Combinations - https://repl.it/GE1s/0

Combinations.rb

def combinations(lengths)
  return nil if lengths.nil?
  (0..lengths.reduce(:*) - 1).map do |line|
    (0..lengths.count - 1).map do |row|
      sub_pos = lengths.slice(row + 1, lengths.count).reduce(:*) || 1
      line / sub_pos % lengths[row]
    end
  end
end

combinations [2,2,2]
# returns [[0, 0, 0], [0, 0, 1], [0, 1, 0], [0, 1, 1], [1, 0, 0], [1, 0, 1], [1, 1, 0], [1, 1, 1]]