# Geometry :

From: Giorgino R (giorginor21_at_[hidden])
Date: 2020-06-15 07:23:22

Hi there,

I have two questions for boost::geometry, any help will be much appreciated:

i) I am trying to use overlap function for two linestrings (
https://postgis.net/docs/ST_Overlaps.html); however, it looks like I am
doing something wrong (?) .

using point_2d = bg::model::point<double, 2, bg::cs::cartesian>;
using linestring_2d = bg::model::linestring<point_2d>;

// overlaps
linestring_2d line_test_1({ point_2d(0.0, 0.0), point_2d(2.0, 0.0) });
linestring_2d line_test_2({ point_2d(0.0, 0.0), point_2d(1.0, 0.0) });
bool res = bg::overlaps(line_test1, line_test2);

This results to false. Should I use a specific strategy? I checked also
tests and I saw that you do test overlap for linestrings.

ii) I have a structure that contains a linestring. I want to use an
unordered set for storing unique objects with respect to their geometry
(linestring). Therefore, I will need to write hash functions as below:

struct Object_hash
{
std::size_t operator()(const Object& obj) const
{
Linestring<Point> linestring = obj.get_linestring();
std::size_t hash = 0;
for (auto iter=linestring.cbegin(); iter!=linestring.cend(); ++iter)
{
boost::hash_combine(hash, iter->x1());
boost::hash_combine(hash, iter->x2());
}
return hash;
}
};

struct Object_compare
{
bool operator()(const Object& obj1, const Object & obj2) const
{
return boost::geometry::equals(obj1.get_linestring(),
obj2.get_linestring());
}
};

As expected, the above is valid for linestrings with the same direction of
their points. If one linestring is reversed this off course will not be
valid. Is there any way that I can find a better hashing function? Is there
any invariant field (for two spatial equaly linestrings) that I could use
to create hash key?