module FieldTest
  class Experiment
    attr_reader :id, :name, :description, :variants, :weights, :winner, :started_at, :ended_at

    def initialize(attributes)
      attributes = attributes.symbolize_keys
      @id = attributes[:id]
      @name = attributes[:name] || @id.to_s.titleize
      @description = attributes[:description]
      @variants = attributes[:variants]
      @weights = @variants.size.times.map { |i| attributes[:weights].to_a[i] || 1 }
      @winner = attributes[:winner]
      @started_at = Time.zone.parse(attributes[:started_at].to_s) if attributes[:started_at]
      @ended_at = Time.zone.parse(attributes[:ended_at].to_s) if attributes[:ended_at]
    end

    def variant(participants, options = {})
      return winner if winner
      return variants.first if options[:exclude]

      participants = FieldTest::Participant.standardize(participants)
      check_participants(participants)
      membership = membership_for(participants) || FieldTest::Membership.new(experiment: id)

      if options[:variant] && variants.include?(options[:variant])
        membership.variant = options[:variant]
      else
        membership.variant ||= weighted_variant
      end

      # upgrade to preferred participant
      membership.participant = participants.first

      if membership.changed?
        begin
          membership.save!

          # log it!
          info = {
            experiment: id,
            variant: membership.variant,
            participant: membership.participant
          }.merge(options.slice(:ip, :user_agent))

          # sorta logfmt :)
          info = info.map { |k, v| v = "\"#{v}\"" if k == :user_agent; "#{k}=#{v}" }.join(" ")
          Rails.logger.info "[field test] #{info}"
        rescue ActiveRecord::RecordNotUnique
          membership = memberships.find_by(participant: participants.first)
        end
      end

      membership.try(:variant) || variants.first
    end

    def convert(participants)
      participants = FieldTest::Participant.standardize(participants)
      check_participants(participants)
      membership = membership_for(participants)

      if membership
        membership.converted = true
        membership.save! if membership.changed?
        true
      else
        false
      end
    end

    def memberships
      FieldTest::Membership.where(experiment: id)
    end

    def results
      data = memberships.group(:variant).group(:converted)
      data = data.where("created_at >= ?", started_at) if started_at
      data = data.where("created_at <= ?", ended_at) if ended_at
      data = data.count
      results = {}
      variants.each do |variant|
        converted = data[[variant, true]].to_i
        participated = converted + data[[variant, false]].to_i
        results[variant] = {
          participated: participated,
          converted: converted,
          conversion_rate: participated > 0 ? converted.to_f / participated : nil
        }
      end
      case variants.size
      when 1, 2, 3
        total = 0.0

        (variants.size - 1).times do |i|
          c = results.values[i]
          b = results.values[(i + 1) % variants.size]
          a = results.values[(i + 2) % variants.size]

          alpha_a = 1 + a[:converted]
          beta_a = 1 + a[:participated] - a[:converted]
          alpha_b = 1 + b[:converted]
          beta_b = 1 + b[:participated] - b[:converted]
          alpha_c = 1 + c[:converted]
          beta_c = 1 + c[:participated] - c[:converted]

          # TODO calculate this incrementally by caching intermediate results
          prob_winning =
            if variants.size == 2
              cache_fetch ["field_test", "prob_b_beats_a", alpha_b, beta_b, alpha_c, beta_c] do
                Calculations.prob_b_beats_a(alpha_b, beta_b, alpha_c, beta_c)
              end
            else
              cache_fetch ["field_test", "prob_c_beats_a_and_b", alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c] do
                Calculations.prob_c_beats_a_and_b(alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c)
              end
            end

          results[variants[i]][:prob_winning] = prob_winning
          total += prob_winning
        end

        results[variants.last][:prob_winning] = 1 - total
      end
      results
    end

    def active?
      !winner
    end

    def self.find(id)
      experiment = all.index_by(&:id)[id.to_s]
      raise FieldTest::ExperimentNotFound unless experiment

      experiment
    end

    def self.all
      FieldTest.config["experiments"].map do |id, settings|
        FieldTest::Experiment.new(settings.merge(id: id.to_s))
      end
    end

    private

      def check_participants(participants)
        raise FieldTest::UnknownParticipant, "Use the :participant option to specify a participant" if participants.empty?
      end

      def membership_for(participants)
        memberships = self.memberships.where(participant: participants).index_by(&:participant)
        participants.map { |part| memberships[part] }.compact.first
      end

      def weighted_variant
        total = weights.sum.to_f
        pick = rand
        n = 0
        weights.map { |w| w / total }.each_with_index do |w, i|
          n += w
          return variants[i] if n >= pick
        end
        variants.last
      end

      def cache_fetch(key)
        if FieldTest.cache
          Rails.cache.fetch(key.join("/")) { yield }
        else
          yield
        end
      end
  end
end