# Copyright (c) 2020 Contrast Security, Inc. See https://www.contrastsecurity.com/enduser-terms-0317a for more details.
# frozen_string_literal: true

cs__scoped_require 'singleton'
cs__scoped_require 'contrast/components/interface'

module Contrast
  module Utils
    module Assess
      # SamplingUtil has methods for tracking the number of requests per time window
      class SamplingUtil
        include Singleton

        include Contrast::Components::Interface
        access_component :sampling

        def initialize
          @requests = {}
        end

        def update
          self.class.reset_sampling_control
          @enabled            = self.class.sampling_enabled?
          @baseline           = self.class.sampling_control[:baseline]
          @request_frequency  = self.class.sampling_control[:request_frequency]
          @response_frequency = self.class.sampling_control[:response_frequency]
          @window_ms          = self.class.sampling_control[:window]

          true
        end

        def sample? request
          history = request_history(request)
          history.hit

          # if sampling isn't enabled, we record all requests and take a
          # default amount of responses
          return [true, true] unless @enabled

          # if we've exceeded this sample window, reset it
          if history.elapsed >= @window_ms
            history.reset_window
            return [true, true]
          end

          # we have to take a baseline of this request/ response combo. we only
          # baseline the first response, but we'll do full request analysis up
          # to the baseline amount
          return [true, history.window_hit == 1] if history.window_hit < @baseline

          # Once the window_hit exceeds the baseline, we limit the analysis
          # based on @request_frequency and @response_frequency.
          non_baseline = history.window_hit - @baseline
          analyze_request = (non_baseline % @request_frequency).zero?
          analyze_response = (non_baseline % @response_frequency).zero?

          [analyze_request, analyze_response]
        end

        private

        # RequestHistory tracks one requests history per time window
        class RequestHistory
          attr_reader :window_start, :window_hit

          def initialize
            @window_start = Contrast::Utils::Timer.now_ms
            @window_hit = 0
          end

          def elapsed
            Contrast::Utils::Timer.now_ms - window_start
          end

          def hit cnt = 1
            @window_hit += cnt
          end

          def reset_window
            @window_start = Contrast::Utils::Timer.now_ms
            @window_hit = 0
          end
        end

        def request_history request
          @requests[request.hash_id] ||= RequestHistory.new
        end
      end
    end
  end
end