struct SN needs a strict weak ordering operator <

...so it can be used with std::sort in impSolve() (further down in
basegfx/source/polygon/b2dpolypolygoncutter.cxx).  At least on macOS with a
LLVM 20 trunk libc++ in hardened mode, JunitTest_sfx2_complex failed with

> ~/llvm/inst/bin/../include/c++/v1/__debug_utils/strict_weak_ordering_check.h:59: assertion __comp(*(__first + __a), *(__first + __b)) failed: Your comparator is not a valid strict-weak ordering

To simplify the new implementation of struct SN operator <, add a B2DPoint
operator <=> (but whose implementation would cause

> In file included from /home/tdf/lode/jenkins/workspace/android_arm/basegfx/source/curve/b2dbeziertools.cxx:21:
> In file included from /home/tdf/lode/jenkins/workspace/android_arm/include/basegfx/curve/b2dcubicbezier.hxx:22:
> /home/tdf/lode/jenkins/workspace/android_arm/include/basegfx/point/b2dpoint.hxx:129:41: error: invalid operands to binary expression ('tuple<const double &, const double &>' and 'tuple<const double &, const double &>')
>         { return std::tie(a.mnX, a.mnY) <=> std::tie(b.mnX, b.mnY); }
>                  ~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~

etc. on Android with NDK 23.2, see
<https://ci.libreoffice.org/job/gerrit_android_arm/43174/>, so work around that
in the implementation for now).

Change-Id: I9f46d39dc9e9024fe9ac59413c44e49642282c8b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175622
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

basegfx/source/polygon/b2dpolypolygoncutter.cxx

@@ -17,7 +17,6 @@
  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
  */
 
-#include <basegfx/numeric/ftools.hxx>
 #include <basegfx/polygon/b2dpolypolygoncutter.hxx>
 #include <basegfx/point/b2dpoint.hxx>
 #include <basegfx/vector/b2dvector.hxx>
@@ -32,6 +31,7 @@
 #include <vector>
 #include <algorithm>
 #include <numeric>
+#include <tuple>
 
 namespace basegfx
 {
@@ -72,23 +72,12 @@ namespace basegfx
         public:
             PN*                     mpPN;
 
+            // For this to be a strict weak ordering, the assumption is that none of the involved
+            // maPoint coordinates are NaN:
             bool operator<(const SN& rComp) const
             {
-                if(fTools::equal(mpPN->maPoint.getX(), rComp.mpPN->maPoint.getX()))
+                return std::tie(mpPN->maPoint, mpPN->mnI)
-                {
+                    < std::tie(rComp.mpPN->maPoint, rComp.mpPN->mnI);
-                    if(fTools::equal(mpPN->maPoint.getY(), rComp.mpPN->maPoint.getY()))
-                    {
-                        return (mpPN->mnI < rComp.mpPN->mnI);
-                    }
-                    else
-                    {
-                        return fTools::less(mpPN->maPoint.getY(), rComp.mpPN->maPoint.getY());
-                    }
-                }
-                else
-                {
-                    return fTools::less(mpPN->maPoint.getX(), rComp.mpPN->maPoint.getX());
-                }
             }
         };
 

include/basegfx/point/b2dpoint.hxx

@@ -19,7 +19,9 @@
 
 #pragma once
 
+#include <compare>
 #include <ostream>
+#include <tuple>
 
 #include <basegfx/tuple/b2dtuple.hxx>
 #include <basegfx/point/b2ipoint.hxx>
@@ -122,6 +124,19 @@ namespace basegfx
         {
             return static_cast<const B2DPoint&>( ::basegfx::B2DTuple::getEmptyTuple() );
         }
+
+        friend auto operator <=>(B2DPoint const & a, B2DPoint const & b)
+        {
+            // Avoid compilation failure with Android NDK 23.2, where std::tuple operator <=> isn't
+            // yet implemented (and where __cpp_lib_three_way_comparison happens to not be defined
+            // in <compare>, so discriminate on that):
+#if defined __cpp_lib_three_way_comparison
+            return std::tie(a.mnX, a.mnY) <=> std::tie(b.mnX, b.mnY);
+#else
+            auto const comp = a.mnX <=> b.mnX;
+            return comp == 0 ? a.mnY <=> b.mnY : comp;
+#endif
+        }
     };
 
     // external operators