Re: [PATCH 09/13] refs: introduce an iterator interface

> On Tue, May 31, 2016 at 3:59 AM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>> On 05/31/2016 07:29 AM, Eric Sunshine wrote:
>>> On Mon, May 30, 2016 at 3:55 AM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>>>> +struct ref_iterator *empty_ref_iterator_begin(void);
>>>> +
>>>> +/*
>>>> + * Return true iff ref_iterator is an empty_ref_iterator.
>>>> + */
>>>> +int is_empty_ref_iterator(struct ref_iterator *ref_iterator);
>>>
>>> I can see that you used this function as an optimization or
>>> convenience in overlay_ref_iterator_begin(), but do you expect it to
>>> be generally useful otherwise? Is it worth publishing? Do you have
>>> other use-cases in mind?
>>
>> It is only "published" within the refs module, in refs/refs-internal.h.
>> This header file is not meant to be used by code outside of the refs module.
> 
> Ah, I forgot about that. In that case, it's probably less of an issue.
> 
>> My thinking was that it might be useful to other reference backends. The
>> function is pretty safe for anybody to call, though I admit that it is
>> not very general.
>>
>> I don't have a strong feeling either way. If nobody else chimes in, I'll
>> remove it from the header file as you suggested. We can always add it
>> back if somebody needs it.
> 
> I don't feel strongly about it either.

>>> Also, can you explain why the merge iterator doesn't also perform the
>>> optimization/convenience of checking if one iterator is an empty
>>> iterator?
>>
>> That's because the merge iterator doesn't know what its select function
>> will do. For example, you could imagine an "intersect" select function
>> that only lets through references that were in *both* sub-iterators. In
>> that case, your suggested "optimization" would be incorrect.
> 
> Makes sense. Thanks for explaining. I wonder if this deserves a
> comment somewhere in code or commit message to make the situation
> clear to a future developer who might think it a good idea to promote
> the "optimization" to the merge iterator.