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Add byte conversions #72

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Add byte conversions #72

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ecbf7c3
Add a crude `to_ne_bytes` implementation
hecatia-elegua Aug 9, 2023
015adbe
Adjust and document `to_ne_bytes`
hecatia-elegua Aug 11, 2023
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36 changes: 15 additions & 21 deletions bilge-impl/src/bitsize_internal.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -233,32 +233,26 @@ fn generate_common(ir: ItemIr, arb_int: &TokenStream) -> TokenStream {
}
impl #name {
pub fn to_ne_bytes(&self) -> [u8; (<Self as Bitsized>::BITS + 7) / 8] {
//this works on little endian, since zeroes are cut out (i.e. u40 has 3 bytes left until u64 but they are cut out)
// 0 0 0 [1 1 1 1 1]
//but not on big endian, since it still copies from the back
// 1 1 1 [1 1 0 0 0]
// unsafe { core::mem::transmute_copy::<Self, [u8; (<Self as Bitsized>::BITS + 7) / 8]>(self) }

//this doesn't work for non-repr enums
// let mut full = unsafe { core::mem::transmute_copy::<Self, [u8; core::mem::size_of::<Self>()]>(self) };

//this doesn't work for non-repr enums either, since `Self > Dst` is still getting size checked in `transmute_copy`
//and `Self` is 1 byte even if bitsize(32)
// let mut full = unsafe {
// core::mem::transmute_copy::<Self, [u8; (<<Self as Bitsized>::ArbitraryInt as Number>::UnderlyingType::BITS as usize) / 8]>(self)
// };

//so, let's use the first one since it's readable:
// u40, as 8 bytes
let mut full = unsafe { core::mem::transmute_copy::<Self, [u8; core::mem::size_of::<Self>()]>(self) };
// u40 = u64 = 8 bytes
const ARB_INT_BYTES: usize = core::mem::size_of::<#name>();
// u40 = (40 + 7) / 8 = 5 bytes
const MIN_BYTES: usize = (<#name as Bitsized>::BITS + 7) / 8;

// u40, as 8 bytes
let mut full = unsafe { core::mem::transmute_copy::<Self, [u8; ARB_INT_BYTES]>(self) };
// u40, as 5 bytes
#[cfg(target_endian = "big")]
{
let size_difference = core::mem::size_of::<Self>() - ((<Self as Bitsized>::BITS + 7) / 8);
full.rotate_left(size_difference);
// = 3
let size_difference = ARB_INT_BYTES - MIN_BYTES;
// 0 0 0 [1 1 1 1 1]
return full[size_difference..].try_into().unwrap()
}
#[cfg(target_endian = "little")]
{
// [1 1 1 1 1] 0 0 0
return full[..MIN_BYTES].try_into().unwrap();
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is this just transmute_copy(self)? because it's already in little endian?

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that's true, that's also how the impl started.
though it's a bit more explicit like this, but I might change it back when doing the other methods

}
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full[0..((<Self as Bitsized>::BITS + 7) / 8)].try_into().unwrap()
}
}
}
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