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 alloc::vec::Vec;
use alloc::boxed::Box;

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

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

/// Trait that allows reading of data into a slice.
pub trait Input {
	/// Read into the provided input slice. Returns the number of bytes read.
	fn read(&mut self, into: &mut [u8]) -> usize;

	/// Read a single byte from the input.
	fn read_byte(&mut self) -> Option<u8> {
		let mut buf = [0u8];
		match self.read(&mut buf[..]) {
			0 => None,
			1 => Some(buf[0]),
			_ => unreachable!(),
		}
	}
}

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

#[cfg(feature = "std")]
impl<R: ::std::io::Read> Input for R {
	fn read(&mut self, into: &mut [u8]) -> usize {
		match (self as &mut ::std::io::Read).read_exact(into) {
			Ok(()) => into.len(),
			Err(_) => 0,
		}
	}
}

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

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

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

	fn push_byte(&mut self, byte: u8) {
		self.write(&[byte]);
	}

	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(bytes);
	}
}

#[cfg(feature = "std")]
impl<W: ::std::io::Write> Output for W {
	fn write(&mut self, bytes: &[u8]) {
		(self as &mut ::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]) {
		for byte in bytes {
			self.push_byte(*byte);
		}
	}

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

/// 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` for allocating types.
pub trait Encode {
	/// 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::new();
		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 zero-copy read of value-references from slices in LE format.
pub trait Decode: Sized {
	/// Attempt to deserialise the value from input.
	fn decode<I: Input>(value: &mut I) -> Option<Self>;
}

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

/// 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>> {
	type As;
	fn encode_as(&self) -> &Self::As;
	fn decode_from(Self::As) -> Self;
}

impl<T> Encode for Compact<T>
where
	T: CompactAs,
	for<'a> CompactRef<'a, <T as CompactAs>::As>: Encode,
{
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(self.0.encode_as()).encode_to(dest)
	}
}

impl<'a, T> Encode for CompactRef<'a, T>
where
	T: CompactAs,
	for<'b> CompactRef<'b, <T as CompactAs>::As>: Encode,
{
	fn encode_to<Out: Output>(&self, dest: &mut Out) {
		CompactRef(self.0.encode_as()).encode_to(dest)
	}
}

impl<T> Decode for Compact<T>
where
	T: CompactAs,
	Compact<<T as CompactAs>::As>: Decode,
{
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		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: 'a>(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 - 1)		(u8)
//   yL yL yL 01 / yH yH yH yL												(2**6 ... 2**14 - 1)	(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 - 1)	(u16, u32)  low LMMH high
//   nn nn nn 11 [ / zz zz zz zz ]{4 + n}									(2**30 ... 2**536 - 1)	(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, u8> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0...0b00111111 => dest.push_byte(self.0 << 2),
			_ => (((*self.0 as u16) << 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 Encode for Compact<u8> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&self.0).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<'a> Encode for CompactRef<'a, u16> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0...0b00111111 => dest.push_byte((*self.0 as u8) << 2),
			0...0b00111111_11111111 => ((*self.0 << 2) | 0b01).encode_to(dest),
			_ => (((*self.0 as u32) << 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 Encode for Compact<u16> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&self.0).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<'a> Encode for CompactRef<'a, u32> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0...0b00111111 => dest.push_byte((*self.0 as u8) << 2),
			0...0b00111111_11111111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0...0b00111111_11111111_11111111_11111111 => ((*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 Encode for Compact<u32> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&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<'a> Encode for CompactRef<'a, u64> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0...0b00111111 => dest.push_byte((*self.0 as u8) << 2),
			0...0b00111111_11111111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0...0b00111111_11111111_11111111_11111111 => (((*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 Encode for Compact<u64> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&self.0).encode_to(dest)
	}

	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<'a> Encode for CompactRef<'a, u128> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self.0 {
			0...0b00111111 => dest.push_byte((*self.0 as u8) << 2),
			0...0b00111111_11111111 => (((*self.0 as u16) << 2) | 0b01).encode_to(dest),
			0...0b00111111_11111111_11111111_11111111 => (((*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 Encode for Compact<u128> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		CompactRef(&self.0).encode_to(dest)
	}

