src.dualinventive.com/go/dinet/ll/decoder.go

package ll

import (
	"bytes"
	"encoding/binary"
	"errors"
	"io"
)

// A Decoder reads and decodes Low-level messages from an input stream
type Decoder struct {
	buf      []byte
	rd       io.Reader
	r, w     int
	s        decoderState
	dataSize int
}

const (
	decoderBufChunkSize = 4096
	decoderBufMaxSize   = 64 * 1024
)

type decoderState uint

const (
	decoderStateStart  decoderState = iota
	decoderStateHeader decoderState = iota
	decoderStateData   decoderState = iota
)

// NewDecoder returns a new decoder that reads from r
// The decoder introduces its own buffering and may read data from r beyond single Messages requested
func NewDecoder(r io.Reader) *Decoder {
	buf := make([]byte, decoderBufChunkSize)
	return &Decoder{buf: buf, rd: r, s: decoderStateStart}
}

func (dec *Decoder) readHeader(msg *Msg) error {
	if len(dec.buf[dec.r:]) < remaningHeaderSize {
		return nil
	}

	r := bytes.NewReader(dec.buf[dec.r:])

	if err := dec.readType(r, &msg.Type); err != nil {
		return err
	}

	if err := dec.readSize(r); err != nil {
		return err
	}

	dec.r += remaningHeaderSize
	dec.s = decoderStateData
	return nil
}

func (dec *Decoder) readType(r io.Reader, t *MsgType) error {
	if err := binary.Read(r, binary.BigEndian, t); err != nil {
		return err
	}
	if !t.Valid() {
		return errors.New("invalid message type")
	}
	return nil
}

func (dec *Decoder) readSize(r io.Reader) error {
	var t uint16
	if err := binary.Read(r, binary.BigEndian, &t); err != nil {
		return err
	}
	dec.dataSize = int(t) - hdrSize
	return nil
}

// readData reads all data into the message when we have enough, it returns true when
//  add data is read, false otherwise
func (dec *Decoder) readData(m *Msg) bool {
	if len(dec.buf[dec.r:]) < dec.dataSize {
		return false
	}

	if dec.dataSize > 0 {
		mData := make([]byte, dec.dataSize)
		copy(mData, dec.buf[dec.r:dec.r+dec.dataSize])
		m.Data = mData
	}
	dec.r += dec.dataSize
	dec.s = decoderStateStart
	return true
}

// Decode reads the next Msg from its input and stores it in the value pointed to by m.
func (dec *Decoder) Decode(m *Msg) error {
	for {
		prevState := dec.s

		switch dec.s {
		case decoderStateStart:
			if r := bytes.Index(dec.buf[dec.r:], []byte(hdrMagic)); r != -1 {
				dec.r += r + len(hdrMagic)
				dec.s = decoderStateHeader
			}
		case decoderStateHeader:
			if err := dec.readHeader(m); err != nil {
				return err
			}
		case decoderStateData:
			if done := dec.readData(m); done {
				return nil
			}
		}

		// When the state changes, first go into the new state before calling fill again
		if prevState == dec.s {
			if err := dec.fill(); err != nil {
				return err
			}
		}
	}
}

func (dec *Decoder) grow() error {
	if cap(dec.buf)+decoderBufChunkSize > decoderBufMaxSize {
		return errors.New("ll: 2^16-1 packet size exceeded")
	}

	newBuf := make([]byte, len(dec.buf), cap(dec.buf)+decoderBufChunkSize)
	copy(newBuf, dec.buf)
	dec.buf = newBuf
	return nil
}

func (dec *Decoder) fill() error {
	// Slide existing data to beginning.
	if dec.r > 0 {
		copy(dec.buf, dec.buf[dec.r:dec.w])
		dec.w -= dec.r
		dec.r = 0
	}

	// Check if we need to grow
	if dec.w >= cap(dec.buf) {
		if err := dec.grow(); err != nil {
			return err
		}
	}

	// Read new data
	n, err := dec.rd.Read(dec.buf[dec.w:cap(dec.buf)])
	if err != nil {
		return err
	}
	dec.w += n
	dec.buf = dec.buf[0:dec.w]
	return nil
}