codec.rs

// Copyright 2017, 2018 Parity Technologies
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Serialisation.

use crate::alloc::vec::Vec;
use crate::alloc::boxed::Box;
use crate::alloc::collections::btree_map::BTreeMap;
use crate::alloc::collections::btree_set::BTreeSet;

#[cfg(any(feature = "std", feature = "full"))]
use crate::alloc::{
	string::String,
	borrow::Cow,
};

use core::{mem, slice, ops::Deref};
use arrayvec::ArrayVec;
use core::marker::PhantomData;

#[cfg(feature = "std")]
use std::fmt;

#[cfg_attr(feature = "std", derive(Debug))]
#[derive(PartialEq)]
#[cfg(feature = "std")]
/// Descriptive error type
pub struct Error(&'static str);

#[cfg(not(feature = "std"))]
#[derive(PartialEq)]
pub struct Error;

impl Error {
	#[cfg(feature = "std")]
	/// Error description
	///
	/// This function returns an actual error str when running in `std`
	/// environment, but `""` on `no_std`.
	pub fn what(&self) -> &'static str {
		self.0
	}

	#[cfg(not(feature = "std"))]
	/// Error description
	///
	/// This function returns an actual error str when running in `std`
	/// environment, but `""` on `no_std`.
	pub fn what(&self) -> &'static str {
		""
	}
}

#[cfg(feature = "std")]
impl std::fmt::Display for Error {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
		write!(f, "{}", self.0)
	}
}

#[cfg(feature = "std")]
impl std::error::Error for Error {
	fn description(&self) -> &str {
		self.0
	}
}

impl From<&'static str> for Error {
	#[cfg(feature = "std")]
	fn from(s: &'static str) -> Error {
		Error(s)
	}

	#[cfg(not(feature = "std"))]
	fn from(_s: &'static str) -> Error {
		Error
	}
}

/// Trait that allows reading of data into a slice.
pub trait Input {
	/// Read into the provided input slice. Returns the number of bytes read.
	///
	/// Note that this function should be more like `std::io::Read::read_exact`
	/// than `std::io::Read::read`. I.e. the buffer should always be filled
	/// with as many bytes as available and if `n < into.len()` is returned
	/// then it should mean that there was not enough bytes available and the
	/// `Input` is drained.
	///
	/// Callers of this function should not need to call again if `n < into.len()`
	/// is returned.
	fn read(&mut self, into: &mut [u8]) -> Result<usize, Error>;

	/// Read a single byte from the input.
	fn read_byte(&mut self) -> Result<u8, Error> {
		let mut buf = [0u8];
		self.read(&mut buf[..])?;
		Ok(buf[0])
	}
}

#[cfg(not(feature = "std"))]
impl<'a> Input for &'a [u8] {
	fn read(&mut self, into: &mut [u8]) -> Result<usize, Error> {
		if into.len() > self.len() {
			return Err("".into());
		}
		let len = ::core::cmp::min(into.len(), self.len());
		into[..len].copy_from_slice(&self[..len]);
		*self = &self[len..];
		Ok(len)
	}
}

#[cfg(feature = "std")]
impl From<std::io::Error> for Error {
	fn from(_err: std::io::Error) -> Self {
		"io error".into()
	}
}

#[cfg(feature = "std")]
impl<R: std::io::Read> Input for R {
	fn read(&mut self, into: &mut [u8]) -> Result<usize, Error> {
		(self as &mut dyn std::io::Read).read_exact(into)?;
		Ok(into.len())
	}
}

/// Prefix another input with a byte.
struct PrefixInput<'a, T> {
	prefix: Option<u8>,
	input: &'a mut T,
}

impl<'a, T: 'a + Input> Input for PrefixInput<'a, T> {
	fn read(&mut self, buffer: &mut [u8]) -> Result<usize, Error> {
		match self.prefix.take() {
			Some(v) if !buffer.is_empty() => {
				buffer[0] = v;
				let res = 1 + self.input.read(&mut buffer[1..])?;
				Ok(res)
			}
			_ => self.input.read(buffer)
		}
	}
}

/// Trait that allows writing of data.
pub trait Output: Sized {
	/// Write to the output.
	fn write(&mut self, bytes: &[u8]);

