📦 specification for the SuperPack extensible schemaless binary encoding format
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SuperPack is a data serialisation format. Due to its design and the optimisations enabled by its extension mechanism, it is particularly suitable for encoding values with repeated structure: arrays of booleans, strings used more than once, objects which have the same shape, etc.

A comparison of schemaless serialisation formats run on 1000 public domain book records from the New York Public Library Digital Collections API:

name encoded size gzip compressed extensible rich built-in types human-readable
SuperPack1 768.1KB 225.8KB YES no no
Sereal 1442.2KB 268.3KB YES no no
MessagePack 2019.7KB 271.7KB YES no no
SuperPack2 2024.5KB 264.5KB YES no no
CBOR 2025.6KB 270.4KB YES YES no
ARSON 2048.1KB 361.5KB theoretically YES arguably
bencode 2208.2KB 273.8KB no no arguably
edn 2257.8KB 261.2KB YES no YES
JSON 2276.0KB 260.9KB no no arguably
BSON 2353.0KB 314.4KB YES YES no
YAML 2444.7KB 275.7KB YES no YES
  1. with built-in optimisations enabled
  2. with no optimisations enabled

SuperPack is designed to

  • achieve a small encoded payload size
  • have a reasonably small transcoder
  • encode data of an (a priori) unknown schema
  • transcode arbitrary data types without consulting the SuperPack authors by using the extension capabilities
  • operate in an environment without access to a lossless data compression algorithm


Terminology and Requirements

The SuperPack format specifies 256 type tags for representing values of various data types.

A SuperPack value is an encoded representation of a value. It starts with a type tag and can be decoded without backtracking. The number of bytes that comprise the SuperPack value may not be known until it is fully decoded.

A SuperPack payload is a byte sequence containing one SuperPack value, prepended by up to one SuperPack value for each enabled extension.

A SuperPack encoder is a function from a value to a SuperPack payload. A SuperPack decoder is a function from a SuperPack payload to a value.

  • When a value has more than one equivalent representation (e.g. the integer 0 as uint6, uint14, nint4, etc.), a SuperPack encoder may produce any applicable representation.
  • When a value has more than one equivalent representaion, a SuperPack encoder should produce a SuperPack value with the shortest length possible.
  • A SuperPack encoder is not required to produce all possible SuperPack values (it may always use str* for strings, etc).
  • A SuperPack decoder must be able to decode any SuperPack value.

Type Tags

Hex Dec Binary Tag Name
0x00 0 00------ uint6
0x40 64 01------ uint14
0x80 128 10000000 RESERVED
0x81 129 1000---- nint4
0x90 144 1001---- barray4
0xA0 160 101----- array5
0xC0 192 110----- str5
0xE0 224 11100000 false
0xE1 225 11100001 true
0xE2 226 11100010 null
0xE3 227 11100011 undefined
0xE4 228 11100100 uint16
0xE5 229 11100101 uint24
0xE6 230 11100110 uint32
0xE7 231 11100111 uint64
0xE8 232 11101000 nint8
0xE9 233 11101001 nint16
0xEA 234 11101010 nint32
0xEB 235 11101011 nint64
0xEC 236 11101100 float32
0xED 237 11101101 double64
0xEE 238 11101110 timestamp
0xEF 239 11101111 binary*
0xF0 240 11110000 cstring
0xF1 241 11110001 str*
0xF2 242 11110010 array*
0xF3 243 11110011 barray*
0xF4 244 11110100 map
0xF5 245 11110101 bmap
0xF6 246 11110110 RESERVED
0xF7 247 11110111 extension*
0xF8 251 11111--- extension3

Type Breakdown

Notational Conventions

  • 0x00 represents a byte as the hexadecimal number following 0x.
  • xxxxxxxx represents a byte as 8 bits. Some xs may instead be 0 or 1 to indicate that those bits are fixed. Remaining x bits may be either 0 or 1.
  • Boxes with -------- borders are drawn around one of the above single byte values.
  • Boxes with ======== borders are drawn around zero or more bytes as described within.
  • Boxes with ~~~~~~~~ borders are drawn around zero or more SuperPack encoded values.

uint family

Stores a big-endian encoding of a non-negative integer in 1, 2, 3, 4, 5, or 9 bytes.



An integer between 0 and 63.



An integer between 64 and 16,383 (214 - 1).


|  0xE4  |xxxxxxxx|xxxxxxxx|

An integer between 16,384 and 65,535 (216 - 1).


|  0xE5  |xxxxxxxx|xxxxxxxx|xxxxxxxx|

An integer between 65,536 and 16,777,215 (224 - 1).


|  0xE6  |xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|

An integer between 16,777,216 and 4,294,967,295 (232 - 1).


|  0xE7  |xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|

An integer between 4,294,967,296 and 264 - 1.

nint family

Stores a big-endian encoding of the magnitude of a non-positive integer in 1, 2, 3, 5, or 9 bytes.



