 | Posted by IntegratedDimensions
in reply to Simen Kjærås |  Permalink Reply |
|
IntegratedDimensions 
Posted in reply to Simen Kjærås
| On Monday, 21 May 2018 at 15:41:21 UTC, Simen Kjærås wrote:
> On Saturday, 19 May 2018 at 18:44:42 UTC, IntegratedDimensions wrote:
>> On Saturday, 19 May 2018 at 18:19:35 UTC, IntegratedDimensions wrote:
>>> Is there any way to create an int24 type that behaves just like any other built in type without having to reimplement everything?
>>
>> In fact, what I'd like to do is create an arbitrary type:
>>
>> struct atype(T)
>> {
>>
>> }
>>
>> where atype(T) is just a "view" in to N_T bits interpreted as T, an enum.
>>
>> If T is bit, then the N = 1 and the interpretation is 1 bit.
>> If T is byte, then the N = 8 and the interpretation is 7 bits followed by 1 signed bit.
>> If T is int24, then the N = 24 and the interpretation is 23 bits followed by 1 signed bit.
>>
>> The idea is the storage of atype is exactly N bits. If this is not possible due to boundary issues then N can always be a multiple of 8(which is for my use cause is the smallest).
>
> D does not support types that take up less than one byte of space. It's possible to make types that represent less than one byte - bool may be considered such an example - but they still take up at least 1 byte.
>
> If you create a custom range type, you could pack more than one element in each byte, see std.bitmanip.BitArray[0] for an example.
>
>
>> The main thing is that I would like to be able to use atype as if it were a built in type.
>>
>> If N = 24, 3 bytes, I want to be able to create arrays of atype!int24[] which work just as if they were arrays of bytes without any exception or special cases.
>>
>> atype!byte would be equivalent to byte and reduce to the compiler internals. I'm not looking to create a "view" of an array. I want a standalone type that can behave as all the desired types needed, which is most of the numerical types of D and some of the ones it neglected like 24-bit ints, 48-bit ints, etc. Ideally, any type could be used and the "most optimal" code is generated while being able to use the types using the standard model.
>
> We already have std.numeric.CustomFloat[1]. As the name implies, it only works for floats.
>
> I hacked together something somewhat equivalent for ints:
>
> https://gist.github.com/Biotronic/f6668d8ac95b70302015fee93ae9c8c1
>
> Usage:
>
> // Two's-complement, native-endian, 24-bit int type:
> CustomInt!24 a;
>
> // Unsigned, native-endian, 15-bit:
> CustomInt!(15, Representation.Unsigned) b;
>
> // Offset (-2..5) 3-bit int:
> CustomInt!(3, Representation.OffsetBinary, 2) c;
>
> // You get the idea:
> CustomInt!(64, Representation.SignedMagnitude, 0, Endianness.BigEndian) d;
>
> Not sure this is what you're looking for, but it's at the very least inspired by your post. :)
>
> If what you want is a type that can represent something a packed array of 13-bit ints, the above is not what you're looking for - you're going to need a custom range type.
>
> --
> Simen
>
> [0]: https://dlang.org/phobos/std_bitmanip#BitArray
> [1]: https://dlang.org/phobos/std_numeric.html#.CustomFloat
Cool. I'll try it as a drop in replacement and if it works then it works! ;) Thanks.
Just to be clear and to make sure this works the way it seems:
All types multiple of a byte are reduced to either internal representation(byte, ubyte, short, ushort, int, uint, long, ulong) directly(becomes an alias) or the most efficient structure for the representation: unsigned 24-bit = 3 bytes and is effectively ubyte[3], usigned 128-bit is ubyte[16], etc?
Non multiples are extended up one byte, so 7-bits is representing using an byte, etc.
This seems to be the case from the code.
Now, what I didn't see was anything to work with non byte aligned arrays of CustomInt. Would it be possible to add? I know you say that we should use bitmanip but code could be extended to support it relatively easily by treating an array of bits as an array of CustomInts and the indexer can compute the appropriate offset using the bit size.
Maybe that will require CustomIntsArray?
The idea is, say one has 7-bit ASCII represented in a ubyte[] then they can map that to a CustomInt!7[] which will be use CustomInt!7(but 7 bits, not 8) as representation. But, of course CustomInt!7[3] would be 8 bits. But basically it retrieves the correct value and stores it by doing the standard masking.
BTW, it looks like you could extend your type to deal with floats and doubles which would make this type very robust in dealing with arbitrary primitive types.
The idea is that any matching language types are aliased to and those that don't are handled appropriately.
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