	/// Write a single byte to the output.
	fn push_byte(&mut self, byte: u8) {
		self.write(&[byte]);
	}

	/// Write encoding of given value to the output.
	fn push<V: Encode + ?Sized>(&mut self, value: &V) {
		value.encode_to(self);
	}
}

#[cfg(not(feature = "std"))]
impl Output for Vec<u8> {
	fn write(&mut self, bytes: &[u8]) {
		self.extend_from_slice(bytes)
	}
}

#[cfg(feature = "std")]
impl<W: std::io::Write> Output for W {
	fn write(&mut self, bytes: &[u8]) {
		(self as &mut dyn std::io::Write).write_all(bytes).expect("Codec outputs are infallible");
	}
}

struct ArrayVecWrapper<T: arrayvec::Array>(ArrayVec<T>);

impl<T: arrayvec::Array<Item=u8>> Output for ArrayVecWrapper<T> {
	fn write(&mut self, bytes: &[u8]) {
		let old_len = self.0.len();
		let new_len = old_len + bytes.len();

		assert!(new_len <= self.0.capacity());
		unsafe {
			self.0.set_len(new_len);
		}

		self.0[old_len..new_len].copy_from_slice(bytes);
	}

	fn push_byte(&mut self, byte: u8) {
		self.0.push(byte);
	}
}

/// This enum must not be exported and must only be instantiable by parity-codec.
/// Because implementation of Encode and Decode for u8 is done in this crate
/// and there is not other usage.
pub enum IsU8 {
	Yes,
	No,
}

/// Trait that allows zero-copy write of value-references to slices in LE format.
///
/// Implementations should override `using_encoded` for value types and `encode_to` and `size_hint` for allocating types.
/// Wrapper types should override all methods.
pub trait Encode {
	#[doc(hidden)]
	// This const is used to optimise implementation of codec for Vec<u8>.
	const IS_U8: IsU8 = IsU8::No;

	/// If possible give a hint of expected size of the encoding.
	///
	/// This method is used inside default implementation of `encode`
	/// to avoid re-allocations.
	fn size_hint(&self) -> usize {
		0
	}

	/// Convert self to a slice and append it to the destination.
	fn encode_to<T: Output>(&self, dest: &mut T) {
		self.using_encoded(|buf| dest.write(buf));
	}

	/// Convert self to an owned vector.
	fn encode(&self) -> Vec<u8> {
		let mut r = Vec::with_capacity(self.size_hint());
		self.encode_to(&mut r);
		r
	}

	/// Convert self to a slice and then invoke the given closure with it.
	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		f(&self.encode())
	}
}

/// Trait that allows to append items to an encoded representation without
/// decoding all previous added items.
pub trait EncodeAppend {
	/// The item that will be appended.
	type Item: Encode;

	/// Append `to_append` items to the given `self_encoded` representation.
	fn append(self_encoded: Vec<u8>, to_append: &[Self::Item]) -> Result<Vec<u8>, Error>;
}

/// Trait that allows zero-copy read of value-references from slices in LE format.
pub trait Decode: Sized {
	#[doc(hidden)]
	const IS_U8: IsU8 = IsU8::No;

	/// Attempt to deserialise the value from input.
	fn decode<I: Input>(value: &mut I) -> Result<Self, Error>;
}

/// Trait that allows zero-copy read/write of value-references to/from slices in LE format.
pub trait Codec: Decode + Encode {}
impl<S: Decode + Encode> Codec for S {}

/// A marker trait for types that wrap other encodable type.
///
/// Such types should not carry any additional information
/// that would require to be encoded, because the encoding
/// is assumed to be the same as the wrapped type.
pub trait WrapperTypeEncode: Deref {}

impl<T> WrapperTypeEncode for Vec<T> {}
impl<T: ?Sized> WrapperTypeEncode for Box<T> {}
impl<'a, T: ?Sized> WrapperTypeEncode for &'a T {}
impl<'a, T: ?Sized> WrapperTypeEncode for &'a mut T {}

