Lesson 08 · Decoding packets, and the round trip

Decoding packets, and the round trip

A codec is a pair. Today you write the RLE decoder that expands packets back to bytes, teaching it both packet types up front, and confirm the run codec round-trips.

The goal

Decode a packet stream back to bytes and show encode then decode is identity.

Start here - the target

TO DO

Scenario: A run packet expands and the round trip holds

Giventhe packet stream 0x85, 0x41

WhenRLE Decode runs over it

Thenthe output is the six bytes AAAAAA

Andfor any input, Decode(Encode(input)) equals input

Background

Decoding walks the packet stream one packet at a time. Read a control byte, look at its high bit to decide the packet kind, act on it, and move to the next control byte. For a run, the low seven bits plus one give the count, and the single value byte that follows is emitted that many times: 0x85, 0x41 expands to AAAAAA.

Teach the decoder both packet kinds now, even though the encoder only emits runs so far. A control byte with its high bit clear is a literal packet: the low seven bits plus one give a count, and that many following bytes are copied verbatim. Building the full decoder up front means the next lesson only has to add the encoder’s choice between the two - the reader never changes again. With decode in place you can state the codec’s real contract: Decode(Encode(input)) returns the original bytes, exactly, for every input.

Make it work

func Decode(p []byte) []byte {
  var out []byte
  for i := 0; i < len(p); {
    ctrl := p[i]; i++
    if ctrl&0x80 != 0 { // run: repeat one byte
      n := int(ctrl&0x7F) + 1
      for k := 0; k < n; k++ { out = append(out, p[i]) }
      i++
    } else { // literal: copy the next n bytes (used next lesson)
      n := int(ctrl) + 1
      out = append(out, p[i:i+n]...); i += n
    }
  }
  return out
}

CheckpointDONE

The run codec round-trips; the decoder already understands both packet kinds. Commit and stop here.