class Protobuf::Decoder

def self.decode_each_field(stream, &block)
  until stream.eof?
    tag, bytes = read_field(stream)
    block.call(tag, bytes)
  end
end

def self.read_field(stream)
  tag, wire_type = read_key(stream)
  bytes = case wire_type
          when ::Protobuf::WireType::VARINT then
            read_varint(stream)
          when ::Protobuf::WireType::FIXED64 then
            read_fixed64(stream)
          when ::Protobuf::WireType::LENGTH_DELIMITED then
            read_length_delimited(stream)
          when ::Protobuf::WireType::FIXED32 then
            read_fixed32(stream)
          when ::Protobuf::WireType::START_GROUP then
            fail NotImplementedError, 'Group is deprecated.'
          when ::Protobuf::WireType::END_GROUP then
            fail NotImplementedError, 'Group is deprecated.'
          else
            fail InvalidWireType, wire_type
          end
  [tag, bytes]
end

def self.read_fixed32(stream)
  stream.read(4)
end

def self.read_fixed64(stream)
  stream.read(8)
end

def self.read_key(stream)
  bits = read_varint(stream)
  wire_type = bits & 0x07
  tag = bits >> 3
  [tag, wire_type]
end

def self.read_length_delimited(stream)
  value_length = read_varint(stream)
  stream.read(value_length)
end

def self.read_varint(stream)
  value = index = 0
  begin
    byte = stream.readbyte
    value |= (byte & 0x7f) << (7 * index)
    index += 1
  end while (byte & 0x80).nonzero?
  value
end