	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 Decode for Compact<u8> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		let prefix = input.read_byte()?;
		Some(Compact(match prefix % 4 {
			0 => prefix as u8 >> 2,
			1 => {
				let x = u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x < 256 {
					x as u8
				} else {
					return None
				}
			}
			_ => return None,
		}))
	}
}

impl Decode for Compact<u16> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		let prefix = input.read_byte()?;
		Some(Compact(match prefix % 4 {
			0 => prefix as u16 >> 2,
			1 => u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u16 >> 2,
			2 => {
				let x = u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? >> 2;
				if x < 65536 {
					x as u16
				} else {
					return None
				}
			}
			_ => return None,
		}))
	}
}

impl Decode for Compact<u32> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		let prefix = input.read_byte()?;
		Some(Compact(match prefix % 4 {
			0 => prefix as u32 >> 2,
			1 => u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u32 >> 2,
			2 => u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u32 >> 2,
			3|_ => {	// |_. yeah, i know.
				if prefix >> 2 == 0 {
					// just 4 bytes. ok.
					u32::decode(input)?
				} else {
					// Out of range for a 32-bit quantity.
					return None
				}
			}
		}))
	}
}

impl Decode for Compact<u64> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		let prefix = input.read_byte()?;
		Some(Compact(match prefix % 4 {
			0 => prefix as u64 >> 2,
			1 => u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u64 >> 2,
			2 => u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u64 >> 2,
			3|_ => match (prefix >> 2) + 4 {
				4 => u32::decode(input)? as u64,
				8 => u64::decode(input)?,
				x if x > 8 => return None,
				bytes_needed => {
					let mut res = 0;
					for i in 0..bytes_needed {
						res |= (input.read_byte()? as u64) << (i * 8);
					}
					res
				}
			}
		}))
	}
}

impl Decode for Compact<u128> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		let prefix = input.read_byte()?;
		Some(Compact(match prefix % 4 {
			0 => prefix as u128 >> 2,
			1 => u16::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u128 >> 2,
			2 => u32::decode(&mut PrefixInput{prefix: Some(prefix), input})? as u128 >> 2,
			3|_ => match (prefix >> 2) + 4 {
				4 => u32::decode(input)? as u128,
				8 => u64::decode(input)? as u128,
				16 => u128::decode(input)?,
				x if x > 16 => return None,
				bytes_needed => {
					let mut res = 0;
					for i in 0..bytes_needed {
						res |= (input.read_byte()? as u128) << (i * 8);
					}
					res
				}
			}
		}))
	}
}

impl<S: Decode + Encode> Codec for S {}

impl<T: Encode, E: Encode> Encode for Result<T, E> {
	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) -> Option<Self> {
		match input.read_byte()? {
			0 => Some(Ok(T::decode(input)?)),
			1 => Some(Err(E::decode(input)?)),
			_ => None,
		}
	}
}

/// 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) -> Option<Self> {
		match input.read_byte()? {
			0 => Some(OptionBool(None)),
			1 => Some(OptionBool(Some(true))),
			2 => Some(OptionBool(Some(false))),
			_ => None,
		}
	}
}

impl<T: Encode> Encode for Option<T> {
	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) -> Option<Self> {
		match input.read_byte()? {
			0 => Some(None),
			1 => Some(Some(T::decode(input)?)),
			_ => None,
		}
	}
}

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);
				}
			}
		}

		impl<T: Decode> Decode for [T; $n] {
			fn decode<I: Input>(input: &mut I) -> Option<Self> {
				let mut r = ArrayVec::new();
				for _ in 0..$n {
					r.push(T::decode(input)?);
				}
				r.into_inner().ok()
			}
		}
	)* }
}

impl_array!(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
	40 48 56 64 72 96 128 160 192 224 256);

impl<T: Encode> Encode for Box<T> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		self.as_ref().encode_to(dest)
	}
}

impl<T: Decode> Decode for Box<T> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		Some(Box::new(T::decode(input)?))
	}
}

impl Encode for [u8] {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		let len = self.len();
		assert!(len <= u32::max_value() as usize, "Attempted to serialize a collection with too many elements.");
		Compact(len as u32).encode_to(dest);
		dest.write(self)
	}
}

impl Encode for Vec<u8> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		self.as_slice().encode_to(dest)
	}
}

