cobs_codec

Fast, dependency-free Consistent Overhead Byte Stuffing (COBS) and COBS/R codecs for Dart and Flutter, with first-class dart:convert integration and stream framing for serial links.

COBS encodes an arbitrary byte sequence into one that contains no zero (0x00) bytes, at a small and predictable cost — at most one extra byte per 254 bytes, plus one. That lets a single 0x00 reliably delimit packets on a byte stream (serial/UART, USB-CDC, TCP, BLE, …), so a receiver can always resynchronise on the next 0x00 even after a corrupt packet. Unlike escape-based schemes such as PPP byte stuffing, COBS never doubles a packet's size.

Features

• ⚡ Basic COBS and COBS/R (Reduced) — pick the standard scheme, or COBS/R to shave the trailing overhead byte off small messages.
• 🧩 dart:convert native — cobs and cobsr are Codecs, so they fuse, transform streams, and compose like json/utf8/base64.
• 🔌 Stream framing built in — turn a raw serial byte stream into a stream of decoded packets with CobsFrameDecoder; chunk boundaries don't have to align with frames.
• 🚩 Configurable delimiter (sentinel) — encode and decode COBS and COBS/R so the output avoids any chosen byte, not just 0x00, letting that byte delimit frames instead.
• 🏷️ Sentinel-aware framing — the framing helpers and the stream decoder can split on the chosen sentinel rather than 0x00.
• ♻️ In-place decode — decode basic COBS inside the same buffer with no second allocation; COBS never expands on decode.
• 🎯 Zero dependencies, all platforms — pure Dart (dart:typed_data), works on mobile, desktop, web, server and CLI. Uses Uint8List throughout.
• 📏 Predictable sizing — maxEncodedLength / encodingOverhead for buffer pre-allocation.
• ✅ Reference-verified — tested against the golden vectors from the original COBS and COBS/R implementations.

Install

dart pub add cobs_codec

dependencies:
  cobs_codec: ^1.1.0

Usage

Encode and decode

import 'package:cobs_codec/cobs_codec.dart';

void main() {
  final data = [0x11, 0x22, 0x00, 0x33];

  final encoded = cobs.encode(data); // [0x03, 0x11, 0x22, 0x02, 0x33] — no 0x00
  final decoded = cobs.decode(encoded); // [0x11, 0x22, 0x00, 0x33]
}

cobs.encode returns a Uint8List and never fails — any input is encodable. cobs.decode throws a CobsDecodeException (a FormatException) if the input is not valid COBS.

COBS/R — save a byte

COBS always adds exactly one byte to messages of 254 bytes or fewer. COBS/R opportunistically avoids that byte when the final data byte allows it:

cobs.encode([0x31, 0x32, 0x33, 0x34, 0x35]);  // [0x06, 0x31, 0x32, 0x33, 0x34, 0x35]
cobsr.encode([0x31, 0x32, 0x33, 0x34, 0x35]); // [0x35, 0x31, 0x32, 0x33, 0x34]  (same size!)

Both round-trip losslessly; just decode with the matching codec.

Framing a packet stream

COBS output has no 0x00, so append one to delimit frames:

final frame = cobsFrame([0x11, 0x00, 0x22]); // [0x02, 0x11, 0x02, 0x22, 0x00]

final packets = cobsUnframe(buffer); // List<Uint8List>, one per 0x00-delimited frame

Reading packets from a serial stream

CobsFrameDecoder is a StreamTransformer that buffers bytes across arbitrarily chunked reads and emits one decoded packet per completed frame — exactly what you want on a UART:

import 'package:cobs_codec/cobs_codec.dart';

// `serialPort` is any Stream<List<int>> of incoming bytes.
serialPort
    .transform(CobsFrameDecoder(
      // Keep receiving even if a frame is corrupted on a noisy link.
      onInvalidFrame: (error, rawFrame) => print('dropped bad frame: $error'),
      // Bound memory if a peer never sends the 0x00 delimiter.
      maxFrameLength: 4096,
    ))
    .listen((packet) => handlePacket(packet));

// Sending: encode + delimit each outgoing packet.
outgoingPackets
    .transform(const CobsFrameEncoder())
    .listen(serialPort.add);

A custom delimiter byte (sentinel)

Need a delimiter other than 0x00 — say a byte your protocol reserves for framing? The …WithSentinel variants make the encoding avoid an arbitrary sentinel byte instead (they XOR the finished encoding with it), so that byte can delimit frames:

final data = [0x11, 0x22, 0x00, 0x33];

final encoded = cobsEncodeWithSentinel(data, 0xAA); // [0xA9, 0xBB, 0x88, 0xA8, 0x99] — no 0xAA
final decoded = cobsDecodeWithSentinel(encoded, 0xAA); // [0x11, 0x22, 0x00, 0x33]

// COBS/R has the same pair.
cobsrEncodeWithSentinel(data, 0xAA); // [0xA9, 0xBB, 0x88, 0x99]

sentinel == 0 is byte-for-byte identical to the plain codec, and decoding never mutates its input.

