# 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 ["Type encoding (SCALE)" page in Substrate docs](https://docs.substrate.io/reference/scale-codec/).

## 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](https://docs.substrate.io/reference/scale-codec/#fn-1).

* `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

```rust
# // Import macros if derive feature is not used.
# #[cfg(not(feature="derive"))]
# use parity_scale_codec_derive::{Encode, Decode};

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);

# fn main() { }
```

### Compact type with HasCompact

```rust
# // Import macros if derive feature is not used.
# #[cfg(not(feature="derive"))]
# use parity_scale_codec_derive::{Encode, Decode};

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);

# fn main() { }
```

### Type with CompactAs

```rust
# // Import macros if derive feature is not used.
# #[cfg(not(feature="derive"))]
# use parity_scale_codec_derive::{Encode, Decode};

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);

# fn main() { }
```

## 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.

## Known issues

Even though this crate supports deserialization of arbitrarily sized array (e.g. `[T; 1024 * 1024 * 1024]`)
using such types is not recommended and will most likely result in a stack overflow. If you have a big
array inside of your structure which you want to decode you should wrap it in a `Box`, e.g. `Box<[T; 1024 * 1024 * 1024]>`.

-------------------------

License: Apache-2.0