Recursively collect all descendants of a Person

descendants_spec.rb

require 'rspec'
require 'person'

describe "descendants" do
  it "returns all its descendants" do
    sonny = Person.new('Sonny')
    pa = Person.new('Pa', sonny)
    grandpa = Person.new('Grandpa', pa)

    expect(Person.all_descendants_of(grandpa).map(&:name)).to eq ['Pa', 'Sonny']
  end

  it "does not return children that aren't directly related" do
    sonny = Person.new('Sonny')
    pa = Person.new('Pa', sonny)
    ma = Person.new('Ma', sonny)
    grandpa = Person.new('Grandpa', pa)

    expect(Person.all_descendants_of(grandpa).map(&:name)).to eq ['Pa', 'Sonny']
    expect(Person.all_descendants_of(ma).map(&:name)).to eq ['Sonny']
  end

  it "does not return other people's children" do
    sonny = Person.new('Sonny')
    pa = Person.new('Pa', sonny)
    cousin_donny = Person.new('Donny')

    expect(Person.all_descendants_of(cousin_donny).map(&:name)).to eq []
  end
end

person.rb

class Person
  attr_reader :name, :children

  def initialize(name, children=[])
    @name, @children = name, Array(children)
  end

  def self.all_descendants_of(person, accumulated_children=[])
    person.children.each do |child|
      accumulated_children << child
      Person.all_descendants_of(child, accumulated_children)
    end
    accumulated_children
  end
end