pearson.rb

class Recommendations::Metrics::Pearson < Recommendations::Metrics::MetricBase
  def compare(user_id, user_rates, sampler_id, sampler_rates)
    return 0 unless user_rates && sampler_rates
    shared_ids = user_rates.keys & sampler_rates.keys
    return 0 if shared_ids.empty?# || shared_ids.size < MinimumShared

    sum1 = sum2 = sum1Sq = sum2Sq = pSum = 0.0

    shared_ids.each do |id|
      prefs1_item = user_rates[id] || 0.0
      prefs2_item = sampler_rates[id] || 0.0

      sum1   += prefs1_item
      sum2   += prefs2_item
      sum1Sq += prefs1_item ** 2
      sum2Sq += prefs2_item ** 2
      pSum   += prefs2_item * prefs1_item
    end

    num = pSum - ( ( sum1 * sum2 ) / shared_ids.size )
    den = Math.sqrt( ( sum1Sq - ( sum1 ** 2 ) / shared_ids.size ) * ( sum2Sq - ( sum2 ** 2 ) / shared_ids.size ) )

    return 0 if den == 0

    num / den
  end
end