CoordinateSequence: Fix logic error when adding another CoordSeq

NEWS.md

@@ -13,8 +13,9 @@
   - Fix LargestEmptyCircle to respect polygonal obstacles (GH-939, Martin Davis)
   - Fix WKTWriter to emit EMPTY elements in multi-geometries (GH-952, Mike Taves)
   - Fix IncrementalDelaunayTriangulator to ensure triangulation boundary is convex (GH-953, Martin Davis)
-  - Fix PreparedLineStringDistance for lines within envelope and polygons (GH-959, Martin Davis) 
+  - Fix PreparedLineStringDistance for lines within envelope and polygons (GH-959, Martin Davis)
   - Improve scale handling for PrecisionModel (GH-956, Martin Davis)
+  - Fix error in CoordinateSequence::add when disallowing repeated points (GH-963, Dan Baston)
 
 
 ## Changes in 3.12.0

src/geom/CoordinateSequence.cpp

@@ -191,8 +191,13 @@ CoordinateSequence::add(const CoordinateSequence& cs, std::size_t from, std::siz
         }
     }
 
+    if (first > to) {
+        // No unique points to add.
+        return;
+    }
+
     std::size_t last = first + 1;
-    const CoordinateXY* last_unique = &cs.front<CoordinateXY>();
+    const CoordinateXY* last_unique = &cs.getAt<CoordinateXY>(first);
     while(last <= to) {
         const CoordinateXY* curr = &cs.getAt<CoordinateXY>(last);
         if (curr->equals2D(*last_unique)) {

tests/unit/geom/CoordinateSequenceTest.cpp

@@ -1493,4 +1493,21 @@ void object::test<54>
     ensure(xyz3_2.equalsIdentical(xyz3));
 }
 
+
+// test add(CoordinateSequence&, false) when last point of receiving sequence is found after the beginning of donor sequence
+template<>
+template<>
+void object::test<55>
+()
+{
+    CoordinateSequence seq1{CoordinateXY(1,2), CoordinateXY(3, 4)};
+    CoordinateSequence seq2{CoordinateXY(3, 4), CoordinateXY(3, 4), CoordinateXY(5, 6), CoordinateXY(3, 4), CoordinateXY(7, 8), CoordinateXY(7, 8), CoordinateXY(9, 10)};
+
+    CoordinateSequence expected{CoordinateXY(1, 2), CoordinateXY(3, 4), CoordinateXY(5, 6), CoordinateXY(3, 4), CoordinateXY(7, 8), CoordinateXY(9, 10)};
+
+    seq1.add(seq2, false);
+
+    ensure_equals(seq1, expected);
+}
+
 } // namespace tut