#[cfg(any(feature = "std", feature = "full"))]
impl<'a, T: ToOwned + ?Sized> WrapperTypeEncode for Cow<'a, T> {}
#[cfg(any(feature = "std", feature = "full"))]
impl<T: ?Sized> WrapperTypeEncode for std::sync::Arc<T> {}
#[cfg(any(feature = "std", feature = "full"))]
impl<T: ?Sized> WrapperTypeEncode for std::rc::Rc<T> {}
#[cfg(any(feature = "std", feature = "full"))]
impl WrapperTypeEncode for String {}

impl<T, X> Encode for X where
	T: Encode + ?Sized,
	X: WrapperTypeEncode<Target=T>,
{
	fn size_hint(&self) -> usize {
		(&**self).size_hint()
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		(&**self).using_encoded(f)
	}

	fn encode(&self) -> Vec<u8> {
		(&**self).encode()
	}

	fn encode_to<W: Output>(&self, dest: &mut W) {
		(&**self).encode_to(dest)
	}
}

/// A marker trait for types that can be created solely from other decodable types.
///
/// The decoding of such type is assumed to be the same as the wrapped type.
pub trait WrapperTypeDecode: Sized {
	/// A wrapped type.
	type Wrapped: Into<Self>;
}
impl<T> WrapperTypeDecode for Box<T> {
	type Wrapped = T;
}
#[cfg(any(feature = "std", feature = "full"))]
impl<T> WrapperTypeDecode for std::sync::Arc<T> {
	type Wrapped = T;
}
#[cfg(any(feature = "std", feature = "full"))]
impl<T> WrapperTypeDecode for std::rc::Rc<T> {
	type Wrapped = T;
}

impl<T, X> Decode for X where
	T: Decode + Into<X>,
	X: WrapperTypeDecode<Wrapped=T>,
{
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		Ok(T::decode(input)?.into())
	}
}

/// Something that can return the compact encoded length for a given value.
pub trait CompactLen<T> {
	/// Returns the compact encoded length for the given value.
	fn compact_len(val: &T) -> usize;
}

/// Compact-encoded variant of T. This is more space-efficient but less compute-efficient.
#[derive(Eq, PartialEq, Clone, Copy, Ord, PartialOrd)]
pub struct Compact<T>(pub T);

impl<T> From<T> for Compact<T> {
	fn from(x: T) -> Compact<T> { Compact(x) }
}

impl<'a, T: Copy> From<&'a T> for Compact<T> {
	fn from(x: &'a T) -> Compact<T> { Compact(*x) }
}

/// Allow foreign structs to be wrap in Compact
pub trait CompactAs: From<Compact<Self>> {
	/// A compact-encodable type that should be used as the encoding.
	type As;

	/// Returns the encodable type.
	fn encode_as(&self) -> &Self::As;

	/// Create `Self` from the decodable type.
	fn decode_from(_: Self::As) -> Self;
}

impl<T> Encode for Compact<T>
where
	for<'a> CompactRef<'a, T>: Encode,
{
	fn size_hint(&self) -> usize {
		CompactRef(&self.0).size_hint()
	}

	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&self.0).encode_to(dest)
	}

	fn encode(&self) -> Vec<u8> {
		CompactRef(&self.0).encode()
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		CompactRef(&self.0).using_encoded(f)
	}
}

impl<'a, T> Encode for CompactRef<'a, T>
where
	T: CompactAs,
	for<'b> CompactRef<'b, T::As>: Encode,
{
	fn size_hint(&self) -> usize {
		CompactRef(self.0.encode_as()).size_hint()
	}

	fn encode_to<Out: Output>(&self, dest: &mut Out) {
		CompactRef(self.0.encode_as()).encode_to(dest)
	}

	fn encode(&self) -> Vec<u8> {
		CompactRef(self.0.encode_as()).encode()
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		CompactRef(self.0.encode_as()).using_encoded(f)
	}
}

impl<T> Decode for Compact<T>
where
	T: CompactAs,
	Compact<T::As>: Decode,
{
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		Compact::<T::As>::decode(input)
			.map(|x| Compact(<T as CompactAs>::decode_from(x.0)))
	}
}

macro_rules! impl_from_compact {
	( $( $ty:ty ),* ) => {
		$(
			impl From<Compact<$ty>> for $ty {
				fn from(x: Compact<$ty>) -> $ty { x.0 }
			}
		)*
	}
}

impl_from_compact! { (), u8, u16, u32, u64, u128 }

/// Compact-encoded variant of &'a T. This is more space-efficient but less compute-efficient.
#[derive(Eq, PartialEq, Clone, Copy)]
pub struct CompactRef<'a, T>(pub &'a T);

impl<'a, T> From<&'a T> for CompactRef<'a, T> {
	fn from(x: &'a T) -> Self { CompactRef(x) }
}

impl<T> ::core::fmt::Debug for Compact<T> where T: ::core::fmt::Debug {
	fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
		self.0.fmt(f)
	}
}

