Clock Exercise Solution

clock.rb

class Clock
  attr_accessor :hours, :minutes

  def initialize(hours, minutes)
    @hours, @minutes = hours, minutes
  end

  def self.at(hours=0, minutes=0)
    new(hours, minutes)
  end

  def to_s
    "#{format(@hours)}:#{format(@minutes)}"
  end

  def +(minutes)
    minutes.times do
      @minutes += 1
      change_hour_and_adjust_minutes(:+, 1) if @minutes == 61
      @hours = 0 if wrap_around?
    end

    self
  end

  def -(minutes)
    minutes.times do
      @minutes -= 1
      change_hour_and_adjust_minutes(:-, 59) if @minutes == -1
      @hours = 23 if wrap_around?
    end

    self
  end

  def ==(other)
    @minutes == other.minutes && @hours == other.hours
  end

  private

  def format(hours_or_minutes)
    result = "00"
    result << hours_or_minutes.to_s
    result = result.slice(-2..-1)
  end

  def change_hour_and_adjust_minutes(operation, minutes)
    @hours = @hours.send(operation, 1)
    @minutes = minutes
  end

  def wrap_around?
    @hours == -1 || @hours == 24
  end
end