segmentIntersectionOrder: shared point case (#4794)

Author: Fedr

source/MRMesh/MRPrecisePredicates2.cpp

@@ -274,6 +274,28 @@ bool segmentIntersectionOrder( const std::array<PreciseVertCoords2, 6> & vs )
     // s=01, sa=23, sb=45
     assert( doSegmentSegmentIntersect( { vs[0], vs[1], vs[2], vs[3] } ) );
     assert( doSegmentSegmentIntersect( { vs[0], vs[1], vs[4], vs[5] } ) );
+
+    // if sa and sb have a shared point
+    auto sa0 = vs[2];
+    auto sa1 = vs[3];
+    auto sb0 = vs[4];
+    auto sb1 = vs[5];
+    if ( sa1.id == sb0.id )
+        std::swap( sa0, sa1 );
+    else if ( sa0.id == sb1.id )
+        std::swap( sb0, sb1 );
+    else if ( sa1.id == sb1.id )
+    {
+        std::swap( sa0, sa1 );
+        std::swap( sb0, sb1 );
+    }
+    if ( sa0.id == sb0.id )
+    {
+        assert( sa0.pt == sb0.pt );
+        assert( sa1.id != sb1.id );
+        return ccw( { sa0, sa1, sb1 } ) == ccw( { sa0, sa1, vs[1] } );
+    }
+
     const auto ds = getPointDegrees( vs );
 
     // res = ( ccw(sa,s[0])*ccw(sb,ds[1])   -   ccw(sb,s[0])*ccw(sa,ds[1]) ) /

source/MRMesh/MRPrecisePredicates2.h

@@ -79,6 +79,7 @@ struct CoordinateConverters2
 /// given line segment s=01 and two other segments sa=23, sb=45 known to intersect it, finds the order of intersection using precise predicates:
 /// true:  s[0], s ^ sa, s ^ sb, s[1]
 /// false: s[0], s ^ sb, s ^ sa, s[1]
+/// segments sa and sb can have at most one shared point, all other points must be unique
 [[nodiscard]] MRMESH_API bool segmentIntersectionOrder( const std::array<PreciseVertCoords2, 6> & vs );
 
 /// finds intersection precise, using high precision int inside

source/MRTest/MRPrecisePredicatesTests.cpp

@@ -113,6 +113,10 @@ TEST( MRMesh, PrecisePredicates2More )
     // intersection of 45 and 03 is closer to 4 than intersection of 45 and 12
     EXPECT_TRUE(  segmentIntersectionOrder( { vs[4], vs[5], vs[0], vs[3], vs[1], vs[2] } ) );
     EXPECT_FALSE( segmentIntersectionOrder( { vs[5], vs[4], vs[0], vs[3], vs[1], vs[2] } ) );
+
+    // intersection of 45 and 03 is closer to 4 than intersection of 45 and 02
+    EXPECT_TRUE(  segmentIntersectionOrder( { vs[4], vs[5], vs[0], vs[3], vs[0], vs[2] } ) );
+    EXPECT_FALSE( segmentIntersectionOrder( { vs[4], vs[5], vs[0], vs[2], vs[0], vs[3] } ) );
 }
 
 TEST( MRMesh, PrecisePredicates2FullDegen )
@@ -221,6 +225,14 @@ TEST( MRMesh, PreciseSegmentIntersectionOrder2 )
     EXPECT_FALSE( segmentIntersectionOrder( { vs[0], vs[1], vs[4], vs[5], vs[3], vs[2] } ) );
     EXPECT_FALSE( segmentIntersectionOrder( { vs[0], vs[1], vs[5], vs[4], vs[3], vs[2] } ) );
     EXPECT_TRUE(  segmentIntersectionOrder( { vs[1], vs[0], vs[5], vs[4], vs[3], vs[2] } ) );
+
+    // shared point in sa and sb
+    EXPECT_TRUE(  segmentIntersectionOrder( { vs[0], vs[1], vs[2], vs[3], vs[2], vs[5] } ) );
+    EXPECT_TRUE(  segmentIntersectionOrder( { vs[0], vs[1], vs[3], vs[2], vs[2], vs[5] } ) );
+    EXPECT_TRUE(  segmentIntersectionOrder( { vs[0], vs[1], vs[2], vs[3], vs[5], vs[2] } ) );
+    EXPECT_FALSE( segmentIntersectionOrder( { vs[0], vs[1], vs[4], vs[5], vs[2], vs[5] } ) );
+    EXPECT_FALSE( segmentIntersectionOrder( { vs[0], vs[1], vs[4], vs[5], vs[5], vs[2] } ) );
+    EXPECT_FALSE( segmentIntersectionOrder( { vs[0], vs[1], vs[5], vs[4], vs[5], vs[2] } ) );
 }
 
 TEST( MRMesh, PrecisePredicates3 )