On Thursday, 26 May 2016 at 22:15:42 UTC, ag0aep6g wrote:
> On 05/26/2016 11:13 PM, Era Scarecrow wrote:
>>    void scan(ref Data[][] data, Condition cond) {
>>      foreach(i; ...) {
>>        if (cond)
>>          data[i] ~= ...
>>      }
>>    }
>
> Sorry, I'm still lost. Why can't you do whatever you're doing in opOpAssign directly there, or in a free function? Does the pseudo-array contain any additional data? Would a wrapper around a built-in array not work?

 Well a few things at work. First the array is originally a static array, so I pass it as a slice; But if you pass it as a slice you get the entire length, and modifying the length requires either changing length or appending, both which may cause allocation (something I want to avoid). By making my own compatible structure I can manage the length and pointer directly and precisely.

 Second I would prefer the length to start at 0 and grow UP TO the size of the static array. If it was just a single array this wouldn't be an issue, but I'm working with a couple hundred of these. Plus I don't want to add more data to the memory or structure than necessary to treat it as an array after the fact. I shouldn't have to.

 Truthfully the same result can be done with shrinking it afterwards, but I was trying to see if I could make it any faster by managing the pointer and length directly and avoiding the large temporaries. So far any speed gains are so minimal it's worth dropping.


> Oh, I see. Yeah, the answer is garbage. Seeing the spec section that Adam D. Ruppe linked, the length field actually comes first.
>
> That CTFE errors out when you move the pointer to a variable screams that something is wrong. The code seems to be seriously misinterpreted in CTFE.
>
> I've filed an issue:
> https://issues.dlang.org/show_bug.cgi?id=16081

 Glad I found a bug! :) And all because [].ptr.offsetof wasn't an option...

 I just wanted to guarantee it was the correct order (in case it gets changed in the future). But ultimately it isn't worth pursuing.

On Thursday, 26 May 2016 at 22:47:02 UTC, Era Scarecrow wrote:
> On Thursday, 26 May 2016 at 22:15:42 UTC, ag0aep6g wrote:
>> Sorry, I'm still lost. Why can't you do whatever you're doing in opOpAssign directly there, or in a free function? Does the pseudo-array contain any additional data? Would a wrapper around a built-in array not work?
>
>  Well a few things at work. First the array is originally a static array, so I pass it as a slice; But if you pass it as a slice you get the entire length, and modifying the length requires either changing length or appending, both which may cause allocation (something I want to avoid). By making my own compatible structure I can manage the length and pointer directly and precisely.

You can do that with arrays, too, without causing allocations:

    assert(slice.length < static_array.length);
    slice = slice.ptr[0 .. slice.length+1];

Of course that's unsafe, but your pointer magic certainly is, too.

On Friday, 27 May 2016 at 09:18:47 UTC, Marc Schütz wrote:
> You can do that with arrays, too, without causing allocations:
>
>     assert(slice.length < static_array.length);
>     slice = slice.ptr[0 .. slice.length+1];
>
> Of course that's unsafe, but your pointer magic certainly is, too.

 Initial impressions suggest it's actually slower. The multiple dereferences to do it since it's a 2d array...

On Thursday, 26 May 2016 at 21:13:14 UTC, Era Scarecrow wrote:
>  To do what I want currently it's something like...
>
>   enum Size = 1024, Other = 128;
>   Data[Size][Other] staticarray;  //stack allocation
>   Data[][] sliced = staticarray[];
>   scan(sliced, condition);
>
>   void scan(ref Data[][] data, Condition cond) {
>     int lengths[Size];
>
>     foreach(i; ...) {
>       if (cond)
>         data[i][lengths[i]++] = ...
>     }
>
>     //cleanup/shrink
>     foreach(i, l; lengths)
>       data[i] = data[i][0 .. l];
>   }

Like this:

  static void rawlength(ref Data[] r, size_t len) {
    r = r.ptr[0..len];
  }

  void scan(ref Data[][] data, Condition cond) {
    foreach(i; ...) {
      if (cond)
        data[i].rawlength = ...
    }
  }