require "date"

class Tariff
  attr_reader :kilowatt_hour_rate # Arbeitspreis
  attr_reader :basic_rate # Grundpreis
  attr_reader :month

  def initialize(kilowatt_hour_rate:, basic_rate:, month:)
    @kilowatt_hour_rate = kilowatt_hour_rate
    @basic_rate = basic_rate
    @month = month
  end
end

class Reading
  attr_reader :date
  attr_reader :kilowatts

  def initialize(date:, kilowatts:)
    @date = date
    @kilowatts = kilowatts
  end
end

class MonthlyConsumption
  def self.calculate_for(month:, tariffs:, readings:)
    start_reading = readings.find { |reading| reading.date == Date.new(month.year, month.month, 1) }
    end_reading = readings.find { |reading| reading.date == Date.new(month.next_month.year, month.next_month.month, 1) }
    kilowatts = end_reading.kilowatts - start_reading.kilowatts

    tariff = tariffs.take_while { |tariff| tariff.month < month }.last

    price = tariff.basic_rate + kilowatts * tariff.kilowatt_hour_rate

    "In #{month.strftime('%B %Y')} you consumed #{kilowatts} kilowatts for #{price / 100} Euro"
  end
end

tariffs = [
  Tariff.new(
    kilowatt_hour_rate: 35,
    basic_rate: 1200,
    month: Date.new(2022, 1, 1)
  ),
  Tariff.new(
    kilowatt_hour_rate: 37,
    basic_rate: 1200,
    month: Date.new(2022, 5, 1)
  ),
  Tariff.new(
    kilowatt_hour_rate: 30,
    basic_rate: 1200,
    month: Date.new(2022, 7, 1)
  )
]

current_reading = 3220

readings = (Date.new(2022, 1, 1)...Date.new(2022, 12, 31)).map do |date|
  current_reading += [3, 4, 6].sample
  Reading.new(
    date: date,
    kilowatts: current_reading
  )
end

puts MonthlyConsumption.calculate_for(month: Date.new(2022, 6), tariffs: tariffs, readings: readings)