module Periodicity
  class Period
    def initialize(period, at: nil)
      @period = period
      @at_hour, @at_min = parse_at(at) if at
    end

    def next
      @period.from_now.change({hour: @at_hour, min: @at_min}.compact)
    end

    def parse_at(at)
      case at
      when String
        time = Time.parse(at)
        return time.hour, time.min
      when Hash
        return at[:hour], at[:min]
      else
        raise "Unknown 'at' format"
      end
    end
  end
end