Geometry Boost List: Re: [geometry] [point_iterator] make point iterator bidirectional

Subject: Re: [geometry] [point_iterator] make point iterator bidirectional
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-05-05 14:50:06

Hi Menelaos,

Menelaos Karavelas wrote On 5-5-2014 9:21:
> Hello all.
>
> Before getting into the details, let me give some background. During 
> the past couple of months I have implemented an iterator for the 
> points of a geometry. The code is already in the develop branch, but 
> it is not yet documented (which means "use at your own risk", at least 
> for now...). The idea is to have an iterator to iterate over all 
> points of a geometry, and the supported geometries are: linestrings, 
> rings, polygons, multi-points, multi-linestrings and multi-polygons. 
> To use the iterator one can simply write:
>     bg::point_iterator<Geometry> first = bg::points_begin(g); // 
> non-const first
>     bg::point_iterator<Geometry> last = bg::points_end(g); // 
> non-const past-the-end
>
>     bg::point_iterator<Geometry const> const_first = 
> bg::points_begin(g); // const first
>     bg::point_iterator<Geometry const> const_last = bg::points_end(g); 
> // const past-the-end
> The points' iterator was implemented as a forward iterator.
> The corresponding implementation file is:
> * 
> https://github.com/boostorg/geometry/blob/develop/include/boost/geometry/iterators/point_iterator.hpp
> but there is also:
> * 
> https://github.com/boostorg/geometry/blob/develop/include/boost/geometry/iterators/point_iterator_type.hpp
> and other source code files involved....
> If people are interested I can describe the implementation design in 
> detail.
>
>
> This email is about follow-up work in this direction. During the past 
> couple of days I changed/modified/updated the implementation to make 
> the point iterator bidirectional. So now one can use operator-- on 
> point iterators, can use it in algorithms that require a bidirectional 
> iterator (such std::reverse), etc. In addition, I have implemented two 
> new free functions (the supported geometries are as above):
> ** points_front: returns the first point of a (non-empty) geometry
> ** points_back: returns the last point of a (non-empty) geometry
> So in order to use them, you may simply write:
>     bg::point_type<Geometry>::type p = bg::points_front(g);
>     bg::point_type<Geometry>::type q = bg::points_back(g);
> (I have omitted the "typename" keyword up-front).
>
> Finally, I have implemented a reverse iterator for points, and the 
> corresponding free functions: points_rbegin and points_rend. To use 
> them simply do:
>     bg::point_reverse_iterator<Geometry> rfirst = 
> bg::points_rbegin(g); // non-const reverse first
>     bg::point_reverse_iterator<Geometry> rlast = bg::points_rend(g); 
> // non-const reverse past-the-end
>
>     bg::point_reverse_iterator<Geometry const> const_rfirst = 
> bg::points_rbegin(g); // const reverse first
>     bg::point_reverse_iterator<Geometry const> const_rlast = 
> bg::points_rend(g); // const reverse past-the-end
>
> I have created a pull request for this: 
> https://github.com/boostorg/geometry/pull/21
>
> In there you will find:
> ** changes in the unit tests (re-arranged/re-factored code; added 
> tests for points_front/back and point_reverse_iterator; added code to 
> test the iterators as bidirectional). The corresponding files are in 
> test/iterators:
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/test/iterators/concatenate_iterator.cpp
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/test/iterators/flatten_iterator.cpp
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/test/iterators/point_iterator.cpp
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/test/iterators/test_iterator_common.hpp 
> (contains common factored-out code)
>
> ** changes in the implementation:
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/include/boost/geometry/iterators/concatenate_iterator.hpp
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/include/boost/geometry/iterators/flatten_iterator.hpp
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/include/boost/geometry/iterators/point_iterator.hpp 
> (contains points_front/points_back)
> * 
> https://github.com/mkaravel/geometry/blob/feature/point_iterator/include/boost/geometry/iterators/point_reverse_iterator.hpp 
> (contains implementation of reverse iterator)
>
> I look forward to comments/suggestions. I would suggest to put your 
> comments/suggestions on GitHub, using GitHub's comments' feature 
> available through the pull request.

Thanks a lot for this new functionality. I already liked the 
points_begin and points_end, and bidirectional functionality makes it 
more complete.

I think the point iterators are very useful for end users too, so if we 
could create end-user documentation and some concise examples, would be 
great.

If I have more remarks I will let you know.

Regards, Barend