package dns

import (
	"crypto/hmac"
	"io"
)

// Need to lookup the actual codes
const (
	HmacMD5 = iota
	HmacSHA1
)

// The following values must be put in wireformat, so that
// the MAC can be calculated
// RFC 2845, section 3.4.2. TSIG Variables
type tsig_generation_fmt struct {
	// From RR_HEADER
	Name  string "domain-name"
	Class uint16
	Ttl   uint32
	// Rdata of the TSIG
	Algorithm  string    "domain-name"
	TimeSigned [3]uint16 "TSIG"
	Fudge      uint16
	// MACSize, MAC and OrigId excluded
	Error     uint16
	OtherLen  uint16
	OtherData string
}

// Generate the HMAC for msg. The TSIG RR is modified
// to include the MAC and MACSize. Note the the msg Id must
// be set, otherwise the MAC is not correct
func (rr *RR_TSIG) GenerateMAC(msg *Msg, secret string) bool {
	buf := make([]byte, 4096) // TODO(mg) bufsize!
	tsigbuf := new(tsig_generation_fmt)

	// Fill the struct and generate the wiredata
	tsigbuf.Name = rr.Header().Name
	tsigbuf.Class = rr.Header().Class
	tsigbuf.Ttl = rr.Header().Ttl
	tsigbuf.Algorithm = rr.Algorithm
	tsigbuf.TimeSigned = rr.TimeSigned
	tsigbuf.Fudge = rr.Fudge
	tsigbuf.Error = rr.Error
	tsigbuf.OtherLen = rr.OtherLen
	tsigbuf.OtherData = rr.OtherData
	packStruct(tsigbuf, buf, 0)

        msgbuf, ok := msg.Pack()
        if !ok {
                return false
        }
        buf = append(buf, msgbuf...)

	//func NewMD5(key []byte) hash.Hash
	hmac := hmac.NewMD5([]byte(secret))
	io.WriteString(hmac, string(buf))
	rr.MAC = string(hmac.Sum())
        rr.MACSize = uint16(len(rr.MAC))
        rr.OrigId = msg.MsgHdr.Id

	return true
}