How to build your own Grader?

Introduction

Graders are plain old ruby class that returns a probability number between 0 and 1 which represent the likely hood of the description being rated to be considered good or not. The probability returned by each grader will be multiplied together to generate a final probability in order for LinkThumbnailer to sort the description candidates between each other.

Here is an example of a "random" grader that return a 100% probability or a 0% probability randomly:

module LinkThumbnailer
  module Graders
    class Random < ::LinkThumbnailer::Graders::Base

      def call
        if rand(2) == 0
          1.0
        else
          0.0
        end
      end

    end
  end
end

There are a couple things to notice here:

• Graders should live in LinkThumbnailer::Graders namespace
• Graders must inherit from LinkThumbnailer::Graders::Base
• Graders must respond to one public method called call
• Graders must return a Float number between 0 and 1 representing a probability.
• Graders have access to global and runtime configurations using a private method called config
• Graders have access to the description being rated using a private method called text

When instantiating a new grader, it should receive a LinkThumbnailer::Models::Description instance. The LinkThumbnailer::Graders::Base class from which all graders should inherit give you access to the description string with the text private method. You can also access the Nokogiri node using the private node method.

Note: Since each graders can specify a probability weight, you don't have to worry about it here. It will be automatically applied to the computation of the final score.