Martian anniversaries

martian_anniversaries.rb

# Happy birthday to you...
#
# @example
#   dates = MartianAnniversaries.from '2015-01-01'
#   dates.take(2)
#   # => ['2016-11-17', '2018-10-05']
#
class MartianAnniversaries
  include Enumerable

  attr_reader :birthday

  def self.from(str)
    new(str)
  end

  def initialize(str)
    @birthday = Date.parse(str).to_time
  end

  def each
    to_enum unless block_given?
    yield format(@anniversary = birthday + martian_year)
    loop { yield format(@anniversary += martian_year) }
  end

  private

  def format(seconds)
    Time.at(seconds).to_date.strftime '%Y-%m-%d'
  end

  def martian_day
    @martian_day ||= 88775.244
  end

  def martian_year
    @martian_year ||= 668.5907 * martian_day
  end
end