SuperMemo2 Repetition Spacing

datum.rb

class Datum < ActiveRecord::Base
  has_many :repetition_intervals
  has_many :difficulty_factors
  has_many :datum_responses

  after_create :initial_datum_setup

  def recent_repetition_interval
    repetition_intervals.last
  end

  def recent_difficulty_factor
    difficulty_factors.last
  end

  def recent_response
    datum_repsonses.last
  end

  private
  def initial_datum_setup
    update_attribute(:repetitions, 0)
    repetition_intervals.create(:state => 'first_repetition')
    difficulty_factors.create(:factor => 2.5)
  end
end

datum_response.rb

class DatumResponse < ActiveRecord::Base
  belongs_to :datum

  after_create :initial_datum_response_setup

  def next_repetition_interval_state
    datum.recent_repetition_interval.next_successful_state
  end
  
  def difficulty_factor
    datum.recent_difficulty_factor.factor
  end

  private
  def initial_datum_response_setup
    handle_difficulty_factor

    case quality
    when 0, 1, 2
      datum.repetitions = 0
      datum.save
      datum.repetition_intervals.create(:state => 'first_repetition')
    else
      datum.repetitions += 1
      datum.save
      datum.repetition_intervals.create(:state => next_repetition_interval_state)
    end
  end
  
  def handle_difficulty_factor
    new_factor = difficulty_factor + (0.1 - (5 - quality) * (0.08 + (5 - quality) * 0.02))
    if new_factor > 1.3
      datum.difficulty_factors.create(:factor => new_factor)
    else
      datum.difficulty_factors.create(:factor => 1.3)
    end
  end
end

difficulty_factor.rb

class DifficultyFactor < ActiveRecord::Base
  belongs_to :datum
end

repetition_interval.rb

class RepetitionInterval < ActiveRecord::Base
  belongs_to :datum

  state_machine :initial => :created do
    state :created
    state :first_repetition do
      def next_repetition_interval;1;end
    end
    
    state :second_repetition do
      def next_repetition_interval;6;end
    end

    state :extended_repetition do
      def next_repetition_interval;calculated_next_repetition_interval;end
    end
  end

  after_create :initial_interval_setup

  def next_successful_state
    (second_repetition? || extended_repetition?) ? 'extended_repetition' : 'second_repetition'
  end

  def calculated_next_repetition_interval
    ((datum.repetitions - 1) * difficulty_factor).round
  end

  def next_repetition_date
    created_at + next_repetition_interval.days
  end

  def difficulty_factor
    datum.recent_difficulty_factor.factor
  end

  private
  def initial_interval_setup
    update_attribute(:interval, next_repetition_interval)
    update_attribute(:next_repetition, next_repetition_date)
  end

end

tekrar_schema.rb

ActiveRecord::Schema.define(:version => 20110911190622) do

  create_table "data", :force => true do |t|
    t.string   "state"
    t.string   "description"
    t.text     "body"
    t.integer  "repetitions"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "datum_responses", :force => true do |t|
    t.integer  "datum_id"
    t.integer  "quality"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "difficulty_factors", :force => true do |t|
    t.integer  "datum_id"
    t.float    "factor"
    t.string   "state"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "repetition_intervals", :force => true do |t|
    t.string   "state"
    t.integer  "datum_id"
    t.integer  "interval"
    t.datetime "next_repetition"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end