people_spec.rb

require 'rspec'

describe 'people joining' do
  it 'has proper return values' do
    expectations = [
      [[''], ''],
      [['johnny'], "johnny"],
      [['johnny', 'teddy'], "johnny and teddy"],
      [['johnny', 'teddy', 'lui'], "johnny, teddy and lui"],
      [['johnny', 'teddy', 'lui', 'marco'], "johnny, teddy, lui and marco"],
    ]

    aggregate_failures do
      expectations.each do |input, expected|
        actual = friendly(input)
        expect(actual).to eql(expected)
      end
    end
  end

  def friendly(array)
    return array[0] if array.size == 1

    ending = array.pop(2).join(' and ')
    friendly = array[0..array.size-1].push(ending)

    friendly.join(', ')
  end
end