Crate parity_scale_codec

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Parity SCALE Codec

Rust implementation of the SCALE (Simple Concatenated Aggregate Little-Endian) data format for types used in the Parity Substrate framework.

SCALE is a light-weight format which allows encoding (and decoding) which makes it highly suitable for resource-constrained execution environments like blockchain runtimes and low-power, low-memory devices.

It is important to note that the encoding context (knowledge of how the types and data structures look) needs to be known separately at both encoding and decoding ends. The encoded data does not include this contextual information.

To get a better understanding of how the encoding is done for different types, take a look at the SCALE Code page at the Substrate Knowledge Base.

Implementation

The codec is implemented using the following traits:

Encode

The Encode trait is used for encoding of data into the SCALE format. The Encode trait contains the following functions:

  • size_hint(&self) -> usize: Gets the capacity (in bytes) required for the encoded data. This is to avoid double-allocation of memory needed for the encoding. It can be an estimate and does not need to be an exact number. If the size is not known, even no good maximum, then we can skip this function from the trait implementation. This is required to be a cheap operation, so should not involve iterations etc.
  • encode_to<T: Output>(&self, dest: &mut T): Encodes the value and appends it to a destination buffer.
  • encode(&self) -> Vec<u8>: Encodes the type data and returns a slice.
  • using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R: Encodes the type data and executes a closure on the encoded value. Returns the result from the executed closure.

Note: Implementations should override using_encoded for value types and encode_to for allocating types. size_hint should be implemented for all types, wherever possible. Wrapper types should override all methods.

Decode

The Decode trait is used for deserialization/decoding of encoded data into the respective types.

  • fn decode<I: Input>(value: &mut I) -> Result<Self, Error>: Tries to decode the value from SCALE format to the type it is called on. Returns an Err if the decoding fails.

CompactAs

The CompactAs trait is used for wrapping custom types/structs as compact types, which makes them even more space/memory efficient. The compact encoding is described here.

  • encode_as(&self) -> &Self::As: Encodes the type (self) as a compact type. The type As is defined in the same trait and its implementation should be compact encode-able.
  • decode_from(_: Self::As) -> Result<Self, Error>: Decodes the type (self) from a compact encode-able type.

HasCompact

The HasCompact trait, if implemented, tells that the corresponding type is a compact encode-able type.

EncodeLike

The EncodeLike trait needs to be implemented for each type manually. When using derive, it is done automatically for you. Basically the trait gives you the opportunity to accept multiple types to a function that all encode to the same representation.

Usage Examples

Following are some examples to demonstrate usage of the codec.

Simple types


use parity_scale_codec::{Encode, Decode};

#[derive(Debug, PartialEq, Encode, Decode)]
enum EnumType {
	#[codec(index = 15)]
	A,
	B(u32, u64),
	C {
		a: u32,
		b: u64,
	},
}

let a = EnumType::A;
let b = EnumType::B(1, 2);
let c = EnumType::C { a: 1, b: 2 };

a.using_encoded(|ref slice| {
    assert_eq!(slice, &b"\x0f");
});

b.using_encoded(|ref slice| {
    assert_eq!(slice, &b"\x01\x01\0\0\0\x02\0\0\0\0\0\0\0");
});

c.using_encoded(|ref slice| {
    assert_eq!(slice, &b"\x02\x01\0\0\0\x02\0\0\0\0\0\0\0");
});

let mut da: &[u8] = b"\x0f";
assert_eq!(EnumType::decode(&mut da).ok(), Some(a));

let mut db: &[u8] = b"\x01\x01\0\0\0\x02\0\0\0\0\0\0\0";
assert_eq!(EnumType::decode(&mut db).ok(), Some(b));

let mut dc: &[u8] = b"\x02\x01\0\0\0\x02\0\0\0\0\0\0\0";
assert_eq!(EnumType::decode(&mut dc).ok(), Some(c));

let mut dz: &[u8] = &[0];
assert_eq!(EnumType::decode(&mut dz).ok(), None);

Compact type with HasCompact


use parity_scale_codec::{Encode, Decode, Compact, HasCompact};

#[derive(Debug, PartialEq, Encode, Decode)]
struct Test1CompactHasCompact<T: HasCompact> {
    #[codec(compact)]
    bar: T,
}

#[derive(Debug, PartialEq, Encode, Decode)]
struct Test1HasCompact<T: HasCompact> {
    #[codec(encoded_as = "<T as HasCompact>::Type")]
    bar: T,
}

let test_val: (u64, usize) = (0u64, 1usize);

let encoded = Test1HasCompact { bar: test_val.0 }.encode();
assert_eq!(encoded.len(), test_val.1);
assert_eq!(<Test1CompactHasCompact<u64>>::decode(&mut &encoded[..]).unwrap().bar, test_val.0);

Type with CompactAs


use serde_derive::{Serialize, Deserialize};
use parity_scale_codec::{Encode, Decode, Compact, HasCompact, CompactAs, Error};

