# frozen_string_literal: true

# == Experimentation
#
# Utility module for A/B testing experimental features. Define your experiments in the `EXPERIMENTS` constant.
# Experiment options:
# - tracking_category (optional, used to set the category when tracking an experiment event)
# - rollout_strategy: default is `:cookie` based rollout. We may also set it to `:user` based rollout
#
# The experiment is controlled by a Feature Flag (https://docs.gitlab.com/ee/development/feature_flags/controls.html),
# which is named "#{experiment_key}_experiment_percentage" and *must* be set with a percentage and not be used for other purposes.
#
# To enable the experiment for 10% of the users:
#
# chatops: `/chatops run feature set experiment_key_experiment_percentage 10`
# console: `Feature.enable_percentage_of_time(:experiment_key_experiment_percentage, 10)`
#
# To disable the experiment:
#
# chatops: `/chatops run feature delete experiment_key_experiment_percentage`
# console: `Feature.remove(:experiment_key_experiment_percentage)`
#
# To check the current rollout percentage:
#
# chatops: `/chatops run feature get experiment_key_experiment_percentage`
# console: `Feature.get(:experiment_key_experiment_percentage).percentage_of_time_value`
#

# TODO: see https://gitlab.com/gitlab-org/gitlab/-/issues/217490
module Gitlab
  module Experimentation
    EXPERIMENTS = {
    }.freeze

    class << self
      def get_experiment(experiment_key)
        return unless EXPERIMENTS.key?(experiment_key)

        ::Gitlab::Experimentation::Experiment.new(experiment_key, **EXPERIMENTS[experiment_key])
      end

      def active?(experiment_key)
        experiment = get_experiment(experiment_key)
        return false unless experiment

        experiment.active?
      end

      def in_experiment_group?(experiment_key, subject:)
        return false if subject.blank?
        return false unless active?(experiment_key)

        log_invalid_rollout(experiment_key, subject)

        experiment = get_experiment(experiment_key)
        return false unless experiment

        experiment.enabled_for_index?(index_for_subject(experiment, subject))
      end

      def rollout_strategy(experiment_key)
        experiment = get_experiment(experiment_key)
        return unless experiment

        experiment.rollout_strategy
      end

      def log_invalid_rollout(experiment_key, subject)
        return if valid_subject_for_rollout_strategy?(experiment_key, subject)

        logger = Gitlab::ExperimentationLogger.build
        logger.warn message: 'Subject must conform to the rollout strategy',
                     experiment_key: experiment_key,
                     subject: subject.class.to_s,
                     rollout_strategy: rollout_strategy(experiment_key)
      end

      def valid_subject_for_rollout_strategy?(experiment_key, subject)
        case rollout_strategy(experiment_key)
        when :user
          subject.is_a?(User)
        when :group
          subject.is_a?(Group)
        when :cookie
          subject.nil? || subject.is_a?(String)
        else
          false
        end
      end

      private

      def index_for_subject(experiment, subject)
        index = Zlib.crc32("#{experiment.key}#{subject_id(subject)}")

        index % 100
      end

      def subject_id(subject)
        if subject.respond_to?(:to_global_id)
          subject.to_global_id.to_s
        elsif subject.respond_to?(:to_s)
          subject.to_s
        else
          raise ArgumentError, 'Subject must respond to `to_global_id` or `to_s`'
        end
      end
    end
  end
end