#[cfg(feature = "std")]
impl<T> serde::Serialize for Compact<T> where T: serde::Serialize {
	fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: serde::Serializer {
		T::serialize(&self.0, serializer)
	}
}

#[cfg(feature = "std")]
impl<'de, T> serde::Deserialize<'de> for Compact<T> where T: serde::Deserialize<'de> {
	fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: serde::Deserializer<'de> {
		T::deserialize(deserializer).map(Compact)
	}
}

#[cfg(feature = "std")]
pub trait MaybeDebugSerde: core::fmt::Debug + serde::Serialize + for<'a> serde::Deserialize<'a> {}
#[cfg(feature = "std")]
impl<T> MaybeDebugSerde for T where T: core::fmt::Debug + serde::Serialize + for<'a> serde::Deserialize<'a> {}

#[cfg(not(feature = "std"))]
pub trait MaybeDebugSerde {}
#[cfg(not(feature = "std"))]
impl<T> MaybeDebugSerde for T {}

/// Trait that tells you if a given type can be encoded/decoded in a compact way.
pub trait HasCompact: Sized {
	/// The compact type; this can be
	type Type: for<'a> EncodeAsRef<'a, Self> + Decode + From<Self> + Into<Self> + Clone +
		PartialEq + Eq + MaybeDebugSerde;
}

/// Something that can be encoded as a reference.
pub trait EncodeAsRef<'a, T: 'a> {
	/// The reference type that is used for encoding.
	type RefType: Encode + From<&'a T>;
}

impl<'a, T: 'a> EncodeAsRef<'a, T> for Compact<T> where CompactRef<'a, T>: Encode + From<&'a T> {
	type RefType = CompactRef<'a, T>;
}

impl<T: 'static> HasCompact for T where
	Compact<T>: for<'a> EncodeAsRef<'a, T> + Decode + From<Self> + Into<Self> + Clone +
		PartialEq + Eq + MaybeDebugSerde,
{
	type Type = Compact<T>;
}

// compact encoding:
// 0b00 00 00 00 / 00 00 00 00 / 00 00 00 00 / 00 00 00 00
//   xx xx xx 00															(0 .. 2**6)		(u8)
//   yL yL yL 01 / yH yH yH yL												(2**6 .. 2**14)	(u8, u16)  low LH high
//   zL zL zL 10 / zM zM zM zL / zM zM zM zM / zH zH zH zM					(2**14 .. 2**30)	(u16, u32)  low LMMH high
//   nn nn nn 11 [ / zz zz zz zz ]{4 + n}									(2**30 .. 2**536)	(u32, u64, u128, U256, U512, U520) straight LE-encoded

// Note: we use *LOW BITS* of the LSB in LE encoding to encode the 2 bit key.

impl<'a> Encode for CompactRef<'a, ()> {
	fn encode_to<W: Output>(&self, _dest: &mut W) {
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		f(&[])
	}

	fn encode(&self) -> Vec<u8> {
		Vec::new()
	}
}

impl<'a> Encode for CompactRef<'a, u8> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0..=0b0011_1111 => dest.push_byte(self.0 << 2),
			_ => ((u16::from(*self.0) << 2) | 0b01).encode_to(dest),
		}
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		let mut r = ArrayVecWrapper(ArrayVec::<[u8; 2]>::new());
		self.encode_to(&mut r);
		f(&r.0)
	}
}

impl CompactLen<u8> for Compact<u8> {
	fn compact_len(val: &u8) -> usize {
		match val {
			0..=0b0011_1111 => 1,
			_ => 2,
		}
	}
}

