# frozen_string_literal: true

require "set"

class Module
  # Error generated by +delegate+ when a method is called on +nil+ and +allow_nil+
  # option is not used.
  class DelegationError < NoMethodError; end

  RUBY_RESERVED_KEYWORDS = %w(__ENCODING__ __LINE__ __FILE__ alias and BEGIN begin break
  case class def defined? do else elsif END end ensure false for if in module next nil
  not or redo rescue retry return self super then true undef unless until when while yield)
  DELEGATION_RESERVED_KEYWORDS = %w(_ arg args block)
  DELEGATION_RESERVED_METHOD_NAMES = Set.new(
    RUBY_RESERVED_KEYWORDS + DELEGATION_RESERVED_KEYWORDS
  ).freeze

  # When building decorators, a common pattern may emerge:
  #
  #   class Partition
  #     def initialize(event)
  #       @event = event
  #     end
  #
  #     def person
  #       detail.person || creator
  #     end
  #
  #     private
  #       def respond_to_missing?(name, include_private = false)
  #         @event.respond_to?(name, include_private)
  #       end
  #
  #       def method_missing(method, *args, &block)
  #         @event.send(method, *args, &block)
  #       end
  #   end
  #
  # With <tt>Module#delegate_missing_to</tt>, the above is condensed to:
  #
  #   class Partition
  #     delegate_missing_to :@event
  #
  #     def initialize(event)
  #       @event = event
  #     end
  #
  #     def person
  #       detail.person || creator
  #     end
  #   end
  #
  # The target can be anything callable within the object, e.g. instance
  # variables, methods, constants, etc.
  #
  # The delegated method must be public on the target, otherwise it will
  # raise +DelegationError+. If you wish to instead return +nil+,
  # use the <tt>:allow_nil</tt> option.
  #
  # The <tt>marshal_dump</tt> and <tt>_dump</tt> methods are exempt from
  # delegation due to possible interference when calling
  # <tt>Marshal.dump(object)</tt>, should the delegation target method
  # of <tt>object</tt> add or remove instance variables.
  def delegate_missing_to(target, allow_nil: nil)
    target = target.to_s
    target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)

    module_eval <<-RUBY, __FILE__, __LINE__ + 1
      def respond_to_missing?(name, include_private = false)
        # It may look like an oversight, but we deliberately do not pass
        # +include_private+, because they do not get delegated.

        return false if name == :marshal_dump || name == :_dump
        #{target}.respond_to?(name) || super
      end

      def method_missing(method, *args, &block)
        if #{target}.respond_to?(method)
          #{target}.public_send(method, *args, &block)
        else
          begin
            super
          rescue NoMethodError
            if #{target}.nil?
              if #{allow_nil == true}
                nil
              else
                raise DelegationError, "\#{method} delegated to #{target}, but #{target} is nil"
              end
            else
              raise
            end
          end
        end
      end
    RUBY
  end
end