module Tins::StringByteOrderMark

def bom_encoding
  prefix = self[0, 4].force_encoding(Encoding::ASCII_8BIT)
  case prefix
  when /\A\xef\xbb\xbf/n                    then Encoding::UTF_8
  when /\A\x00\x00\xff\xfe/n                then Encoding::UTF_32BE
  when /\A\xff\xfe\x00\x00/n                then Encoding::UTF_32LE
  when /\A\xfe\xff/n                        then Encoding::UTF_16BE
  when /\A\xff\xfe/n                        then Encoding::UTF_16LE
  when /\A\x2b\x2f\x76[\x38-\x39\x2b\x2f]/n then Encoding::UTF_7
  when /\A\x84\x31\x95\x33/n                then Encoding::GB18030
  end
end