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SCALE Codec

The SCALE (Simple Concatenated Aggregate Little-Endian) Codec is a lightweight, efficient, binary serialization and deserialization codec.

It is designed for high-performance, copy-free encoding and decoding of data in resource-constrained execution contexts, like the Substrate runtime. It is not self-describing in any way and assumes the decoding context has all type knowledge about the encoded data.

SCALE for Substrate

Substrate uses the parity-scale-codec, a Rust implementation of the SCALE Codec. This library and the SCALE codec are advantageous for Substrate and blockchain systems because:

  • It is lightweight relative to generic serialization frameworks like serde, which add significant boilerplate that can bloat the size of the binary.
  • It does not use Rust STD, and thus can compile to Wasm for the Substrate runtime.
  • It is built to have great support in Rust for deriving codec logic for new types:
    #[derive(Encode, Decode)]
    

It is important to define the encoding scheme used on Substrate rather than reuse an existing Rust codec library because this codec needs to be re-implemented on other platforms and languages that want to support interoperability.

Codec definition

Here you will find how the SCALE codec encodes different types.

Fixed-width integers

Basic integers are encoded using a fixed-width little-endian (LE) format.

Example

  • signed 8-bit integer 69: 0x45
  • unsigned 16-bit integer 42: 0x2a00
  • unsigned 32-bit integer 16777215: 0xffffff00

Compact/general integers

A "compact" or general integer encoding is sufficient for encoding large integers (up to 2**536) and is more efficient at encoding most values than the fixed-width version. (Though for single-byte values, the fixed-width integer is never worse.)

It is encoded with the two least significant bits denoting the mode:

  • 0b00: single-byte mode; upper six bits are the LE encoding of the value (valid only for values of 0-63).
  • 0b01: two-byte mode: upper six bits and the following byte is the LE encoding of the value (valid only for values 64-(2**14-1)).
  • 0b10: four-byte mode: upper six bits and the following three bytes are the LE encoding of the value (valid only for values (2**14)-(2**30-1)).
  • 0b11: Big-integer mode: The upper six bits are the number of bytes following, less four. The value is contained, LE encoded, in the bytes following. The final (most significant) byte must be non-zero. Valid only for values (2**30)-(2**536-1).

Example

  • unsigned integer 0: 0x00
  • unsigned integer 1: 0x04
  • unsigned integer 42: 0xa8
  • unsigned integer 69: 0x1501

Error:

  • 0x0100: Zero encoded in mode 1

Boolean

Boolean values are encoded using the least significant bit of a single byte.

Example

  • boolean false: 0x00
  • boolean true: 0x01

Options

One or zero values of a particular type. Encoded as:

  • 0x00 if it is None ("empty" or "null").
  • 0x01 followed by the encoded value if it is Some.

As an exception, in the case that the type is a boolean, then it is always one byte:

  • 0x00 if it is None ("empty" or "null").
  • 0x01 if it is the true value.
  • 0x02 if it is the false value.

Results

Results are commonly used enumerations which indicate whether certain operations were successful or unsuccessful. Encoded as:

  • 0x00 if the operation was successful, followed by the encoded value.
  • 0x01 if the operation was unsuccessful, followed by the encoded error.

Example

// A custom result type used in a crate.
let Result = std::result::Result<u8, bool>;
  • Ok(42): 0x002a
  • Err(false): 0x0100

Vectors (lists, series, sets)

A collection of same-typed values is encoded, prefixed with a compact encoding of the number of items, followed by each item's encoding concatenated in turn.

Example

Vector of unsigned 16-bit integers:

[4, 8, 15, 16, 23, 42]

SCALE Bytes:

0x18040008000f00100017002a00

Strings

Strings are Vectors of bytes (Vec<u8>) containing a valid UTF8 sequence.

Tuples

A fixed-size series of values, each with a possibly different but predetermined and fixed type. This is simply the concatenation of each encoded value.

Example

Tuple of compact unsigned integer and boolean:

(3, false): 0x0c00

Data structures

For structures, the values are named, but that is irrelevant for the encoding (names are ignored - only order matters). All containers store elements consecutively. The order of the elements is not fixed, depends on the container, and cannot be relied on at decoding.

This implicitly means that decoding some byte-array into a specified structure that enforces an order and then re-encoding it could result in a different byte array than the original that was decoded.

Example

Imagine a SortedVecAsc<u8> structure that always has byte-elements in ascending order and you have [3, 5, 2, 8], where the first element is the number of bytes following (i.e. [3, 5, 2] would be invalid).

SortedVecAsc::from([3, 5, 2, 8]) would decode to [3, 2, 5, 8], which does not match the original encoding.

Enumerations (tagged-unions)

A fixed number of variants, each mutually exclusive and potentially implying a further value or series of values.

Encoded as the first byte identifying the index of the variant that the value is. Any further bytes are used to encode any data that the variant implies. Thus, no more than 256 variants are supported.

Example

enum IntOrBool {
  Int(u8),
  Bool(bool),
}
  • Int(42): 0x002a
  • Bool(true): 0x0101

Implementations

The Parity SCALE Codec has been implemented in many languages, including a reference implementation that is written in Rust and maintained by Parity Technologies.

References

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