60 lines
1.7 KiB
Go
60 lines
1.7 KiB
Go
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package packets
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import (
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"bytes"
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)
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// FixedHeader contains the values of the fixed header portion of the MQTT packet.
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type FixedHeader struct {
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Remaining int // the number of remaining bytes in the payload.
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Type byte // the type of the packet (PUBLISH, SUBSCRIBE, etc) from bits 7 - 4 (byte 1).
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Qos byte // indicates the quality of service expected.
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Dup bool // indicates if the packet was already sent at an earlier time.
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Retain bool // whether the message should be retained.
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}
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// Encode encodes the FixedHeader and returns a bytes buffer.
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func (fh *FixedHeader) Encode(buf *bytes.Buffer) {
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buf.WriteByte(fh.Type<<4 | encodeBool(fh.Dup)<<3 | fh.Qos<<1 | encodeBool(fh.Retain))
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encodeLength(buf, int64(fh.Remaining))
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}
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// Decode extracts the specification bits from the header byte.
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func (fh *FixedHeader) Decode(headerByte byte) error {
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fh.Type = headerByte >> 4 // Get the message type from the first 4 bytes.
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switch fh.Type {
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case Publish:
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fh.Dup = (headerByte>>3)&0x01 > 0 // Extract flags. Check if message is duplicate.
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fh.Qos = (headerByte >> 1) & 0x03 // Extract QoS flag.
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fh.Retain = headerByte&0x01 > 0 // Extract retain flag.
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case Pubrel:
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fh.Qos = (headerByte >> 1) & 0x03
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case Subscribe:
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fh.Qos = (headerByte >> 1) & 0x03
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case Unsubscribe:
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fh.Qos = (headerByte >> 1) & 0x03
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default:
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if (headerByte>>3)&0x01 > 0 || (headerByte>>1)&0x03 > 0 || headerByte&0x01 > 0 {
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return ErrInvalidFlags
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}
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}
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return nil
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}
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// encodeLength writes length bits for the header.
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func encodeLength(buf *bytes.Buffer, length int64) {
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for {
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digit := byte(length % 128)
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length /= 128
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if length > 0 {
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digit |= 0x80
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}
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buf.WriteByte(digit)
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if length == 0 {
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break
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}
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}
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}
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