RingIntersection.hpp

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#pragma once
#include <ranges>
#include <fishnet/IGeometry.hpp>
 
namespace fishnet::geometry {
 
template<typename R, LinearGeometry L>
constexpr static bool ringIntersects(const R & ring, const L & linearFeature) noexcept {
    auto segments = ring.getSegments();
    using difference_type = std::ranges::range_difference_t<decltype(segments)>;
    auto size = std::ranges::ssize(segments);
    auto segmentAt = [&segments](difference_type index){return *(std::ranges::begin(segments) + index);};
    // Helper lambda to check whether two points are on the same side of the linearFeature (or on the line)
    auto onSameSide = [&linearFeature](const auto & lhs, const auto & rhs) {
        auto line = linearFeature.toLine();
        if(line.contains(rhs) || line.contains(lhs)){
            return true;
        }
        return line.isLeft(lhs) == line.isLeft(rhs);
    };
    for(difference_type i = 0; i < size; ++i){
        auto current = segmentAt(i);
        auto inter = current.intersection(linearFeature);
        if constexpr(ISegment<L>){
            if(inter && (linearFeature.isEndpoint(inter.value()) && current.isEndpoint(inter.value())))
                 continue;
            if(inter && (linearFeature.isEndpoint(inter.value())) && (ring.isOutside(linearFeature.p()) || ring.isOutside(linearFeature.q())))
                return true;
        }
        if constexpr(IRay<L>){
            if(inter && inter.value() == linearFeature.origin())
                 continue;
        }
        if(inter && current.isEndpoint(inter.value())) { // intersection is vertex of ring
            if(current.p() == inter.value()){
                // i+size-1 rather than i-1: the latter underflows for i == 0 and then only wraps
                // to size-1 when the number of segments happens to be a power of two
                if(not onSameSide(current.q(),segmentAt((i+size-1)%size).p()))
                    return true;
            }else{ // inter.value() == current.q()
                if(not onSameSide(current.p(),segmentAt((i+1)%size).q()))
                    return true;
            }
        }else if(inter){
            if(not onSameSide(current.p(),current.q())) return true;
        }
    }
    return false;
}
} // namespace fishnet::geometry