electroenergy payment calculation

payment.rb

# encoding: utf-8
class Payment
  RATE = 3.8
  
  attr_accessor :previous, :current
  
  def initialize(params = {})
    @previous = params.delete(:previous)
    @current  = params.delete(:current)
  end
  
  def difference
    @current - @previous
  end
  
  def cost
    difference * RATE
  end
  
  def to_s
    "(#{@previous} -> #{@current}): #{difference} КВт * #{RATE} руб/КВт = #{cost} руб"
  end
  
  def self.for(amount, params = {})
    max_difference = (amount / RATE).floor
    if params[:previous]
      params[:current] = params[:previous] + max_difference
    elsif params[:current]
      params[:previous] = params[:current] - max_difference
    end
    
    new(params)
  end
end

puts Payment.new(previous: 38565, current: 39354)
puts Payment.for(3000, previous: 38565)
puts Payment.for(5000, current: 39354)