subarray / 2d-array / cross-array combinations in ruby

subarray_combinations.rb

# subarray_combinations([['a', 'b'], [1, 2]])
# # => [['a', 1], ['a', 2], ['b', 1], ['b', 2]]

def subarray_combinations(array_of_arrays, index = 0)
  return [] if index == array_of_arrays.size

  subarray = array_of_arrays[index]
  tails = subarray_combinations(array_of_arrays, index + 1)

  if tails.empty?
    subarray.map { |item| [item] }
  else
    subarray.each_with_object([]) do |item, paths|
      tails.each { |tail| paths << [item, *tail] }
    end
  end
end