[union] fix issue 1100

Author: barendgehrels

doc/release_notes.qbk

@@ -19,6 +19,20 @@
 
 [section:release_notes Release Notes]
 
+[/=================]
+[heading Boost 1.83]
+[/=================]
+
+[*Major improvements]
+
+[*Improvements]
+
+[*Solved issues]
+
+* [@https://github.com/boostorg/geometry/issues/1100 1100] Fix for union
+
+[*Breaking changes]
+
 [/=================]
 [heading Boost 1.82]
 [/=================]

include/boost/geometry/algorithms/detail/overlay/get_turn_info.hpp

@@ -557,6 +557,7 @@ struct touch : public base_turn_handler
     >
     static inline bool handle_imperfect_touch(UniqueSubRange1 const& range_p,
                                               UniqueSubRange2 const& range_q,
+                                              int side_pk_q2,
                                               UmbrellaStrategy const& umbrella_strategy,
                                               TurnInfo& ti)
     {
@@ -595,10 +596,11 @@ struct touch : public base_turn_handler
             return d1.measure > 0 && d2.measure > 0;
         };
 
-        if (has_distance(range_p, range_q))
+        if (side_pk_q2 == -1 && has_distance(range_p, range_q))
         {
             // Even though there is a touch, Q(j) is left of P1
             // and P(i) is still left from Q2.
+            // Q continues to the right.
             // It can continue.
             ti.operations[0].operation = operation_blocked;
             // Q turns right -> union (both independent),
@@ -608,14 +610,11 @@ struct touch : public base_turn_handler
             return true;
         }
 
-        if (has_distance(range_q, range_p))
+        if (side_pk_q2 == 1 && has_distance(range_q, range_p))
         {
-            // Even though there is a touch, Q(j) is left of P1
-            // and P(i) is still left from Q2.
-            // It can continue.
+            // Similarly, but the other way round.
+            // Q continues to the left.
             ti.operations[0].operation = operation_union;
-            // Q turns right -> union (both independent),
-            // Q turns left -> intersection
             ti.operations[1].operation = operation_blocked;
             ti.touch_only = true;
             return true;
@@ -676,8 +675,8 @@ struct touch : public base_turn_handler
                 || (side_qi_p1 == 0 && side_qk_p1 == 0 && side_pk_p != -1))
             {
                 if (side_qk_p1 == 0 && side_pk_q1 == 0
-                    && has_qk && has_qk
-                    && handle_imperfect_touch(range_p, range_q, umbrella_strategy, ti))
+                    && has_pk && has_qk
+                    && handle_imperfect_touch(range_p, range_q, side_pk_q2, umbrella_strategy, ti))
                 {
                     // If q continues collinearly (opposite) with p, it should be blocked
                     // but (FP) not if there is just a tiny space in between

test/algorithms/overlay/overlay_cases.hpp

@@ -1101,6 +1101,12 @@ static std::string issue_1081c[2] =
     "POLYGON((423.217316892426425 67.5751428875900331,414.40388971336813 96.7777620478938587,410.429206867910466 100.02249750988706,412.848579034710781 101.010025694314706,412.502954439453561 102.98508206238165,416.996074177797027 109.05132662251431,427.364812035512671 72.6538592632950468,423.217316892426425 67.5751428875900331))"
 };
 
+static std::string issue_1100[2] =
+{
+    "POLYGON((1.5300545419548890819e-16 2101,1 2101,1 2100,1.1102230246251565404e-16 2100,1.5300545419548890819e-16 2101))",
+    "POLYGON((-0.19761611601674899941 2101,0 2100.6135231738085167,0 2100,-0.5 2100,-0.5 2101,-0.19761611601674899941 2101))"
+};
+
 static std::string issue_1108[2] =
 {
     "POLYGON((17 -2,18 -1.269679093926235014,12 0,22 0,17 -2))",

test/algorithms/set_operations/union/union.cpp

@@ -459,6 +459,9 @@ void test_areal()
     TEST_UNION(issue_1081c, 1, 1, -1, 2338.08);
     TEST_UNION_REV(issue_1081c, 1, 1, -1, 2338.08);
 
+    TEST_UNION(issue_1100, BG_IF_RESCALED(2, 1), 0, -1, 1.46181);
+    TEST_UNION_REV(issue_1100, BG_IF_RESCALED(2, 1), 0, -1, 1.46181);
+
     TEST_UNION(issue_1108, 1, 0, -1, 12.1742);
     TEST_UNION_REV(issue_1108, 1, 0, -1, 12.1742);