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package p2p

import (
    "bytes"
    "encoding/binary"
    "io"
    "io/ioutil"
    "math/big"

    "github.com/ethereum/go-ethereum/ethutil"
    "github.com/ethereum/go-ethereum/rlp"
)

// Msg defines the structure of a p2p message.
//
// Note that a Msg can only be sent once since the Payload reader is
// consumed during sending. It is not possible to create a Msg and
// send it any number of times. If you want to reuse an encoded
// structure, encode the payload into a byte array and create a
// separate Msg with a bytes.Reader as Payload for each send.
type Msg struct {
    Code    uint64
    Size    uint32 // size of the paylod
    Payload io.Reader
}

// NewMsg creates an RLP-encoded message with the given code.
func NewMsg(code uint64, params ...interface{}) Msg {
    buf := new(bytes.Buffer)
    for _, p := range params {
        buf.Write(ethutil.Encode(p))
    }
    return Msg{Code: code, Size: uint32(buf.Len()), Payload: buf}
}

func encodePayload(params ...interface{}) []byte {
    buf := new(bytes.Buffer)
    for _, p := range params {
        buf.Write(ethutil.Encode(p))
    }
    return buf.Bytes()
}

// Data returns the decoded RLP payload items in a message.
func (msg Msg) Data() (*ethutil.Value, error) {
    s := rlp.NewListStream(msg.Payload, uint64(msg.Size))
    var v []interface{}
    err := s.Decode(&v)
    return ethutil.NewValue(v), err
}

// Discard reads any remaining payload data into a black hole.
func (msg Msg) Discard() error {
    _, err := io.Copy(ioutil.Discard, msg.Payload)
    return err
}

type MsgReader interface {
    ReadMsg() (Msg, error)
}

type MsgWriter interface {
    // WriteMsg sends an existing message.
    // The Payload reader of the message is consumed.
    // Note that messages can be sent only once.
    WriteMsg(Msg) error

    // EncodeMsg writes an RLP-encoded message with the given
    // code and data elements.
    EncodeMsg(code uint64, data ...interface{}) error
}

// MsgReadWriter provides reading and writing of encoded messages.
type MsgReadWriter interface {
    MsgReader
    MsgWriter
}

// MsgLoop reads messages off the given reader and
// calls the handler function for each decoded message until
// it returns an error or the peer connection is closed.
//
// If a message is larger than the given maximum size,
// MsgLoop returns an appropriate error.
func MsgLoop(r MsgReader, maxsize uint32, f func(code uint64, data *ethutil.Value) error) error {
    for {
        msg, err := r.ReadMsg()
        if err != nil {
            return err
        }
        if msg.Size > maxsize {
            return newPeerError(errInvalidMsg, "size %d exceeds maximum size of %d", msg.Size, maxsize)
        }
        value, err := msg.Data()
        if err != nil {
            return err
        }
        if err := f(msg.Code, value); err != nil {
            return err
        }
    }
}

var magicToken = []byte{34, 64, 8, 145}

func writeMsg(w io.Writer, msg Msg) error {
    // TODO: handle case when Size + len(code) + len(listhdr) overflows uint32
    code := ethutil.Encode(uint32(msg.Code))
    listhdr := makeListHeader(msg.Size + uint32(len(code)))
    payloadLen := uint32(len(listhdr)) + uint32(len(code)) + msg.Size

    start := make([]byte, 8)
    copy(start, magicToken)
    binary.BigEndian.PutUint32(start[4:], payloadLen)

    for _, b := range [][]byte{start, listhdr, code} {
        if _, err := w.Write(b); err != nil {
            return err
        }
    }
    _, err := io.CopyN(w, msg.Payload, int64(msg.Size))
    return err
}

func makeListHeader(length uint32) []byte {
    if length < 56 {
        return []byte{byte(length + 0xc0)}
    }
    enc := big.NewInt(int64(length)).Bytes()
    lenb := byte(len(enc)) + 0xf7
    return append([]byte{lenb}, enc...)
}

// readMsg reads a message header from r.
// It takes an rlp.ByteReader to ensure that the decoding doesn't buffer.
func readMsg(r rlp.ByteReader) (msg Msg, err error) {
    // read magic and payload size
    start := make([]byte, 8)
    if _, err = io.ReadFull(r, start); err != nil {
        return msg, newPeerError(errRead, "%v", err)
    }
    if !bytes.HasPrefix(start, magicToken) {
        return msg, newPeerError(errMagicTokenMismatch, "got %x, want %x", start[:4], magicToken)
    }
    size := binary.BigEndian.Uint32(start[4:])

    // decode start of RLP message to get the message code
    posr := &postrack{r, 0}
    s := rlp.NewStream(posr)
    if _, err := s.List(); err != nil {
        return msg, err
    }
    code, err := s.Uint()
    if err != nil {
        return msg, err
    }
    payloadsize := size - posr.p
    return Msg{code, payloadsize, io.LimitReader(r, int64(payloadsize))}, nil
}

// postrack wraps an rlp.ByteReader with a position counter.
type postrack struct {
    r rlp.ByteReader
    p uint32
}

func (r *postrack) Read(buf []byte) (int, error) {
    n, err := r.r.Read(buf)
    r.p += uint32(n)
    return n, err
}

func (r *postrack) ReadByte() (byte, error) {
    b, err := r.r.ReadByte()
    if err == nil {
        r.p++
    }
    return b, err
}