Keep valid non-manifold subtraction result instead of dropping the cut (#1015)

util::boolean_operation requires a manifold result: success is
!is_manifold(a) || is_manifold(result). When a real subtraction (e.g. a
mitered IfcHalfSpaceSolid beam-end cut that grazes fine end detailing,
retry also fails the manifold test, boolean_operation returns false, and
the caller silently keeps the un-subtracted first operand. The entire
subtraction chain is dropped, so booleans-on output is identical to
booleans-off.

Remember a BRepCheck-valid CUT result that was rejected solely for being
non-manifold, and use it as a last resort once all fuzziness retries are
exhausted. A new defaulted param allow_nonmanifold_fallback is passed
false into the retry recursion so the decision is deferred to the finest
(most accurate) top-level frame. New diagnostic GEO 155.

The success path returns early exactly as before, so the change is inert
for every currently-passing cut. Verified on OCC 7.9.2: models that
already convert (IFC4 #4118, #5473; IFC2X3 #5186, #511) are byte-identical
before and after, while the #1015 beam goes from an un-subtracted 40-vert
prism to a correctly mitered 514-vert solid (end wedge of 0.87 m removed,
matching the fitted half-space distance).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcgeom/kernels/opencascade/boolean_utils.cpp

@@ -832,9 +832,18 @@ bool IfcGeom::util::points_on_planar_face_generator::operator()(gp_Pnt& p) {
 }
 
 
-bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const TopoDS_Shape& a_input, const NCollection_List<TopoDS_Shape>& b_input, BOPAlgo_Operation op, TopoDS_Shape& result, double fuzziness) {
+bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const TopoDS_Shape& a_input, const NCollection_List<TopoDS_Shape>& b_input, BOPAlgo_Operation op, TopoDS_Shape& result, double fuzziness, bool allow_nonmanifold_fallback) {
 	using namespace std::string_literals;
 
+	// A geometrically valid (BRepCheck_Analyzer) subtraction result that is
+	// rejected only for being non-manifold is remembered here. Some real models
+	// (e.g. a mitered/half-space beam end cut that grazes fine end detailing,
+	// see #1015) can only be resolved into such a result. Silently discarding
+	// the whole cut and returning the un-subtracted first operand is worse than
+	// keeping this result, so it is used as a last resort once the fuzziness
+	// retries are exhausted.
+	TopoDS_Shape nonmanifold_fallback;
+
 	const bool do_unify = true;
 	const bool do_subtraction_eliminate_disjoint_bbox = true;
 	const bool do_subtraction_eliminate_touching = true;
@@ -1390,6 +1399,12 @@ bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const To
 
 				} else {
 					settings.log().Notice("GEO", 151, "Boolean operation yields non-manifold result");
+					// r passed BRepCheck_Analyzer above and was rejected solely for
+					// being non-manifold. Remember it as a last-resort fallback so a
+					// difficult but valid subtraction is not dropped entirely (#1015).
+					if (op == BOPAlgo_CUT && nonmanifold_fallback.IsNull() && !r.IsNull()) {
+						nonmanifold_fallback = r;
+					}
 				}
 			}
 		}
@@ -1417,10 +1432,25 @@ bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const To
 	}
 	if (!success) {
 		if (allow_retry) {
-			return boolean_operation(settings, a, b, op, result, new_fuzziness);
+			// Retry at a higher fuzziness in the hope of a clean manifold result.
+			// The retry is not allowed to apply the non-manifold fallback itself:
+			// that decision is deferred to this (finer-fuzziness) frame so that,
+			// if every retry also fails, the most accurate fallback is used.
+			if (boolean_operation(settings, a, b, op, result, new_fuzziness, false)) {
+				return true;
+			}
 		} else {
 			settings.log().Notice("GEO", 154, "No longer attempting boolean operation with higher fuzziness");
 		}
+
+		if (allow_nonmanifold_fallback && !nonmanifold_fallback.IsNull()) {
+			// Every attempt failed the manifold check but produced a geometrically
+			// valid subtraction. Keeping it preserves the boolean cut instead of
+			// silently returning the un-subtracted first operand (#1015).
+			settings.log().Message(Logger::LOG_WARNING, "GEO", 155, "Boolean subtraction result is non-manifold, using it regardless to preserve the cut");
+			result = nonmanifold_fallback;
+			return true;
+		}
 	}
 	return success && !result.IsNull();
 }

src/ifcgeom/kernels/opencascade/boolean_utils.h

@@ -104,7 +104,7 @@ namespace IfcGeom {
 			Logger& log() const { return logger ? *logger : Logger::Root(); }
 		};
 
-		bool boolean_operation(const boolean_settings& settings, const TopoDS_Shape&, const NCollection_List<TopoDS_Shape>&, BOPAlgo_Operation, TopoDS_Shape&, double fuzziness = -1.);
+		bool boolean_operation(const boolean_settings& settings, const TopoDS_Shape&, const NCollection_List<TopoDS_Shape>&, BOPAlgo_Operation, TopoDS_Shape&, double fuzziness = -1., bool allow_nonmanifold_fallback = true);
 
 		bool boolean_operation(const boolean_settings& settings, const TopoDS_Shape&, const TopoDS_Shape&, BOPAlgo_Operation, TopoDS_Shape&, double fuzziness = -1.);