CSV#determine_encoding

# File tmp/rubies/ruby-3.3.0/lib/csv.rb, line 2730
def determine_encoding(encoding, internal_encoding)
  # honor the IO encoding if we can, otherwise default to ASCII-8BIT
  io_encoding = raw_encoding
  return io_encoding if io_encoding

  return Encoding.find(internal_encoding) if internal_encoding

  if encoding
    encoding, = encoding.split(":", 2) if encoding.is_a?(String)
    return Encoding.find(encoding)
  end

  Encoding.default_internal || Encoding.default_external
end