chapter_3.rb

############## Page 36 ##############
class Gear
  attr_reader :chainring, :cog, :rim, :tire
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog       = cog
    @rim       = rim
    @tire      = tire
  end

  def gear_inches
    ratio * Wheel.new(rim, tire).diameter
  end

  def ratio
    chainring / cog.to_f
  end
# ...
end

class Wheel
  attr_reader :rim, :tire
  def initialize(rim, tire)
    @rim       = rim
    @tire      = tire
  end

  def diameter
    rim + (tire * 2)
  end
# ...
end

Gear.new(52, 11, 26, 1.5).gear_inches

############## Page 39 ##############
class Gear
  attr_reader :chainring, :cog, :rim, :tire
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog       = cog
    @rim       = rim
    @tire      = tire
  end

  def gear_inches
    ratio * Wheel.new(rim, tire).diameter
  end
# ...
end

Gear.new(52, 11, 26, 1.5).gear_inches

############## Page 41 ##############
class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(chainring, cog, wheel)
    @chainring = chainring
    @cog       = cog
    @wheel     = wheel
  end

  def gear_inches
    ratio * wheel.diameter
  end
# ...
end

# Gear expects a 'Duck' that knows 'diameter'
Gear.new(52, 11, Wheel.new(26, 1.5)).gear_inches

############## Page 43 ##############
class Gear
  attr_reader :chainring, :cog, :rim, :tire
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog       = cog
    @wheel     = Wheel.new(rim, tire)
  end

  def gear_inches
    ratio * wheel.diameter
  end
# ...

############## Page 42 ##############
class Gear
  attr_reader :chainring, :cog, :rim, :tire
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog       = cog
    @rim       = rim
    @tire      = tire
  end

  def gear_inches
    ratio * wheel.diameter
  end

  def wheel
    @wheel ||= Wheel.new(rim, tire)
  end
# ...

############## Page 44 ##############
def gear_inches
  ratio * wheel.diameter
end

############## Page 44 ##############
def gear_inches
  #... a few lines of scary math
  foo = some_intermediate_result * wheel.diameter
  #... more lines of scary math
end

############## Page 45 ##############
def gear_inches
  #... a few lines of scary math
  foo = some_intermediate_result * diameter
  #... more lines of scary math
end

def diameter
  wheel.diameter
end

############## Page 46 ##############
# The point illustrated in this is to remove argument-order dependencies.
# That is, initialize must be run prior to any other methods in order for
# the other methods that are DEPENDENT on them to work properly.
class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(chainring, cog, wheel)
    @chainring = chainring
    @cog       = cog
    @wheel     = wheel
  end
  ...
end

Gear.new(
  52,
  11,
  Wheel.new(26, 1.5)).gear_inches

############## Page 47 ##############
# This problem is solved in the below example. This uses the special "args" hash 
# and passes in the arguments that need to be initialized.
class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(args)
    @chainring = args[:chainring]
    @cog       = args[:cog]
    @wheel     = args[:wheel]
  end
  ...
end

Gear.new(
  :chainring => 52,
  :cog       => 11,
  :wheel     => Wheel.new(26, 1.5)).gear_inches

############## Page 48 ##############
  # specifying defaults using ||
  def initialize(args)
    @chainring = args[:chainring] || 40
    @cog       = args[:cog]       || 18
    @wheel     = args[:wheel]
  end

############## Page 49 ##############
  # specifying defaults using fetch
  # this is the preferred method for setting a default because it anticipates that the element is in the hash and is extant
  def initialize(args)
    @chainring = args.fetch(:chainring, 40)
    @cog       = args.fetch(:cog, 18)
    @wheel     = args[:wheel]
  end

############## Page 49 ##############
  # specifying defaults by merging a defaults hash
  def initialize(args)
    args = defaults.merge(args)
    @chainring = args[:chainring]
#   ...
  end

  def defaults
    {:chainring => 40, :cog => 18}
  end

############## Page 50 ##############
# Isolating multiparameter initialization is a tactic that helps when interacting with methods that you do not own
# (and therefore cannot change) and require fixed-order arguments 

# When Gear is part of an external interface
module SomeFramework
  class Gear
    attr_reader :chainring, :cog, :wheel
    def initialize(chainring, cog, wheel)
      @chainring = chainring
      @cog       = cog
      @wheel     = wheel
    end
  # ...
  end
end

# wrap the interface to protect yourself from changes
module GearWrapper
  def self.gear(args)
    SomeFramework::Gear.new(args[:chainring],
                            args[:cog],
                            args[:wheel])
  end
end

# Now you can create a new Gear using an arguments hash.
GearWrapper.gear(
  :chainring => 52,
  :cog       => 11,
  :wheel     => Wheel.new(26, 1.5)).gear_inches

# An object that's sole purpose is to create other objects is called a factory.

############## Page ?? ##############
# This is the complete code for example above
module SomeFramework
  class Gear
    attr_reader :chainring, :cog, :wheel
    def initialize(chainring, cog, wheel)
      @chainring = chainring
      @cog       = cog
      @wheel     = wheel
    end

    def gear_inches
      ratio * wheel.diameter
    end

    def ratio
      chainring / cog.to_f
    end
  end
end

class Wheel
  attr_reader :rim, :tire
  def initialize(rim, tire)
    @rim       = rim
    @tire      = tire
  end

  def diameter
    rim + (tire * 2)
  end
end

module GearWrapper
  def self.gear(args)
    SomeFramework::Gear.new(args[:chainring],
                            args[:cog],
                            args[:wheel])
  end
end

GearWrapper.gear(
  :chainring => 52,
  :cog       => 11,
  :wheel     => Wheel.new(26, 1.5)).gear_inches

############## Page 52 ##############
# Dependencies always have a direction - one way to manage this is to reverse the direction.

class Gear
  attr_reader :chainring, :cog
  def initialize(chainring, cog)
    @chainring = chainring
    @cog       = cog
  end

  def gear_inches(diameter)
    ratio * diameter
  end

  def ratio
    chainring / cog.to_f
  end
#  ...
end

class Wheel
  attr_reader :rim, :tire, :gear
  def initialize(rim, tire, chainring, cog)
    @rim       = rim
    @tire      = tire
    @gear      = Gear.new(chainring, cog)
  end

  def diameter
    rim + (tire * 2)
  end

  def gear_inches
    gear.gear_inches(diameter)
  end
#  ...
end

Wheel.new(26, 1.5, 52, 11).gear_inches

# IMPORTANT - Depend on things that change less often than you do