module RSpec::Core::SharedExampleGroup

def self.created_from_caller(other_caller)
  @caller_line == other_caller.last
end

def self.included(kls)
  kls.describe(&@shared_block)
  kls.children.first.metadata[:shared_group_name] = name
end

def key? candidate
  [String, Symbol, Module].any? { |cls| cls === candidate }
end

def key_taken? key
  RSpec.world.shared_example_groups.has_key?(key)
end

def raise_key_taken key
  raise ArgumentError, "Shared example group '#{key}' already exists"
end

def raise_name_error
  raise NameError, "The first argument (#{name}) to share_as must be a legal name for a constant not already in use."
end

def share_as(name, &block)
  if Object.const_defined?(name)
    mod = Object.const_get(name)
    raise_name_error unless mod.created_from_caller(caller)
  end
  mod = Module.new do
    @shared_block = block
    @caller_line = caller.last
    def self.created_from_caller(other_caller)
      @caller_line == other_caller.last
    end
    def self.included(kls)
      kls.describe(&@shared_block)
      kls.children.first.metadata[:shared_group_name] = name
    end
  end
  shared_const = Object.const_set(name, mod)
  RSpec.world.shared_example_groups[shared_const] = block
end

def shared_examples *args, &block
  if key? args.first
    key = args.shift
    raise_key_taken key if key_taken? key
    RSpec.world.shared_example_groups[key] = block
  end
  unless args.empty?
    mod = Module.new
    (class << mod; self; end).send :define_method, :extended  do |host|
      host.class_eval &block
    end
    RSpec.configuration.extend mod, *args
  end
end