impl<'a> Encode for CompactRef<'a, u16> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0..=0b0011_1111 => dest.push_byte((*self.0 as u8) << 2),
			0..=0b0011_1111_1111_1111 => ((*self.0 << 2) | 0b01).encode_to(dest),
			_ => ((u32::from(*self.0) << 2) | 0b10).encode_to(dest),
		}
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		let mut r = ArrayVecWrapper(ArrayVec::<[u8; 4]>::new());
		self.encode_to(&mut r);
		f(&r.0)
	}
}

impl CompactLen<u16> for Compact<u16> {
	fn compact_len(val: &u16) -> usize {
		match val {
			0..=0b0011_1111 => 1,
			0..=0b0011_1111_1111_1111 => 2,
			_ => 4,
		}
	}
}

impl<'a> Encode for CompactRef<'a, u32> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0..=0b0011_1111 => dest.push_byte((*self.0 as u8) << 2),
			0..=0b0011_1111_1111_1111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0..=0b0011_1111_1111_1111_1111_1111_1111_1111 => ((*self.0 << 2) | 0b10).encode_to(dest),
			_ => {
				dest.push_byte(0b11);
				self.0.encode_to(dest);
			}
		}
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		let mut r = ArrayVecWrapper(ArrayVec::<[u8; 5]>::new());
		self.encode_to(&mut r);
		f(&r.0)
	}
}

impl CompactLen<u32> for Compact<u32> {
	fn compact_len(val: &u32) -> usize {
		match val {
			0..=0b0011_1111 => 1,
			0..=0b0011_1111_1111_1111 => 2,
			0..=0b0011_1111_1111_1111_1111_1111_1111_1111 => 4,
			_ => 5,
		}
	}
}

impl<'a> Encode for CompactRef<'a, u64> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0..=0b0011_1111 => dest.push_byte((*self.0 as u8) << 2),
			0..=0b0011_1111_1111_1111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0..=0b0011_1111_1111_1111_1111_1111_1111_1111 => (((*self.0 as u32) << 2) | 0b10).encode_to(dest),
			_ => {
				let bytes_needed = 8 - self.0.leading_zeros() / 8;
				assert!(bytes_needed >= 4, "Previous match arm matches anyting less than 2^30; qed");
				dest.push_byte(0b11 + ((bytes_needed - 4) << 2) as u8);
				let mut v = *self.0;
				for _ in 0..bytes_needed {
					dest.push_byte(v as u8);
					v >>= 8;
				}
				assert_eq!(v, 0, "shifted sufficient bits right to lead only leading zeros; qed")
			}
		}
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		let mut r = ArrayVecWrapper(ArrayVec::<[u8; 9]>::new());
		self.encode_to(&mut r);
		f(&r.0)
	}
}

impl CompactLen<u64> for Compact<u64> {
	fn compact_len(val: &u64) -> usize {
		match val {
			0..=0b0011_1111 => 1,
			0..=0b0011_1111_1111_1111 => 2,
			0..=0b0011_1111_1111_1111_1111_1111_1111_1111 => 4,
			_ => {
				(8 - val.leading_zeros() / 8) as usize + 1
			},
		}
	}
}

impl<'a> Encode for CompactRef<'a, u128> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0..=0b0011_1111 => dest.push_byte((*self.0 as u8) << 2),
			0..=0b0011_1111_1111_1111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0..=0b0011_1111_1111_1111_11111_111_1111_1111 => (((*self.0 as u32) << 2) | 0b10).encode_to(dest),
			_ => {
				let bytes_needed = 16 - self.0.leading_zeros() / 8;
				assert!(bytes_needed >= 4, "Previous match arm matches anyting less than 2^30; qed");
				dest.push_byte(0b11 + ((bytes_needed - 4) << 2) as u8);
				let mut v = *self.0;
				for _ in 0..bytes_needed {
					dest.push_byte(v as u8);
					v >>= 8;
				}
				assert_eq!(v, 0, "shifted sufficient bits right to lead only leading zeros; qed")
			}
		}
	}

