collection_literal_in_loop.rb

# frozen_string_literal: true

require 'set'

module RuboCop
  module Cop
    module Performance
      # This cop identifies places where Array and Hash literals are used
      # within loops. It is better to extract them into a local variable or constant
      # to avoid unnecessary allocations on each iteration.
      #
      # You can set the minimum number of elements to consider
      # an offense with `MinSize`.
      #
      # @example
      #   # bad
      #   users.select do |user|
      #     %i[superadmin admin].include?(user.role)
      #   end
      #
      #   # good
      #   admin_roles = %i[superadmin admin]
      #   users.select do |user|
      #     admin_roles.include?(user.role)
      #   end
      #
      #   # good
      #   ADMIN_ROLES = %i[superadmin admin]
      #   ...
      #   users.select do |user|
      #     ADMIN_ROLES.include?(user.role)
      #   end
      #
      class CollectionLiteralInLoop < Base
        MSG = 'Avoid immutable %<literal_class>s literals in loops. '\
          'It is better to extract it into a local variable or a constant.'

        POST_CONDITION_LOOP_TYPES = %i[while_post until_post].freeze
        LOOP_TYPES = (POST_CONDITION_LOOP_TYPES + %i[while until for]).freeze

        ENUMERABLE_METHOD_NAMES = (Enumerable.instance_methods + [:each]).to_set.freeze
        NONMUTATING_ARRAY_METHODS = %i[& * + - <=> == [] all? any? assoc at
                                       bsearch bsearch_index collect combination
                                       compact count cycle deconstruct difference dig
                                       drop drop_while each each_index empty? eql?
                                       fetch filter find_index first flatten hash
                                       include? index inspect intersection join
                                       last length map max min minmax none? one? pack
                                       permutation product rassoc reject
                                       repeated_combination repeated_permutation reverse
                                       reverse_each rindex rotate sample select shuffle
                                       size slice sort sum take take_while
                                       to_a to_ary to_h to_s transpose union uniq
                                       values_at zip |].freeze

        ARRAY_METHODS = (ENUMERABLE_METHOD_NAMES | NONMUTATING_ARRAY_METHODS).to_set.freeze

        NONMUTATING_HASH_METHODS = %i[< <= == > >= [] any? assoc compact dig
                                      each each_key each_pair each_value empty?
                                      eql? fetch fetch_values filter flatten has_key?
                                      has_value? hash include? inspect invert key key?
                                      keys? length member? merge rassoc rehash reject
                                      select size slice to_a to_h to_hash to_proc to_s
                                      transform_keys transform_values value? values values_at].freeze

        HASH_METHODS = (ENUMERABLE_METHOD_NAMES | NONMUTATING_HASH_METHODS).to_set.freeze

        def_node_matcher :kernel_loop?, <<~PATTERN
          (block
            (send {nil? (const nil? :Kernel)} :loop)
            ...)
        PATTERN

        def_node_matcher :enumerable_loop?, <<~PATTERN
          (block
            (send $_ #enumerable_method? ...)
            ...)
        PATTERN

        def on_send(node)
          receiver, method, = *node.children
          return unless check_literal?(receiver, method) && parent_is_loop?(receiver)

          message = format(MSG, literal_class: literal_class(receiver))
          add_offense(receiver, message: message)
        end

        private

        def check_literal?(node, method)
          !node.nil? &&
            nonmutable_method_of_array_or_hash?(node, method) &&
            node.children.size >= min_size &&
            node.recursive_basic_literal?
        end

        def nonmutable_method_of_array_or_hash?(node, method)
          (node.array_type? && ARRAY_METHODS.include?(method)) ||
            (node.hash_type? && HASH_METHODS.include?(method))
        end

        def parent_is_loop?(node)
          node.each_ancestor.any? { |ancestor| loop?(ancestor, node) }
        end

        def loop?(ancestor, node)
          keyword_loop?(ancestor.type) ||
            kernel_loop?(ancestor) ||
            node_within_enumerable_loop?(node, ancestor)
        end

        def keyword_loop?(type)
          LOOP_TYPES.include?(type)
        end

        def node_within_enumerable_loop?(node, ancestor)
          enumerable_loop?(ancestor) do |receiver|
            receiver != node && !receiver.descendants.include?(node)
          end
        end

        def literal_class(node)
          if node.array_type?
            'Array'
          elsif node.hash_type?
            'Hash'
          end
        end

        def enumerable_method?(method_name)
          ENUMERABLE_METHOD_NAMES.include?(method_name)
        end

        def min_size
          Integer(cop_config['MinSize'] || 1)
        end
      end
    end
  end
end