module NetUsage
  class CalcDelta
    def initialize(iface)
      @iface = iface
      @filePath = '/proc/net/dev'
      @rxCount, @txCount = readCounts
    end

    def readCounts
      # Read the current bytecounts from the specified file.
      pndFile = File.open(@filePath)
      pndFile.each_line(){|line|
        if line.index(@iface)
          # There may or may not be a space before the first number
          # depending on how many digits the number has. Hence the
          # more complicated split regexp.
          fields = line.strip.split(/[:\s]+/)
          rxCount = fields[1].to_i
          txCount = fields[9].to_i
          return [rxCount, txCount]
        end
      }
      return nil
    end

    def usage
      rxCount, txCount = readCounts

      rxUsage = rxCount - @rxCount
      # Handle wraparound:
      if rxCount < @rxCount
        rxUsage += 2**32
      end

      txUsage = txCount - @txCount
      # Handle wraparound:
      if txCount < @txCount
        txUsage += 2**32
      end

      @rxCount = rxCount
      @txCount = txCount

      return rxUsage + txUsage
    end
  end
end