You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: signature.go
    path of file: ./repos/sealer/vendor/golang.org/x/crypto/openpgp/packet
    
    the code of the file until where you have to start completion: // Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package packet

import (
	"bytes"
	"crypto"
	"crypto/dsa"
	"crypto/ecdsa"
	"encoding/asn1"
	"encoding/binary"
	"hash"
	"io"
	"math/big"
	"strconv"
	"time"

	"golang.org/x/crypto/openpgp/errors"
	"gol
    
func (sig *Signature) Serialize(w io.Writer) (err error) {
	if len(sig.outSubpackets) == 0 {
		sig.outSubpackets = sig.rawSubpackets
	}
	if sig.RSASignature.bytes == nil && sig.DSASigR.bytes == nil && sig.ECDSASigR.bytes == nil {
		return errors.InvalidArgumentError("Signature: need to call Sign, SignUserId or SignKey before Serialize")
	}

	sigLength := 0
	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		sigLength = 2 + len(sig.RSASignature.bytes)
	case PubKeyAlgoDSA:
		sigLength = 2 + len(sig.DSASigR.bytes)
		sigLength += 2 + len(sig.DSASigS.bytes)
	case PubKeyAlgoECDSA:
		sigLength = 2 + len(sig.ECDSASigR.bytes)
		sigLength += 2 + len(sig.ECDSASigS.bytes)
	default:
		panic("impossible")
	}

	unhashedSubpacketsLen := subpacketsLength(sig.outSubpackets, false)
	length := len(sig.HashSuffix) - 6 /* trailer not included */ +
		2 /* length of unhashed subpackets */ + unhashedSubpacketsLen +
		2 /* hash tag */ + sigLength
	err = serializeHeader(w, packetTypeSignature, length)
	if err != nil {
		return
	}

	_, err = w.Write(sig.HashSuffix[:len(sig.HashSuffix)-6])
	if err != nil {
		return
	}

	unhashedSubpackets := make([]byte, 2+unhashedSubpacketsLen)
	unhashedSubpackets[0] = byte(unhashedSubpacketsLen >> 8)
	unhashedSubpackets[1] = byte(unhashedSubpacketsLen)
	serializeSubpackets(unhashedSubpackets[2:], sig.outSubpackets, false)

	_, err = w.Write(unhashedSubpackets)
	if err != nil {
		return
	}
	_, err = w.Write(sig.HashTag[:])
	if err != nil {
		return
	}

	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		err = writeMPIs(w, sig.RSASignature)
	case PubKeyAlgoDSA:
		err = writeMPIs(w, sig.DSASigR, sig.DSASigS)
	case PubKeyAlgoECDSA:
		err = writeMPIs(w, sig.ECDSASigR, sig.ECDSASigS)
	default:
		panic("impossible")
	}
	return
}