module OEmbed::Formatter::Base

def already_loaded?(new_backend)
  begin
    self::Backends.const_defined?(new_backend, false)
  rescue ArgumentError # we're dealing with ruby < 1.9 where const_defined? only takes 1 argument, but behaves the way we want it to.
    self::Backends.const_defined?(new_backend)
  rescue NameError # no backends have been loaded yet
    false
  end
end

def backend=(new_backend)
  new_backend_obj = case new_backend
  when String
    unless already_loaded?(new_backend)
      load "oembed/formatter/#{backend_path}/#{new_backend.downcase}.rb"
    end
    self::Backends.const_get(new_backend)
  else
    new_backend
  end
  test_backend(new_backend_obj)
  @backend = new_backend_obj
rescue
  raise LoadError, "There was an error setting the backend: #{new_backend.inspect} - #{$!.message}"
end

def backend_path
  raise "This method must be defined by a format-specific OEmbed::Formatter sub-class."
end

def decode(value)
  backend.decode(value)
end

def supported?
  !!backend
end

def test_backend(new_backend)
  raise LoadError, "The given backend must respond to the decode method: #{new_backend.inspect}" unless new_backend.respond_to?(:decode)
  expected = {
    "version"=>1.0,
    "string"=>"test",
    "int"=>42,
    "html"=>"<i>Cool's</i>\n the \"word\"!",
  }
  actual = new_backend.decode(test_value)
  # For the test to be true the actual output Hash should have the
  # exact same list of keys _and_ the values should be the same
  # if we ignoring typecasting.
  if(
    actual.keys.sort != expected.keys.sort ||
    actual.detect { |key, value| value.to_s != expected[key].to_s }
  )
    msg = new_backend.decode_fail_msg rescue nil
    msg ||= "The given backend failed to decode the test string correctly"
    raise LoadError, "#{msg}: #{new_backend.inspect}"
  end
end

def test_value
  raise "This method must be defined by a format-specific OEmbed::Formatter sub-class."
end

def with_backend(new_backend)
  old_backend, self.backend = backend, new_backend
  yield
ensure
  self.backend = old_backend
end