Secrete agent observes a PIN, but may not have it right. So where are the arrays of arrays of arrays ...?

luke.rb

# Here is a slightly earlier version of the code with added variables for clarity (I hope)
  def pins(str)
    variations = str.split('').map { |c| @board[c] }
    return variations.first if str.length <= 1
    position, *positions = *(-str.length..-1).to_a
    nested_arrays = positions.inject(variations[position]) do |result, index|
      result.product(variations[index])
    end
    nested_arrays.map(&:flatten).map(&:join)
  end

# nested_arrays before flatten and join above
[[[["1", "3"], "5"], "7"],
 [[["1", "3"], "5"], "4"],
 [[["1", "3"], "5"], "8"],
  ...
 [[["4", "6"], "8"], "7"],
 [[["4", "6"], "8"], "4"],
 [[["4", "6"], "8"], "8"]]