Range length property

[Steven Schveighoffer]

On 4/10/18 6:07 PM, Cym13 wrote:
> On Tuesday, 10 April 2018 at 20:08:14 UTC, Jonathan M Davis wrote:
>> On Tuesday, April 10, 2018 19:47:10 Nordlöw via Digitalmars-d-learn 
>> wrote:
>>> On Tuesday, 10 April 2018 at 14:34:40 UTC, Adam D. Ruppe wrote:
>>> > On Tuesday, 10 April 2018 at 14:25:52 UTC, Nordlöw wrote:
>>> >> Should ranges always provide a length property?
>>> >
>>> > No.
>>> >
>>> >> If so, in which cases is a length property an advantage or >> a 
>>> requirement?
>>> >
>>> > Just provide it whenever it is cheap to do so. If you need > to do 
>>> complex calculations or especially loop over contents > to figure out 
>>> the length, do NOT provide it.
>>> >
>>> > But if it is as simple as returning some value, provide it > and 
>>> algorithms can take advantage of it for optimizations > etc. as needed.
>>>
>>> I'm thinking of my own container Hashmap having its range ByKeyValue 
>>> requiring one extra word of memory to store the iteration count 
>>> which, in turn, can be used to calculate the length of the remaining 
>>> range. Is this motivated?
>>
>> That would depend entirely on what you're trying to do, but in 
>> general, if a range has length, then some algorithms will be more 
>> efficient, and some algorithms do require length. So, if you can 
>> provide length, then the range will be more useful, just like a 
>> bidirectional range can be more useful than a forward range or a 
>> random-access range can be more useful than either. However, if you're 
>> not doing anything that ever benefits from it having length, then it 
>> doesn't buy you anything. So, it ultimately depends on what you're 
>> doing. In a general purpose library, I'd say that it should have 
>> length if it can do so in O(1), but if it's just for you, then it may 
>> or may not be worth it.
>>
>> The other thing to consider is what happens when the container is 
>> mutated. I don't think that ranges necessarily behave all that well 
>> when an underlying container is mutated, but it is something that has 
>> to be considered when dealing with a range over a container. Even if 
>> mutating the underlying container doesn't necessarily invalidate a 
>> range, maintaining the length in the manner that you're suggesting 
>> probably makes it so that it would be invalidated in more cases, since 
>> if any elements are added or removed in the portion that was already 
>> popped off the range, then the iteration count couldn't be used to 
>> calculate the length in the same way anymore. Now, with a hash map, 
>> the range is probably fully invalidated when anything gets added or 
>> removed anyway, since that probably screws with the order of the 
>> elements in the range, but how the range is going to behave when the 
>> underlying container is mutated and how having the length property 
>> does or doesn't affect that is something that you'll need to consider.
>>
>> - Jonathan M Davis
> 
> I find that discussion very interesting as I had never considered that 
> because of design by introspection having a costly length method would 
> lead to unexpected calls by generic algorithms making it a disadventage 
> if present.
> 
> On the other hand I don't think the end user should have to scratch his 
> head to find the length of a range, especially if it's not trivial to 
> get (say, O(log n) kind of case). Therefore exposing a method in any 
> case seems the best from an API perspective.

O(lg n) is fine for .length, it doesn't need to be O(1). It just can't 
be O(n). I think we established that "fast" operations are O(lg n) or 
better.

That being said, I don't know of a use case where you can get the length 
in O(lg n). It's usually O(1) or O(n).

> But to avoid the performance issues mentionned earlier it means it 
> should bear a different name (get/setLength comes to mind). I believe 
> this is the same kind of issue that lead to having "in" for associative 
> arrays but not regular ones. However this also leads to less coherent 
> APIs in contradiction with the principle of least surprise.

It's definitely a tradeoff. It pushes some implementation details to the 
user, but it also makes the runtime complexity more predictable.

> In retrospect since only "unexpected" calls to such methods cause the 
> issue I wonder if it wouldn't be best to have an UDA saying "Hey, 
> please, this method is costly, if you're a generic template performing 
> introspection you should probably not call me". And writing that 
> Andrei's work on complexity annotations comes to mind. Anyway, I don't 
> think the user should use different names just to alleviate an issue on 
> the library side but the alternative would be costly to put in place...

Potentially, but remember at the time length and walkLength were 
conceived, UDA's didn't exist!

Using UDAs would also have the unfortunate side effect of eliminating 
self-documentation. When you see walkLength right now, you know it's 
"slow", when you see length, you know "fast". If you have to look at 
UDAs to figure that out, then reading the code is that much harder.

-Steve