112 lines
2.6 KiB
Go
112 lines
2.6 KiB
Go
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package astits
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import (
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"fmt"
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"io"
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"github.com/pkg/errors"
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)
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// packetBuffer represents a packet buffer
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type packetBuffer struct {
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b []*Packet
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packetSize int
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r io.Reader
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}
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// newPacketBuffer creates a new packet buffer
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func newPacketBuffer(r io.Reader, packetSize int) (pb *packetBuffer, err error) {
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// Init
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pb = &packetBuffer{
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packetSize: packetSize,
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r: r,
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}
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// Packet size is not set
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if pb.packetSize == 0 {
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// Auto detect packet size
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if pb.packetSize, err = autoDetectPacketSize(r); err != nil {
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err = errors.Wrap(err, "astits: auto detecting packet size failed")
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return
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}
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}
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return
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}
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// autoDetectPacketSize updates the packet size based on the first bytes
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// Minimum packet size is 188 and is bounded by 2 sync bytes
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// Assumption is made that the first byte of the reader is a sync byte
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func autoDetectPacketSize(r io.Reader) (packetSize int, err error) {
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// Read first bytes
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const l = 193
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var b = make([]byte, l)
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if _, err = r.Read(b); err != nil {
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err = errors.Wrapf(err, "astits: reading first %d bytes failed", l)
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return
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}
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// Packet must start with a sync byte
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if b[0] != syncByte {
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err = ErrPacketMustStartWithASyncByte
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return
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}
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// Look for sync bytes
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for idx, b := range b {
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if b == syncByte && idx >= 188 {
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// Update packet size
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packetSize = idx
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// Rewind or sync reader
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var n int64
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if n, err = rewind(r); err != nil {
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err = errors.Wrap(err, "astits: rewinding failed")
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return
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} else if n == -1 {
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var ls = packetSize - (l - packetSize)
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if _, err = r.Read(make([]byte, ls)); err != nil {
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err = errors.Wrapf(err, "astits: reading %d bytes to sync reader failed", ls)
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return
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}
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}
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return
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}
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}
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err = fmt.Errorf("astits: only one sync byte detected in first %d bytes", l)
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return
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}
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// rewind rewinds the reader if possible, otherwise n = -1
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func rewind(r io.Reader) (n int64, err error) {
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if s, ok := r.(io.Seeker); ok {
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if n, err = s.Seek(0, 0); err != nil {
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err = errors.Wrap(err, "astits: seeking to 0 failed")
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return
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}
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return
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}
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n = -1
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return
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}
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// next fetches the next packet from the buffer
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func (pb *packetBuffer) next() (p *Packet, err error) {
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// Read
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var b = make([]byte, pb.packetSize)
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if _, err = io.ReadFull(pb.r, b); err != nil {
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if err == io.EOF || err == io.ErrUnexpectedEOF {
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err = ErrNoMorePackets
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} else {
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err = errors.Wrapf(err, "astits: reading %d bytes failed", pb.packetSize)
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}
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return
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}
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// Parse packet
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if p, err = parsePacket(b); err != nil {
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err = errors.Wrap(err, "astits: building packet failed")
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return
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}
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return
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}
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