impl Decode for Vec<u8> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		<Compact<u32>>::decode(input).and_then(move |Compact(len)| {
			let len = len as usize;
			let mut vec = vec![0; len];
			if input.read(&mut vec[..len]) != len {
				None
			} else {
				Some(vec)
			}
		})
	}
}

impl<'a> Encode for &'a str {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		self.as_bytes().encode_to(dest)
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl<'a, T: ToOwned + ?Sized + 'a> Encode for Cow<'a, T> where
	&'a T: Encode,
	<T as ToOwned>::Owned: Encode
{
	fn encode_to<W: Output>(&self, dest: &mut W) {
		match self {
			Cow::Owned(ref x) => x.encode_to(dest),
			Cow::Borrowed(x) => x.encode_to(dest),
		}
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl<'a, T: ToOwned + ?Sized> Decode for Cow<'a, T> where
	<T as ToOwned>::Owned: Decode
{
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		Some(Cow::Owned(Decode::decode(input)?))
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl<T> Encode for PhantomData<T> {
	fn encode_to<W: Output>(&self, _dest: &mut W) {
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl<T> Decode for PhantomData<T> {
	fn decode<I: Input>(_input: &mut I) -> Option<Self> {
		Some(PhantomData)
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl Encode for String {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		self.as_bytes().encode_to(dest)
	}
}

#[cfg(any(feature = "std", feature = "full"))]
impl Decode for String {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		Some(Self::from_utf8_lossy(&Vec::decode(input)?).into())
	}
}

impl<T: Encode> Encode for [T] {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		let len = self.len();
		assert!(len <= u32::max_value() as usize, "Attempted to serialize a collection with too many elements.");
		Compact(len as u32).encode_to(dest);
		for item in self {
			item.encode_to(dest);
		}
	}
}

impl<T: Encode> Encode for Vec<T> {
	fn encode_to<W: Output>(&self, dest: &mut W) {
		self.as_slice().encode_to(dest)
	}
}

impl<T: Decode> Decode for Vec<T> {
	fn decode<I: Input>(input: &mut I) -> Option<Self> {
		<Compact<u32>>::decode(input).and_then(move |Compact(len)| {
			let mut r = Vec::with_capacity(len as usize);
			for _ in 0..len {
				r.push(T::decode(input)?);
			}
			Some(r)
		})
	}
}

impl Encode for () {
	fn encode_to<T: Output>(&self, _dest: &mut T) {
	}

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

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

impl<'a, T: 'a + Encode + ?Sized> Encode for &'a T {
	fn encode_to<D: Output>(&self, dest: &mut D) {
		(&**self).encode_to(dest)
	}

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

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

impl Decode for () {
	fn decode<I: Input>(_: &mut I) -> Option<()> {
		Some(())
	}
}

macro_rules! tuple_impl {
	($one:ident,) => {
		impl<$one: Encode> Encode for ($one,) {
			fn encode_to<T: Output>(&self, dest: &mut T) {
				self.0.encode_to(dest);
			}
		}

		impl<$one: Decode> Decode for ($one,) {
			fn decode<I: Input>(input: &mut I) -> Option<Self> {
				match $one::decode(input) {
					None => None,
					Some($one) => Some(($one,)),
				}
			}
		}
	};
	($first:ident, $($rest:ident,)+) => {
		impl<$first: Encode, $($rest: Encode),+>
		Encode for
		($first, $($rest),+) {
			fn encode_to<T: Output>(&self, dest: &mut T) {
				let (
					ref $first,
					$(ref $rest),+
				) = *self;

				$first.encode_to(dest);
				$($rest.encode_to(dest);)+
			}
		}

		impl<$first: Decode, $($rest: Decode),+>
		Decode for
		($first, $($rest),+) {
			fn decode<INPUT: Input>(input: &mut INPUT) -> Option<Self> {
				Some((
					match $first::decode(input) {
						Some(x) => x,
						None => return None,
					},
					$(match $rest::decode(input) {
						Some(x) => x,
						None => return None,
					},)+
				))
			}
		}

		tuple_impl!($($rest,)+);
	}
}

#[allow(non_snake_case)]
mod inner_tuple_impl {
	use super::{Input, Output, Decode, Encode};
	tuple_impl!(A, B, C, D, E, F, G, H, I, J, K,);
}

