require 'ganymed/sampler/datasource'

module Ganymed
  class Sampler

    ##
    # A Derive {DataSource data source} will store the derivative of the line
    # going from the last to the current value of the data source. Derive is
    # similar to a {Counter} {DataSource data source} in that it produces a
    # rate/s gauge, but only accepts absolute values and produces the
    # derivative instead of summation of all samples.
    #
    # In order to get (meaningful) realtime events from a Derive {DataSource}
    # samples must be sent multiple times per second. Derive does not store the
    # previously flushed value for performance reasons and therefore cannot
    # infer a rate/s gauge with only one event per flush.
    #
    class Derive < DataSource
      def flush(tick, &block)
        each(tick) do |ns, origin, events|
          values = events.sort_by do |event| # sort by timestamp
            event[0]
          end.each_cons(2).map do |a, b| # derive each consective sample
            (b[1] - a[1])/(b[0] - a[0]) # ∆value / ∆ts
          end

          yield ns, origin, values
        end
      end

      def feed(ns, origin, ts, value)
        add(ns, origin, [ts, value])
      end
    end
  end
end