Array append proposal

[KennyTM~]

Steven Schveighoffer wrote:
> "Steven Schveighoffer" wrote
>> "Sergey Gromov" wrote
>>> Fri, 10 Oct 2008 11:11:40 -0400,
>>> Steven Schveighoffer wrote:
>>>> First, store the allocated length of the array on the heap along with 
>>>> the
>>>> array data.  On a 32-bit system, the size will always be 4 bytes, but 
>>>> there
>>>> can be padding bytes added to keep the alignment for the first element 
>>>> in
>>>> the array correct.  The maximum padding, I am not sure about.  I 
>>>> remember
>>>> people saying that for certain operations, 16 byte alignment is 
>>>> required?
>>>> Or is it just 8?  So for instance an array of doubles, there would be an
>>>> 8-byte field storing the length in the first 4 bytes, and padding in the
>>>> second 4 bytes.
>>>>
>>>> Perhaps someone with more knowledge of alignment requirements can 
>>>> comment.
>>>> I'd also appreciate some insight on how this would look on a 64 bit 
>>>> system.
>>>>
>>>> The final detail is how to tell whether the element before an array is 
>>>> the
>>>> length or not.  I propose to use the most significant bit in the array
>>>> length field.  This prevents having arrays of > 2GB, but that's probably
>>>> impossible anyways ;)
>>>>
>>>> If this bit is set, then the array can be appended to in place if its 
>>>> length
>>>> equals the length stored in the heap (and of course, the memory block
>>>> contains enough space).  If not, then a new array is allocated, and the 
>>>> data
>>>> copied, the length stored in the heap is left alone.
>>> How do you implement a slice?  It points in the middle of another array,
>>> data around that point is arbitrary.  There is no way you can check if
>>> you are at the beginning of an array or not.
>> If the first element of the slice is not the first element of the 
>> allocated array, then it does not have its 'at the beginning' bit set in 
>> its local length.
> 
> An example:
> 
> int[] array = new int[10];
> 
> array's length field is set to 0x8000_000a -> most significant bit set means 
> it can possibly be appended to.
> 
> int[] slice = array[0..$];
> 
> slice's length field is set to 0x8000_000a -> most significant bit set means 
> it can possibly be appended to.
> 
> int[] slice2 = array[1..5];
> 
> slice2's length field is set to 0x0000_0004 -> most significant bit unset. 
> Will always allocate a new array on an append.
> 
> -Steve
> 
> 
> 

That means the highest bit of .length will be set (not visible to coder 
of course) if the array is of the form [x..$] right?

--

BTW, I think this design is like a hack. What about a separate bool field?