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.
    #
    class Derive < DataSource
      def flush
        each do |ns, origin, ts|
          values = ts.each_cons(2).map do |a, b|
            (b[1].last - a[1].last)/(b[0] - a[0]) # ∆value / ∆ts
          end.reject do |value|
            value < 0 # ignore overflow/counter wrap
          end

          yield ns, origin, values
        end
      end
    end
  end
end