#[cfg_attr(feature = "std", derive(Serialize, Deserialize, Debug))]
#[derive(PartialEq, Eq, Clone)]
struct StructHasCompact(u32);

impl CompactAs for StructHasCompact {
    type As = u32;

    fn encode_as(&self) -> &Self::As {
        &12
    }

    fn decode_from(_: Self::As) -> Result<Self, Error> {
        Ok(StructHasCompact(12))
    }
}

impl From<Compact<StructHasCompact>> for StructHasCompact {
    fn from(_: Compact<StructHasCompact>) -> Self {
        StructHasCompact(12)
    }
}

#[derive(Debug, PartialEq, Encode, Decode)]
enum TestGenericHasCompact<T> {
    A {
        #[codec(compact)] a: T
    },
}

let a = TestGenericHasCompact::A::<StructHasCompact> {
    a: StructHasCompact(12325678),
};

let encoded = a.encode();
assert_eq!(encoded.len(), 2);

Derive attributes

The derive implementation supports the following attributes:

  • codec(dumb_trait_bound): This attribute needs to be placed above the type that one of the trait should be implemented for. It will make the algorithm that determines the to-add trait bounds fall back to just use the type parameters of the type. This can be useful for situation where the algorithm includes private types in the public interface. By using this attribute, you should not get this error/warning again.
  • codec(skip): Needs to be placed above a field or variant and makes it to be skipped while encoding/decoding.
  • codec(compact): Needs to be placed above a field and makes the field use compact encoding. (The type needs to support compact encoding.)
  • codec(encoded_as = "OtherType"): Needs to be placed above a field and makes the field being encoded by using OtherType.
  • codec(index = 0): Needs to be placed above an enum variant to make the variant use the given index when encoded. By default the index is determined by counting from 0 beginning wth the first variant.
  • codec(encode_bound), codec(decode_bound) and codec(mel_bound): All 3 attributes take in a where clause for the Encode, Decode and MaxEncodedLen trait implementation for the annotated type respectively.
  • codec(encode_bound(skip_type_params)), codec(decode_bound(skip_type_params)) and codec(mel_bound(skip_type_params)): All 3 sub-attributes take in types as arguments to skip trait derivation of the corresponding trait, e.g. T in codec(encode_bound(skip_type_params(T))) will not contain a Encode trait bound while Encode is being derived for the annotated type.

Structs

  • Compact
    Compact-encoded variant of T. This is more space-efficient but less compute-efficient.
  • CompactRef
    Compact-encoded variant of &’a T. This is more space-efficient but less compute-efficient.
  • Error
    Error type.
  • IoReader
    Wrapper that implements Input for any Read type.
  • OptionBool
    Shim type because we can’t do a specialised implementation for Option<bool> directly.
  • Ref
    Reference wrapper that implement encode like any type that is encoded like its inner type.

Traits

  • Codec
    Trait that allows zero-copy read/write of value-references to/from slices in LE format.
  • CompactAs
    Allow foreign structs to be wrap in Compact
  • CompactLen
    Something that can return the compact encoded length for a given value.
  • Decode
    Trait that allows zero-copy read of value-references from slices in LE format.
  • DecodeAll
    Extension trait to Decode that ensures that the given input data is consumed completly while decoding.
  • DecodeLength
    Trait that allows the length of a collection to be read, without having to read and decode the entire elements.
  • DecodeLimit
    Extension trait to Decode for decoding with a maximum recursion depth.
  • Encode
    Trait that allows zero-copy write of value-references to slices in LE format.
  • EncodeAppend
    Trait that allows to append items to an encoded representation without decoding all previous added items.
  • EncodeAsRef
    Something that can be encoded as a reference.
  • EncodeLike
    A marker trait that tells the compiler that a type encode to the same representation as another type.
  • FullCodec
    Trait that bound EncodeLike along with Codec. Usefull for generic being used in function with EncodeLike parameters.
  • FullEncode
    Trait that bound EncodeLike along with Encode. Usefull for generic being used in function with EncodeLike parameters.
  • HasCompact
    Trait that tells you if a given type can be encoded/decoded in a compact way.
  • Input
    Trait that allows reading of data into a slice.
  • Joiner
    Trait to allow itself to be serialised into a value which can be extended by bytes.
  • KeyedVec
    Trait to allow itself to be serialised and prepended by a given slice.
  • MaxEncodedLen
    Items implementing MaxEncodedLen have a statically known maximum encoded size.
  • Output
    Trait that allows writing of data.
  • WrapperTypeDecode
    A marker trait for types that can be created solely from other decodable types.
  • WrapperTypeEncode
    A marker trait for types that wrap other encodable type.

Functions

Derive Macros

  • CompactAs
    Derive parity_scale_codec::Compact and parity_scale_codec::CompactAs for struct with single field.
  • Decode
    Derive parity_scale_codec::Decode and for struct and enum.
  • Encode
    Derive parity_scale_codec::Encode and parity_scale_codec::EncodeLike for struct and enum.
  • MaxEncodedLen
    Derive macro for MaxEncodedLen.