Bitfields

[Russel Winder]

On Tue, 2019-05-21 at 19:14 +0000, Era Scarecrow via Digitalmars-d-learn
wrote:
> 
[…]
>   I worked on/with bitfields in the past, the limit sizes is more 
> or less for natural int types that D supports.

Rust bitfield crate and it's macros are the same, the underlying type for a
bitfield must be a primitive integer type. Fortunately, Rust has i128 and u128
which is enough for my 112 bit EIT header.

Boris Barboris suggested using BitArray and I willinvestigate but the
size_t/byte problem would need to go away.

>   However this limitation is kinda arbitrary, as for simplicity it 
> relies on shifting bits, going larger or any byte size is 
> possible depending on what needs to be stored, but ti's the speed 
> that really takes a penalty when you aren't using native types or 
> you have to do a lot of shifting to get the job done.
> 
>   What's the layout of what you need? I'll see if i can't make 
> something that would work for you.
> 
>   Would be better if you can use a object that breaks the parts 
> down and you can actually fully access those parts, then just 
> re-store it into the limited space you want for storage, which 
> then would be faster than bitfields (although not by much)

I found an interesting way forward in the source code of dvbsnoop. It
basically uses the byte sequence as a backing store and then has a function to
do the necessary accesses to treat it as a bit array.

If D's Bit Array can work with bytes instead of size_t then it is exactly what
dvbsnoop does (in C) but adds writing as well as reading.

The Rust solution using bitfield with a u128 backing it seems to work, but it
is all very clumsy even if it is efficacious. 

-- 
Russel.
===========================================
Dr Russel Winder      t: +44 20 7585 2200
41 Buckmaster Road    m: +44 7770 465 077
London SW11 1EN, UK   w: www.russel.org.uk

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