class Protobuf::Field::Sfixed64Field

def decode(bytes)
  # we don't use 'Q' for pack/unpack. 'Q' is machine-dependent.
  values = bytes.unpack('VV')
  value  = values[0] + (values[1] << 32)
  value -= 0x1_0000_0000_0000_0000 if (value & 0x8000_0000_0000_0000).nonzero?
  value
end