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 anErr
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 typeAs
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 usingOtherType
.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 from0
beginning wth the first variant.codec(encode_bound)
,codec(decode_bound)
andcodec(mel_bound)
: All 3 attributes take in awhere
clause for theEncode
,Decode
andMaxEncodedLen
trait implementation for the annotated type respectively.codec(encode_bound(skip_type_params))
,codec(decode_bound(skip_type_params))
andcodec(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 incodec(encode_bound(skip_type_params(T)))
will not contain aEncode
trait bound whileEncode
is being derived for the annotated type.
Structs
- Compact-encoded variant of T. This is more space-efficient but less compute-efficient.
- Compact-encoded variant of &’a T. This is more space-efficient but less compute-efficient.
- Error type.
- Wrapper that implements Input for any
Read
type. - Shim type because we can’t do a specialised implementation for
Option<bool>
directly. - Reference wrapper that implement encode like any type that is encoded like its inner type.
Traits
- Trait that allows zero-copy read/write of value-references to/from slices in LE format.
- Allow foreign structs to be wrap in Compact
- Something that can return the compact encoded length for a given value.
- Trait that allows zero-copy read of value-references from slices in LE format.
- Extension trait to
Decode
that ensures that the given input data is consumed completly while decoding. - Trait that allows the length of a collection to be read, without having to read and decode the entire elements.
- Extension trait to
Decode
for decoding with a maximum recursion depth. - Trait that allows zero-copy write of value-references to slices in LE format.
- Trait that allows to append items to an encoded representation without decoding all previous added items.
- Something that can be encoded as a reference.
- A marker trait that tells the compiler that a type encode to the same representation as another type.
- Trait that bound
EncodeLike
along withCodec
. Usefull for generic being used in function withEncodeLike
parameters. - Trait that bound
EncodeLike
along withEncode
. Usefull for generic being used in function withEncodeLike
parameters. - Trait that tells you if a given type can be encoded/decoded in a compact way.
- Trait that allows reading of data into a slice.
- Trait to allow itself to be serialised into a value which can be extended by bytes.
- Trait to allow itself to be serialised and prepended by a given slice.
- Items implementing
MaxEncodedLen
have a statically known maximum encoded size. - Trait that allows writing of data.
- A marker trait for types that can be created solely from other decodable types.
- A marker trait for types that wrap other encodable type.
Functions
- Decodes a given
T
fromBytes
. - Decode the vec (without a prepended len).
Derive Macros
- Derive
parity_scale_codec::Compact
andparity_scale_codec::CompactAs
for struct with single field. - Derive
parity_scale_codec::Decode
and for struct and enum. - Derive
parity_scale_codec::Encode
andparity_scale_codec::EncodeLike
for struct and enum. - Derive macro for
MaxEncodedLen
.