^^ Use `drop` instead of `slice` with semi-infinite range.
+        array.slice(..2)
+        ^^^^^^^^^^^^^^^^ Use `take` instead of `slice` with semi-infinite range.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        array.drop(2)
+        array.take(3)
+      RUBY
+    end
+
+    it 'does not register an offense when using `[]` with full range' do
+      expect_no_offenses(<<~RUBY)
+        array[0..2]
+      RUBY
+    end
+
+    it 'does not register an offense when using `[]` with semi-infinite range with non literal' do
+      expect_no_offenses(<<~RUBY)
+        array[..index]
+        array[index..]
+      RUBY
+    end
+
+    it 'does not register an offense when using `[]` with semi-infinite range with negative int' do
+      expect_no_offenses(<<~RUBY)
+        array[..-2]
+        array[-2..]
+      RUBY
+    end
+  end
+end