Retrieving the traversed range

[Steven Schveighoffer]

On Wed, 25 Aug 2010 04:08:00 -0400, Peter Alexander  
<peter.alexander.au at gmail.com> wrote:

> Maybe I'm missing something, but I can't think of anyway *at all* to do  
> this generically.
>
> Lets say I have some arbitrary bidirectional range, R, and I want to  
> find the first element that satisfies some predicate. After that, I want  
> to reverse the part of the range up to that element.
>
> Essentially, I'd like to do something along the lines of:
>
>    reverse(until!(pred)(R));
>
> but Until is (correctly) not bidirectional, so I can't do that.
>
> Use indexing and slicing is not an option because the range isn't  
> necessary random-access.
>
> This isn't about what std.algorithm and std.range can do -- I can't even  
> think of a way to do this using primitive range operations.
>
> Also note that popping off from the back of the range is not an option  
> either because the range could be arbitrarily long and the predicate is  
> usually satisfied very early in the range.
>
> Thanks in advance.

I don't think this is possible generically, because there is no generic  
way to dissect a range into its end points or compose a range from end  
points.  Ranges only provide common interface to traverse them.  I would  
imagine something might be possible like this:

auto range2 = find!(pred)(range1);
auto range3 = range1.allBefore(range2);

where allBefore (or whatever you want to call it) verifies trivially that  
range2 and range1 have identical ends.  But this has to be implemented by  
the range, since a generic representation of a range end point is not  
defined.

dcollections' ranges provide a method to compose them using cursors, and  
to decompose them into two cursors, so what you want is possible, but  
there are some limitations.  I never want to be able to create an invalid  
range, so I throw an exception when you try to compose a range that I  
can't verify in at least O(lgn) time.

For example:

reverse(container[container.begin..find!(pred)(container[]).begin]);

This should work no matter what collection you are using.  If you want to  
slice arbitrary ranges, only ArrayList and Tree* work because of the quick  
verification requirement.

-Steve