Split range into range of overlapping ranges

I attempting to use the Ranges-V3 library to slice up an container of values into a range of ranges such that neighboring ranges share boundary elements.

Consider the following:

using namespace ranges;

std::vector<int> v = { 1, 2, 3, 0, 4, 0, 5, 0, 6, 7, 8, 0, 0, 9 };
auto myRanges = v | /* something like adjacent split */
for_each( myRanges, []( auto&& range ){ std::cout << range << std::endl;} );

I would like to divide the range into overlapping subranges based whether the region fullfills two criteria:

  • whether the element has a value of zero
  • or is adjacent to one or more elements with a value of zero
  • Desired output:

    [1,2,3]
    [3,0,4,0,5,0,6]
    [6,7,8]
    [8,0,0,9]
    

    my attempt:

    auto degenerate =
      []( auto&& arg ){
        return distance( arg ) < 2;  
      };
    
    auto myRanges = v | view::split(0) | view::remove_if( degenerate );
    for_each( myRanges, []( auto&& range ){ std::cout << range << std::endl;} );
    

    Output:

    [1,2,3]
    [6,7,8]
    

    I'm at a loss on how I might

  • "insert" the range from 3 to 6
  • "append" the range from 8 to 9

  • If I understand your requirements correctly, you can implement a generator in terms of adjacent_find :

    namespace detail {
        template<typename IterT, typename SentT>
        struct seg_gen_fn {
            IterT it_;
            SentT end_;
            bool parity_ = true;
    
            ranges::iterator_range<IterT> operator ()() {
                if (it_ == end_) {
                    return {it_, it_};
                }
    
                auto n = ranges::adjacent_find(
                    it_, end_,
                    [p = std::exchange(parity_, !parity_)](auto const a, auto const b) {
                        return a && !b == p;
                    }
                );
                return {
                    std::exchange(it_, n),
                    n != end_ ? ranges::next(std::move(n)) : std::move(n)
                };
            }
        };
    
        template<typename RngT>
        constexpr auto seg_gen(RngT&& rng)
         -> seg_gen_fn<ranges::iterator_t<RngT>, ranges::sentinel_t<RngT>>
        { return {ranges::begin(rng), ranges::end(rng)}; }
    } // namespace detail
    
    auto const segmented_view = [](auto&& rng) {
        return ranges::view::generate(detail::seg_gen(decltype(rng)(rng)))
             | ranges::view::take_while([](auto const& seg) { return !seg.empty(); });
    };
    
    int main() {
        auto const ns = {1, 2, 3, 0, 4, 0, 5, 0, 6, 7, 8, 0, 0, 9};
        ranges::copy(segmented_view(ns), ranges::ostream_iterator<>{std::cout, "n"});
    }
    

    Online Demo
    Not exactly as succinct as one might hope... :-[


    That's probably fine for one-off code, but a little more work and it can be a lot more reusable:

    namespace detail {
        namespace tag = ranges::tag;
    
        template<
            typename RngT, typename PredT, typename IterT = ranges::iterator_t<RngT>,
            typename StateT = ranges::tagged_compressed_tuple<
                tag::begin(IterT), tag::end(ranges::sentinel_t<RngT>),
                tag::current(bool), tag::fun(ranges::semiregular_t<PredT>)
            >
        >
        struct seg_gen_fn : private StateT {
            constexpr seg_gen_fn(RngT&& rng, PredT pred)
              : StateT{ranges::begin(rng), ranges::end(rng), true, std::move(pred)}
            { }
    
            ranges::iterator_range<IterT> operator ()() {
                StateT& state = *this;
                auto& it = state.begin();
                if (it == state.end()) {
                    return {it, it};
                }
    
                auto& parity = state.current();
                auto n = ranges::adjacent_find(
                    it, state.end(),
                    [p = std::exchange(parity, !parity), &pred = state.fun()]
                    (auto const& a, auto const& b) {
                        return !pred(a) && pred(b) == p;
                    }
                );
                return {
                    std::exchange(it, n),
                    n != state.end() ? ranges::next(std::move(n)) : std::move(n)
                };
            }
        };
    
        template<typename RngT, typename PredT>
        constexpr seg_gen_fn<RngT, PredT> seg_gen(RngT&& rng, PredT pred) {
            return {std::forward<RngT>(rng), std::move(pred)};
        }
    } // namespace detail
    
    auto const segmented_view = [](auto&& rng, auto pred) {
        return ranges::view::generate(detail::seg_gen(decltype(rng)(rng), std::move(pred)))
             | ranges::view::take_while([](auto const& seg) { return !seg.empty(); });
    };
    
    int main() {
        auto const ns = {1, 2, 3, 0, 4, 0, 5, 0, 6, 7, 8, 0, 0, 9};
        ranges::copy(
            segmented_view(ns, [](auto const n) { return n == 0; }),
            ranges::ostream_iterator<>{std::cout, "n"}
        );
    }
    

    Online Demo

    Concept checking and projections are left as an exercise.

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