	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		let mut r = ArrayVecWrapper(ArrayVec::<[u8; 17]>::new());
		self.encode_to(&mut r);
		f(&r.0)
	}
}

impl CompactLen<u128> for Compact<u128> {
	fn compact_len(val: &u128) -> usize {
		match val {
			0..=0b0011_1111 => 1,
			0..=0b0011_1111_1111_1111 => 2,
			0..=0b0011_1111_1111_1111_1111_1111_1111_1111 => 4,
			_ => {
				(16 - val.leading_zeros() / 8) as usize + 1
			},
		}
	}
}

impl Decode for Compact<()> {
	fn decode<I: Input>(_input: &mut I) -> Result<Self, Error> {
		Ok(Compact(()))
	}
}

const U8_OUT_OF_RANGE: &'static str = "out of range decoding Compact<u8>";
const U16_OUT_OF_RANGE: &'static str = "out of range decoding Compact<u16>";
const U32_OUT_OF_RANGE: &'static str = "out of range decoding Compact<u32>";
const U64_OUT_OF_RANGE: &'static str = "out of range decoding Compact<u64>";
const U128_OUT_OF_RANGE: &'static str = "out of range decoding Compact<u128>";

impl Decode for Compact<u8> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		let prefix = input.read_byte()?;
		Ok(Compact(match prefix % 4 {
			0 => prefix >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111 && x <= 255 {
					x as u8
				} else {
					return Err(U8_OUT_OF_RANGE.into());
				}
			},
			_ => return Err("unexpected prefix decoding Compact<u8>".into()),
		}))
	}
}

impl Decode for Compact<u16> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		let prefix = input.read_byte()?;
		Ok(Compact(match prefix % 4 {
			0 => u16::from(prefix) >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111 && x <= 0b00111111_11111111 {
					u16::from(x)
				} else {
					return Err(U16_OUT_OF_RANGE.into());
				}
			},
			2 => {
				let x = u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111_11111111 && x < 65536 {
					x as u16
				} else {
					return Err(U16_OUT_OF_RANGE.into());
				}
			},
			_ => return Err("unexpected prefix decoding Compact<u16>".into()),
		}))
	}
}

impl Decode for Compact<u32> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		let prefix = input.read_byte()?;
		Ok(Compact(match prefix % 4 {
			0 => u32::from(prefix) >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111 && x <= 0b00111111_11111111 {
					u32::from(x)
				} else {
					return Err(U32_OUT_OF_RANGE.into());
				}
			},
			2 => {
				let x = u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111_11111111 && x <= u32::max_value() >> 2 {
					u32::from(x)
				} else {
					return Err(U32_OUT_OF_RANGE.into());
				}
			},
			3|_ => {	// |_. yeah, i know.
				if prefix >> 2 == 0 {
					// just 4 bytes. ok.
					let x = u32::decode(input)?;
					if x > u32::max_value() >> 2 {
						u32::from(x)
					} else {
						return Err(U32_OUT_OF_RANGE.into());
					}
				} else {
					// Out of range for a 32-bit quantity.
					return Err(U32_OUT_OF_RANGE.into());
				}
			}
		}))
	}
}

impl Decode for Compact<u64> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		let prefix = input.read_byte()?;
		Ok(Compact(match prefix % 4 {
			0 => u64::from(prefix) >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111 && x <= 0b00111111_11111111 {
					u64::from(x)
				} else {
					return Err(U64_OUT_OF_RANGE.into());
				}
			},
			2 => {
				let x = u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111_11111111 && x <= u32::max_value() >> 2 {
					u64::from(x)
				} else {
					return Err(U64_OUT_OF_RANGE.into());
				}
			},
			3|_ => match (prefix >> 2) + 4 {
				4 => {
					let x = u32::decode(input)?;
					if x > u32::max_value() >> 2 {
						u64::from(x)
					} else {
						return Err(U64_OUT_OF_RANGE.into());
					}
				},
				8 => {
					let x = u64::decode(input)?;
					if x > u64::max_value() >> 8 {
						x
					} else {
						return Err(U64_OUT_OF_RANGE.into());
					}
				},
				x if x > 8 => return Err("unexpected prefix decoding Compact<u64>".into()),
				bytes_needed => {
					let mut res = 0;
					for i in 0..bytes_needed {
						res |= u64::from(input.read_byte()?) << (i * 8);
					}
					if res > u64::max_value() >> (8 - bytes_needed + 1) * 8 {
						res
					} else {
						return Err(U64_OUT_OF_RANGE.into());
					}
				},
			},
		}))
	}
}

