From d89cf214c104b4bfba3a53a8b8aca45134aa60fa Mon Sep 17 00:00:00 2001
From: Dan Baston <dbaston@gmail.com>
Date: Thu, 21 Sep 2023 21:21:05 -0400
Subject: [PATCH] CoordinateSequence: Fix logic error when adding another
 CoordSeq (#963)

---
 NEWS.md                                    |  5 ++---
 src/geom/CoordinateSequence.cpp            |  7 ++++++-
 tests/unit/geom/CoordinateSequenceTest.cpp | 17 +++++++++++++++++
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index cce2cdc682..759aafc8d6 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -6,9 +6,8 @@
   - Remove undefined behaviour in use of null PrecisionModel (GH-931, Jeff Walton)
   - Explicitly set endianness for some tests so that output matches expected (GH-934, Paul Ramsey)
   - 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)
+  - Fix error in CoordinateSequence::add when disallowing repeated points (GH-963, Dan Baston)
 
 
 ## Changes in 3.12.0
diff --git a/src/geom/CoordinateSequence.cpp b/src/geom/CoordinateSequence.cpp
index a0ff679449..fc36b4bb35 100644
--- a/src/geom/CoordinateSequence.cpp
+++ b/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)) {
diff --git a/tests/unit/geom/CoordinateSequenceTest.cpp b/tests/unit/geom/CoordinateSequenceTest.cpp
index 7ec843badc..1d9c08782b 100644
--- a/tests/unit/geom/CoordinateSequenceTest.cpp
+++ b/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