# Tetsy SCALE Codec
Rust implementation of the SCALE (Simple Concatenated Aggregate Little-Endian) data format
for types used in the Tetsy Tetcore 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
[low-level data formats overview page at the Tetcore docs site](https://dev.tetcoin.org/docs/en/overview/low-level-data-format).
## 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://substrate.dev/docs/en/overview/low-level-data-format#compact-general-integers).
* `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) -> Self`: 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
use tetsy_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
```rust
use tetsy_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
```rust
use serde_derive::{Serialize, Deserialize};
use tetsy_scale_codec::{Encode, Decode, Compact, HasCompact, CompactAs};
#[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) -> Self {
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);
```
License: Apache-2.0