impl Decode for Compact<u128> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		let prefix = input.read_byte()?;
		Ok(Compact(match prefix % 4 {
			0 => u128::from(prefix) >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111 && x <= 0b00111111_11111111 {
					u128::from(x)
				} else {
					return Err(U128_OUT_OF_RANGE.into());
				}
			},
			2 => {
				let x = u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x > 0b00111111_11111111 && x <= u32::max_value() >> 2 {
					u128::from(x)
				} else {
					return Err(U128_OUT_OF_RANGE.into());
				}
			},
			3|_ => match (prefix >> 2) + 4 {
				4 => {
					let x = u32::decode(input)?;
					if x > u32::max_value() >> 2 {
						u128::from(x)
					} else {
						return Err(U128_OUT_OF_RANGE.into());
					}
				},
				8 => {
					let x = u64::decode(input)?;
					if x > u64::max_value() >> 8 {
						u128::from(x)
					} else {
						return Err(U128_OUT_OF_RANGE.into());
					}
				},
				16 => {
					let x = u128::decode(input)?;
					if x > u128::max_value() >> 8 {
						x
					} else {
						return Err(U128_OUT_OF_RANGE.into());
					}
				},
				x if x > 16 => return Err("unexpected prefix decoding Compact<u128>".into()),
				bytes_needed => {
					let mut res = 0;
					for i in 0..bytes_needed {
						res |= u128::from(input.read_byte()?) << (i * 8);
					}
					if res > u128::max_value() >> (16 - bytes_needed + 1) * 8 {
						res
					} else {
						return Err(U128_OUT_OF_RANGE.into());
					}
				},
			},
		}))
	}
}

impl<T: Encode, E: Encode> Encode for Result<T, E> {
	fn size_hint(&self) -> usize {
		1 + match *self {
			Ok(ref t) => t.size_hint(),
			Err(ref t) => t.size_hint(),
		}
	}

	fn encode_to<W: Output>(&self, dest: &mut W) {
		match *self {
			Ok(ref t) => {
				dest.push_byte(0);
				t.encode_to(dest);
			}
			Err(ref e) => {
				dest.push_byte(1);
				e.encode_to(dest);
			}
		}
	}
}

impl<T: Decode, E: Decode> Decode for Result<T, E> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		match input.read_byte()? {
			0 => Ok(Ok(T::decode(input)?)),
			1 => Ok(Err(E::decode(input)?)),
			_ => Err("unexpected first byte decoding Result".into()),
		}
	}
}

/// Shim type because we can't do a specialised implementation for `Option<bool>` directly.
#[derive(Eq, PartialEq, Clone, Copy)]
pub struct OptionBool(pub Option<bool>);

impl core::fmt::Debug for OptionBool {
	fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
		self.0.fmt(f)
	}
}

impl Encode for OptionBool {
	fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
		f(&[match *self {
			OptionBool(None) => 0u8,
			OptionBool(Some(true)) => 1u8,
			OptionBool(Some(false)) => 2u8,
		}])
	}
}

impl Decode for OptionBool {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		match input.read_byte()? {
			0 => Ok(OptionBool(None)),
			1 => Ok(OptionBool(Some(true))),
			2 => Ok(OptionBool(Some(false))),
			_ => Err("unexpected first byte decoding OptionBool".into()),
		}
	}
}

impl<T: Encode> Encode for Option<T> {
	fn size_hint(&self) -> usize {
		1 + match *self {
			Some(ref t) => t.size_hint(),
			None => 0,
		}
	}

	fn encode_to<W: Output>(&self, dest: &mut W) {
		match *self {
			Some(ref t) => {
				dest.push_byte(1);
				t.encode_to(dest);
			}
			None => dest.push_byte(0),
		}
	}
}

impl<T: Decode> Decode for Option<T> {
	fn decode<I: Input>(input: &mut I) -> Result<Self, Error> {
		match input.read_byte()? {
			0 => Ok(None),
			1 => Ok(Some(T::decode(input)?)),
			_ => Err("unexpecded first byte decoding Option".into()),
		}
	}
}

macro_rules! impl_array {
	( $( $n:expr, )* ) => { $(
		impl<T: Encode> Encode for [T; $n] {
			fn encode_to<W: Output>(&self, dest: &mut W) {
				for item in self.iter() {
					item.encode_to(dest);
				}
			}
		}