Describing Equivalence Classes in Ruby with RSpec

Here’s an article I wrote a while ago but didn’t get around to releasing.  Enjoy. 

Describing Equivalence Classes in Ruby with RSpec

Since writing that article, I’ve made good use of the code. Here’s an example of it in action: grouping the cards in a deck into their types. A second method then uses those groups to reassemble the cards into an ordered list.

  CARD_TYPES = ['basic land', 'land', 'creature', 'artifact',
                'enchantment', 'sorcery', 'instant', '']

  def maindeck_cards_grouped
    maindeck_cards.equivalence_classes(*CARD_TYPES) do |card, the_type|
      card.magic_card.card_type.downcase.include?(the_type)
    end
  end

  def maindeck_cards_ordered
    cards_by_class = maindeck_cards_grouped
    (CARD_TYPES.inject([]) {|cards, type| cards << cards_by_class[type]}).flatten
  end