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Add read/write implems for generic arrays #235

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Oct 5, 2021
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Add read/write implems for generic arrays #235

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149 changes: 99 additions & 50 deletions src/impls/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -167,61 +167,57 @@ mod pre_const_generics_impl {
mod const_generics_impl {
use super::*;

macro_rules! ImplDekuSliceTraits {
($($typ:ty),+ $(,)?) => {
$(
impl<Ctx: Copy> DekuWrite<Ctx> for &[$typ]
where
$typ: DekuWrite<Ctx>,
{
fn write(&self, output: &mut BitVec<Msb0, u8>, ctx: Ctx) -> Result<(), DekuError> {
for v in *self {
v.write(output, ctx)?;
}
Ok(())
}
}

use std::mem::MaybeUninit;

impl<'a, Ctx: Copy, const N: usize> DekuRead<'a, Ctx> for [$typ; N]
where
$typ: DekuRead<'a, Ctx>,
{
fn read(
input: &'a BitSlice<Msb0, u8>,
ctx: Ctx,
) -> Result<(&'a BitSlice<Msb0, u8>, Self), DekuError>
where
Self: Sized,
{
let mut slice: [$typ; N] = [<$typ>::default(); N];
let mut rest = input;
for i in 0..N {
let (new_rest, value) = <$typ>::read(rest, ctx)?;
slice[i] = value;
rest = new_rest;
}
impl<'a, Ctx: Copy, T, const N: usize> DekuRead<'a, Ctx> for [T; N]
where
T: DekuRead<'a, Ctx>,
{
fn read(
input: &'a BitSlice<Msb0, u8>,
ctx: Ctx,
) -> Result<(&'a BitSlice<Msb0, u8>, Self), DekuError>
where
Self: Sized,
{
#[allow(clippy::uninit_assumed_init)]
// This is safe because we initialize the array immediately after,
// and never return it in case of error
let mut slice: [T; N] = unsafe { MaybeUninit::uninit().assume_init() };
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The reason for this is because of potential performance impacts of default'ing here? let mut slice: [$typ; N] = [<$typ>::default(); N];

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@xlambein xlambein Sep 27, 2021 •
edited
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Using default() requires that T implements Default, which imo is an unnecessary restriction. To me the safe alternative would be to do:

let mut v = Vec::with_capacity(N);
let mut rest = input;
for i = 0..N {
    let (new_rest, value) = T::read(rest, ctx)?;
    v.push(value);
    rest = new_rest;
}
Ok((rest, v.try_into().unwrap()))

That makes a few unnecessary allocations and checks, which are avoided with the unsafe section. It doesn't seem very critical, so if you'd rather do that I think it's fine! :-)

let mut rest = input;
for item in slice.iter_mut() {
let (new_rest, value) = T::read(rest, ctx)?;
*item = value;
rest = new_rest;
}

Ok((rest, slice))
}
}
Ok((rest, slice))
}
}

impl<Ctx: Copy, const N: usize> DekuWrite<Ctx> for [$typ; N]
where
$typ: DekuWrite<Ctx>,
{
fn write(&self, output: &mut BitVec<Msb0, u8>, ctx: Ctx) -> Result<(), DekuError> {
for v in self {
v.write(output, ctx)?;
}
Ok(())
}
}
)+
};
impl<Ctx: Copy, T, const N: usize> DekuWrite<Ctx> for [T; N]
where
T: DekuWrite<Ctx>,
{
fn write(&self, output: &mut BitVec<Msb0, u8>, ctx: Ctx) -> Result<(), DekuError> {
for v in self {
v.write(output, ctx)?;
}
Ok(())
}
}

ImplDekuSliceTraits!(i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, f32, f64);
impl<Ctx: Copy, T> DekuWrite<Ctx> for &[T]
where
T: DekuWrite<Ctx>,
{
fn write(&self, output: &mut BitVec<Msb0, u8>, ctx: Ctx) -> Result<(), DekuError> {
for v in *self {
v.write(output, ctx)?;
}
Ok(())
}
}
}

#[cfg(test)]
Expand Down Expand Up @@ -263,4 +259,57 @@ mod tests {
input.write(&mut res_write, endian).unwrap();
assert_eq!(expected, res_write.into_vec());
}

#[cfg(feature = "const_generics")]
#[rstest(input,endian,expected,expected_rest,
case::normal_le(
[0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88, 0x77, 0x66].as_ref(),
Endian::Little,
[[0xCCDD, 0xAABB], [0x8899, 0x6677]],
bits![Msb0, u8;],
),
case::normal_le(
[0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88, 0x77, 0x66].as_ref(),
Endian::Big,
[[0xDDCC, 0xBBAA], [0x9988, 0x7766]],
bits![Msb0, u8;],
),
)]
fn test_nested_array_bit_read(
input: &[u8],
endian: Endian,
expected: [[u16; 2]; 2],
expected_rest: &BitSlice<Msb0, u8>,
) {
let bit_slice = input.view_bits::<Msb0>();

let (rest, res_read) = <[[u16; 2]; 2]>::read(bit_slice, endian).unwrap();
assert_eq!(expected, res_read);
assert_eq!(expected_rest, rest);
}

#[cfg(feature = "const_generics")]
#[rstest(input,endian,expected,
case::normal_le(
[[0xCCDD, 0xAABB], [0x8899, 0x6677]],
Endian::Little,
vec![0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88, 0x77, 0x66],
),
case::normal_be(
[[0xDDCC, 0xBBAA], [0x9988, 0x7766]],
Endian::Big,
vec![0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88, 0x77, 0x66],
),
)]
fn test_nested_array_bit_write(input: [[u16; 2]; 2], endian: Endian, expected: Vec<u8>) {
let mut res_write = bitvec![Msb0, u8;];
input.write(&mut res_write, endian).unwrap();
assert_eq!(expected, res_write.into_vec());

// test &slice
let input = input.as_ref();
let mut res_write = bitvec![Msb0, u8;];
input.write(&mut res_write, endian).unwrap();
assert_eq!(expected, res_write.into_vec());
}
}