Decoding in place

Basic COBS never expands on decode, so it can be decoded within the same buffer — no second allocation. cobsDecodeInPlace overwrites the buffer with the decoded bytes and returns their length:

final buffer = Uint8List.fromList(cobs.encode([0x11, 0x22, 0x00, 0x33]));
// buffer == [0x03, 0x11, 0x22, 0x02, 0x33]

final n = cobsDecodeInPlace(buffer);                 // 4
final decoded = Uint8List.sublistView(buffer, 0, n); // [0x11, 0x22, 0x00, 0x33]

cobsDecodeInPlaceWithSentinel(buffer, sentinel) does the same for a custom delimiter. (COBS/R can expand on decode, so it has no in-place form.)

Framing on a custom sentinel

cobsFrame / cobsUnframe and both stream transformers take the same optional sentinel, so a whole link can be delimited by a byte other than 0x00:

final frame = cobsFrame([0x11, 0x00, 0x22], sentinel: 0xAA); // [0xA8, 0xBB, 0xA8, 0x88, 0xAA]
final packets = cobsUnframe(frame, sentinel: 0xAA);          // [[0x11, 0x00, 0x22]]

// The stream decoder frames on the same byte:
serialPort.transform(const CobsFrameDecoder(sentinel: 0xAA));

Composing with other codecs

Because they are Codecs, you can fuse COBS with anything:

// Encode a Dart object to JSON, to UTF-8 bytes, then COBS-frame it.
final pipeline = json.fuse(utf8).fuse(cobs);
final wire = pipeline.encode({'id': 7, 'ok': true});
final obj = pipeline.decode(wire);

Sizing buffers

encodingOverhead(0);      // 1
encodingOverhead(254);    // 1
encodingOverhead(255);    // 2
maxEncodedLength(1000);   // 1004

How much overhead?

Input length n	Max encoded length	Overhead
0 (empty)	1	+1 byte
1 – 254	n + 1	+1 byte
255 – 508	n + 2	+2 bytes
n	n + ⌈n / 254⌉	≤ ~0.4%

The overhead is data-independent: worst case and average case are almost the same. Compare PPP/SLIP escape stuffing, whose worst case doubles the packet.

API overview

Symbol	Description
cobs / cobsr	Shared Codec instances (basic COBS and COBS/R).
cobsEncode / cobsDecode	Direct basic-COBS functions.
cobsrEncode / cobsrDecode	Direct COBS/R functions.
cobsEncodeWithSentinel / cobsDecodeWithSentinel	Basic COBS against an arbitrary delimiter byte.
cobsrEncodeWithSentinel / cobsrDecodeWithSentinel	COBS/R against an arbitrary delimiter byte.
cobsDecodeInPlace / cobsDecodeInPlaceWithSentinel	Decode basic COBS within the buffer; returns the length.
CobsCodec, CobsEncoder, CobsDecoder	dart:convert classes for basic COBS.
CobsrCodec, CobsrEncoder, CobsrDecoder	dart:convert classes for COBS/R.
cobsFrame / cobsUnframe	Add / split the frame delimiter (0x00 or a custom sentinel).
CobsFrameEncoder / CobsFrameDecoder	Stream transformers for framed links (optional sentinel).
cobsDelimiter	The frame delimiter byte (0x00).
encodingOverhead / maxEncodedLength	Size bounds.
cobsMaxBlockLength	Max data bytes per COBS block (254).
CobsDecodeException	Thrown on invalid encoded input.

Note on decoding. decode expects a single encoded packet with no surrounding 0x00 delimiter — split a delimited stream into frames first (with cobsUnframe or CobsFrameDecoder). This matches the reference COBS implementations, where framing is the application's responsibility.

Benchmarks

Single-threaded throughput on a 1 KiB payload (dart run benchmark/cobs_benchmark.dart), Dart 3.x on an AMD Ryzen 7 3800XT under WSL2 — indicative, not a controlled benchmark:

Operation	Throughput
cobsEncode	~580 MB/s
cobsDecode	~570 MB/s
cobsrEncode	~610 MB/s

Background

COBS is described in:

Stuart Cheshire and Mary Baker, "Consistent Overhead Byte Stuffing," IEEE/ACM Transactions on Networking, Vol. 7, No. 2, April 1999.

COBS/R ("Reduced") was devised by Craig McQueen, whose C and Python reference implementations were used to validate this package's test vectors. The COBS/ZPE and COBS/ZRE variants are not implemented.

Contributing

Contributions are welcome — see CONTRIBUTING.md.

License

MIT © 2026 Alexander Salas Bastidas (Firechip). See LICENSE.