From Best of Ruby Quiz by James Edward Gray II.

secret_santa.rb

# Randomly assigns secret santas. Will not match up two people with the same last name.

class SecretSanta
  attr_accessor :names

  def initialize(names_array)
    @names = names_array.map { |name| name.split(' ') }
    raise "Odd number of names!" if @names.size.odd?
    @assignments = {}
  end

  def run
    name_check
    assign
    display
    save
  end

  private

  def name_check
    last_name_count = {}
    last_names = @names.map { |name| name[1] }
    last_names.each { |name| last_name_count[name] ? last_name_count[name] += 1 : last_name_count[name] = 1 }
    raise "Incompatible last name count!" if last_name_count.values.any? { |x| x > last_names.size / 2 }
  end

  def assign
    santas = @names.shuffle
    @names.each do |name|
      santas.shuffle! until santas[-1][1] != name[1]
      assignment = santas.pop
      @assignments[name.join(" ")] = assignment.join(" ")
    end
  end

  def display
    @assignments.each { |name, assignment| puts "#{name} --> #{assignment}" }
  end

  def save
    File.open "secret_santa.txt", 'w' do |f|
      f.write "Name --> Assignment \n"
      @assignments.each { |name, assignment| f.write "#{name} --> #{assignment}" +  "\n" }
    end
    puts "Secret santa list saved as secret_santa.txt"
  end
end

names = []
while true
  name = gets.chomp
  break if name == ''
  names << name
end

list1 = SecretSanta.new(names)
list1.run

The assign function will occasionally go into an endless loop if the shuffle works out such that the only remaining names have the same last name. I need to figure out how to fix this.