Generates 90-ball UK bingo / 'housie' cards. The cards contain three rows and nine columns. Each row contains five numbers and four blank spaces randomly distributed along the row. Numbers are apportioned by column (1–9, 10–19, 20–29, 30–39, 40–49, 50–59, 60–69, 70–79 and 80–90).

bingo-card.rb

require 'matrix'

class Card 
    def self.generate
        collection = []
        for group in 0..8
          set = []
          while set.length < 3
            x = rand(10) + (group * 10) + 1
            set.push(x) unless set.include?(x)
          end
          collection += set
        end

        collection.sort!
        grouped_collections = ::Matrix.rows(collection.in_groups_of(3)).t.to_a
        nulled_collection = []
        
        grouped_collections.each do |grouped_collection|
            null_indexes = (0...9).sort_by{rand}[0,4].sort     
            nulled_group = []
            grouped_collection.each_with_index do |n, index|
                if null_indexes.include?(index)
                    nulled_group.push(nil)
                else
                    nulled_group.push(n)
                end 
            end
            nulled_collection.push(nulled_group)
        end
        ::Matrix.rows(nulled_collection).t.to_a.flatten
    end
end