An integer between -1 and -15. The type tag 0x80 is reserved.


|  0xE8  |xxxxxxxx|

An integer between -16 and -255.


|  0xE9  |xxxxxxxx|xxxxxxxx|

An integer between -256 and -65,535.


|  0xEA  |xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|

An integer between -65,536 and -4,294,967,295.


|  0xEB  |xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|

An integer between -4,294,967,296 and -(264 - 1).

float family

Stores an IEEE-754 floating point number in 5 or 9 bytes.

s, e, and m bits represent sign, exponent, and mantissa, respectively.


|  0xEC  |seeeeeee|emmmmmmm|mmmmmmmm|mmmmmmmm|


|  0xED  |seeeeeee|eeeemmmm|mmmmmmmm|mmmmmmmm|mmmmmmmm|mmmmmmmm|mmmmmmmm|mmmmmmmm|

timestamp family


Stores a timestamp (in milliseconds since 1970-01-01T00:00:00.000Z) in 7 bytes. Essentially an int48. The first bit is the sign bit and the bytes are in big-endian order.

|  0xEE  |xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx|

simple values family

Stores false, true, null, or undefined in one byte.


|  0xE0  |


|  0xE1  |


|  0xE2  |


|  0xE3  |

binary family

Stores a binary blob.


|  0xEF  | uint | binary data |

A binary blob of any uint number of bytes.

string family

Stores a UTF-8 encoded string.


|110xxxxx| utf8 data |

A string of up to 31 UTF-8 bytes.


|  0xF1  | uint | utf8 data |

A string of any uint number of UTF-8 bytes.


|  0xF0  | utf8 data |  0x00  |

A null-terminated string of any number of UTF-8 bytes. The UTF-8 bytes must not contain 0x00.

array family


|101xxxxx| values |

An array of up to 31 values.


|  0xF2  | uint | values |

An array of any uint number of values.


|1001xxxx| booleans |

An array of up to 15 boolean values.

The boolean values of the array are stored as one bit each, right-padded with 0 bits to the next full byte if necessary. true is represented as a 1 bit and false is represented as a 0 bit.


|  0xF3  | uint | booleans |

An array of any uint number of boolean values.

The boolean values of the array are stored as one bit each, right-padded with 0 bits to the next full byte if necessary. true is represented as a 1 bit and false is represented as a 0 bit.

map family

Stores a map structure.


|  0xF4  | keys | values |

A map whose keys are given by an array of strings. The strings in the array must be unique. The values of the map follow in the same order as the keys.


|  0xF5  | keys | booleans |

A boolean-valued map whose keys are given by an array of strings. The strings in the array must be unique.

The boolean values of the map are stored as one bit each, right-padded with 0 bits to the next full byte if necessary, in the same order as the keys. true is represented as a 1 bit and false is represented as a 0 bit.

extension family

Represents an otherwise unsupported value in terms of a supported value, identified by the extension point. For more information, see Extensions.


|11111xxx| value |

An extension point between 0 and 7 followed by any SuperPack value.


|  0xF7  | uint | value |

A uint extension point followed by any SuperPack value.


SuperPack encoders should provide an interface for a consumer to specify a serialisation and deserialisation mechanism for a given type of data. For example, since SuperPack does not have built-in support for representation of regular expressions, a regular expression may instead be represented as a two-element array containing its source and flags. When the SuperPack value that represents this intermediate value is decoded, it will be converted back to a regular expression.

let transcoder = new SuperPackTranscoder;
  // extension point
  class {
    // detect values which require this custom serialisation
    isCandidate(x) { return x instanceof RegExp; },
    // return an intermediate value which will be encoded instead
    serialise(r) { return [r.source, r.flags]; },
    // from the intermediate value, reconstruct the original value
    deserialise([source, flags]) { return RegExp(source, flags); },

If the extension uses state to keep track of values it has marked as candidates, it can use that state in conjunction with the optional shouldSerialise method to determine if an extension should be applied to the value. shouldSerialise will only ever be called once all candidates have been determined (see Extensions and Recursion for an exception to this). This is useful for optimisations that need to see all of the data being encoded before being effective. Most extensions will always return true from this function.

Extension memos

Extension memos allow for the storage of a side table used in the decoding of values encoded by an extension. This is mostly useful for deduplication and optimisation.

// preserves ECMAScript Symbol identity and description
class SymbolExtension {
  constructor() {
    this.symbols = [];
  isCandidate(x) {
    return typeof x === 'symbol' || x && x.constructor === Symbol;
  serialise(s) {
    let i = this.symbols.indexOf(s);
    if (i >= 0) return i;
    return this.symbols.push(s) - 1;
  deserialise(n, memo) {
    if (this.symbols[n] == null) {
      let s = Symbol(memo[n]);
      this.symbols[n] = s;
      return s;
    return this.symbols[n];
  memo() { return this.symbols.map(sym => String(sym).slice(7, -1)); }

let encoded = encode(data, { extensions: { 0: SymbolExtension } });

Memos are encoded and prepended to the SuperPack payload in descending extension point order. Memos are only prepended for extensions that indicate that they require a memo. During the encoding of a memo, extensions that use a memo must not be applied if their extension point is greater than that of the extension whose memo is being encoded.


  • String deduplication
  • Map keyset deduplication
  • Representation of values with identity
  • Deduplication of common integers, timestamps, floats, or other potentially large data types

... and many domain-specific usages.


As with any compression-like scheme, highly-compressed inputs which result in a much larger decoded value are a concern. SuperPack decoders are advised to assume that their input is potentially adversarial and be resistant to attacks based on expansion.

Extensions and Recursion

Some extensions will serialise to values which could have themselves been identified as a candidate for applying the extension. In most cases, it is not desirable to apply the extension again to the serialised value, and by default, this is how a SuperPack encoder will work. But for some extensions, the recursion is desirable, and those extensions may supply a shouldApplyRecursively function which returns true to opt in to this behaviour. Be very careful when writing extensions that utilise this functionality, as it can easily lead to infinite recursion.