mtortonesi/sisfc

# frozen_string_literal: true

require_relative './request'


module SISFC
  class Statistics
    attr_reader :mean, :n, :received, :longer_than
    alias_method :closed, :n

    # see http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Online_algorithm
    # and https://www.johndcook.com/blog/standard_deviation/
    def initialize(opts={})
      @n    = 0 # number of requests
      @mean = 0.0
      @m_2  = 0.0
      @longer_than = init_counters_for_longer_than_stats(opts)
      @received = 0
    end

    def request_received
      @received += 1
    end

    def record_request(req)
      # get new sample
      x = req.ttr
      raise "TTR #{x} for request #{req.rid} invalid!" unless x > 0.0

      @longer_than.each_key do |k|
        @longer_than[k] += 1 if x > k
      end

      # update counters
      @n += 1
      delta = x - @mean
      @mean += delta / @n
      @m_2  += delta * (x - @mean)
    end

    def variance
      @m_2 / (@n - 1)
    end

    def to_s
      "received: #{@received}, closed: #{@n}, " +
      "(mean: #{@mean}, variance: #{variance}, longer_than: #{@longer_than.to_s})"
    end

    private
      def init_counters_for_longer_than_stats(custom_kpis_config)
        # prepare an infinite length enumerator that always returns zero
        zeros = Enumerator.new(){|x| loop do x << 0 end }

        Hash[
          # wrap the values in custom_kpis_config[:longer_than] in an array
          Array(custom_kpis_config[:longer_than]).
            # and interval the numbers contained in that array with zeroes
            zip(zeros) ]
      end

  end
end