# encoding: utf-8

# http://xlinux.nist.gov/dads/HTML/reservoirSampling.html

require 'one_apm/support/event_buffer'

module OneApm
  module Agent
    class SampledBuffer < EventBuffer
      attr_reader :seen_lifetime, :captured_lifetime

      def initialize(capacity)
        super
        @captured_lifetime = 0
        @seen_lifetime     = 0
      end

      def reset!
        @captured_lifetime += @items.size
        @seen_lifetime     += @seen
        super
      end

      def append_event(x)
        if @items.size < @capacity
          @items << x
          return x
        else
          m = rand(@seen) # [0, @seen)
          if m < @capacity
            @items[m] = x
            return x
          else
            # discard current sample
            return nil
          end
        end
      end

      def sample_rate_lifetime
        @captured_lifetime > 0 ? (@captured_lifetime.to_f / @seen_lifetime) : 0.0
      end
    end
  end
end