# Returns stats on the current SLA performance of queues in a sidekiq instance.
# Assumes workers are not bound to queues
require 'sidekiq/api'
require 'active_support'
require 'active_support/core_ext/numeric'
module RooOnRails
  module Sidekiq
    class SlaMetric
      def self.queue
        queues = ::Sidekiq::Queue.all.map { |q| SidekiqQueueMetrics.new(q) }
        global_stats = SidekiqWorkerCount.new(queues)
        global_stats.requested_processes
      end

      class SidekiqQueueMetrics
        extend Forwardable
        def_delegators :@queue, :size, :latency, :name
        attr_reader :queue

        def initialize(queue)
          @queue = queue
        end

        def normalised_latency
          metric = queue.latency.to_f / permitted_latency
          metric.round(3)
        end

        def permitted_latency
          prefix, number, unit = queue.name.partition(/[0-9]+/)
          case prefix
          when 'monitoring', 'realtime' then 10.seconds.to_i
          when 'default' then 1.day.to_i
          when 'within' then number.to_i.public_send(unit.to_sym).to_i
          else raise "Cannot determine permitted latency for queue #{queue.name}"
          end
        end
      end

      class SidekiqWorkerCount
        def initialize(metrics)
          @metrics = metrics
        end

        def current_processes
          ::Sidekiq::ProcessSet.new.count
        end

        def max_normalised_latency
          @metrics.any? ? @metrics.map(&:normalised_latency).max : 0
        end

        def requested_processes
          if max_normalised_latency > increasing_latency
            current_processes + 1
          elsif max_normalised_latency < decreasing_latency
            [current_processes - 1, 1].max
          else
            current_processes
          end
        end

        protected

        def increasing_latency
          ENV.fetch('WORKER_INCREASE_THRESHOLD', 0.5).to_f
        end

        def decreasing_latency
          ENV.fetch('WORKER_DECREASE_THRESHOLD', 0.1).to_f
        end
      end
    end
  end
end