/// Trait to allow conversion to a know endian representation when sensitive.
/// Types implementing this trait must have a size > 0.
// note: the copy bound and static lifetimes are necessary for safety of `Codec` blanket
// implementation.
trait EndianSensitive: Copy + 'static {
	fn to_le(self) -> Self { self }
	fn to_be(self) -> Self { self }
	fn from_le(self) -> Self { self }
	fn from_be(self) -> Self { self }
	fn as_be_then<T, F: FnOnce(&Self) -> T>(&self, f: F) -> T { f(&self) }
	fn as_le_then<T, F: FnOnce(&Self) -> T>(&self, f: F) -> T { f(&self) }
}

macro_rules! impl_endians {
	( $( $t:ty ),* ) => { $(
		impl EndianSensitive for $t {
			fn to_le(self) -> Self { <$t>::to_le(self) }
			fn to_be(self) -> Self { <$t>::to_be(self) }
			fn from_le(self) -> Self { <$t>::from_le(self) }
			fn from_be(self) -> Self { <$t>::from_be(self) }
			fn as_be_then<T, F: FnOnce(&Self) -> T>(&self, f: F) -> T { let d = self.to_be(); f(&d) }
			fn as_le_then<T, F: FnOnce(&Self) -> T>(&self, f: F) -> T { let d = self.to_le(); f(&d) }
		}

		impl Encode for $t {
			fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
				self.as_le_then(|le| {
					let size = mem::size_of::<$t>();
					let value_slice = unsafe {
						let ptr = le as *const _ as *const u8;
						if size != 0 {
							slice::from_raw_parts(ptr, size)
						} else {
							&[]
						}
					};

					f(value_slice)
				})
			}
		}

		impl Decode for $t {
			fn decode<I: Input>(input: &mut I) -> Option<Self> {
				let size = mem::size_of::<$t>();
				assert!(size > 0, "EndianSensitive can never be implemented for a zero-sized type.");
				let mut val: $t = unsafe { mem::zeroed() };

				unsafe {
					let raw: &mut [u8] = slice::from_raw_parts_mut(
						&mut val as *mut $t as *mut u8,
						size
					);
					if input.read(raw) != size { return None }
				}
				Some(val.from_le())
			}
		}
	)* }
}
macro_rules! impl_non_endians {
	( $( $t:ty ),* ) => { $(
		impl EndianSensitive for $t {}

		impl Encode for $t {
			fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
				self.as_le_then(|le| {
					let size = mem::size_of::<$t>();
					let value_slice = unsafe {
						let ptr = le as *const _ as *const u8;
						if size != 0 {
							slice::from_raw_parts(ptr, size)
						} else {
							&[]
						}
					};

					f(value_slice)
				})
			}
		}

		impl Decode for $t {
			fn decode<I: Input>(input: &mut I) -> Option<Self> {
				let size = mem::size_of::<$t>();
				assert!(size > 0, "EndianSensitive can never be implemented for a zero-sized type.");
				let mut val: $t = unsafe { mem::zeroed() };

				unsafe {
					let raw: &mut [u8] = slice::from_raw_parts_mut(
						&mut val as *mut $t as *mut u8,
						size
					);
					if input.read(raw) != size { return None }
				}
				Some(val.from_le())
			}
		}
	)* }
}

impl_endians!(u16, u32, u64, u128, usize, i16, i32, i64, i128, isize);
impl_non_endians!(i8, [u8; 1], [u8; 2], [u8; 3], [u8; 4], [u8; 5], [u8; 6], [u8; 7], [u8; 8],
	[u8; 10], [u8; 12], [u8; 14], [u8; 16], [u8; 20], [u8; 24], [u8; 28], [u8; 32], [u8; 40],
	[u8; 48], [u8; 56], [u8; 64], [u8; 80], [u8; 96], [u8; 112], [u8; 128], bool);


#[cfg(test)]
mod tests {
	use super::*;
	use std::borrow::Cow;

	#[test]
	fn vec_is_slicable() {
		let v = b"Hello world".to_vec();
		v.using_encoded(|ref slice|
			assert_eq!(slice, &b"\x2cHello world")
		);
	}

	#[test]