defmodule Fast.Guards do
  defguard is_uuid4(s)
           # hyphens at 8, 13, 18, 23
           # version nibble '4' at pos 14
           # variant nibble 8|9|a|b (case-insensitive) at pos 19
           when is_binary(s) and byte_size(s) == 36 and
                  binary_part(s, 8, 1) == "-" and
                  binary_part(s, 13, 1) == "-" and
                  binary_part(s, 18, 1) == "-" and
                  binary_part(s, 23, 1) == "-" and
                  binary_part(s, 14, 1) == "4" and
                  (binary_part(s, 19, 1) == "8" or
                     binary_part(s, 19, 1) == "9" or
                     binary_part(s, 19, 1) == "a" or
                     binary_part(s, 19, 1) == "A" or
                     binary_part(s, 19, 1) == "b" or
                     binary_part(s, 19, 1) == "B")
end