Implement depth-awareness when enforcing precedence between union/intersection operators (#3730)

Co-authored-by: GuySten <guyste@post.bgu.ac.il>

tests/cpp_unit_tests/test_region.cpp

@@ -0,0 +1,101 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "openmc/cell.h"
+#include "openmc/surface.h"
+
+#include <pugixml.hpp>
+
+namespace {
+
+// Helper class to set up and tear down test surfaces
+class SurfaceFixture {
+public:
+  SurfaceFixture()
+  {
+    pugi::xml_document doc;
+    pugi::xml_node surf_node = doc.append_child("surface");
+    surf_node.set_name("surface");
+    surf_node.append_attribute("id") = "0";
+    surf_node.append_attribute("type") = "x-plane";
+    surf_node.append_attribute("coeffs") = "1";
+
+    for (int i = 1; i < 10; ++i) {
+      surf_node.attribute("id") = i;
+      openmc::model::surfaces.push_back(
+        std::make_unique<openmc::SurfaceXPlane>(surf_node));
+      openmc::model::surface_map[i] = i - 1;
+    }
+  }
+
+  ~SurfaceFixture()
+  {
+    openmc::model::surfaces.clear();
+    openmc::model::surface_map.clear();
+  }
+};
+
+} // anonymous namespace
+
+TEST_CASE("Test region simplification")
+{
+  SurfaceFixture fixture;
+
+  SECTION("Original bug case from issue #3685")
+  {
+    // Input: "-1 2 (-3 4) | (-5 6)" was being incorrectly interpreted
+    auto region = openmc::Region("(-1 2 (-3 4) | (-5 6))", 0);
+    REQUIRE(region.str() == " ( ( -1 2 ( -3 4 ) ) | ( -5 6 ) )");
+  }
+
+  SECTION("Simple union - no extra parentheses needed")
+  {
+    auto region = openmc::Region("1 | 2", 0);
+    REQUIRE(region.str() == " 1 | 2");
+  }
+
+  SECTION("Intersection then union")
+  {
+    // Intersection should have higher precedence, so (1 2) grouped
+    auto region = openmc::Region("1 2 | 3", 0);
+    REQUIRE(region.str() == " ( 1 2 ) | 3");
+  }
+
+  SECTION("Union then intersection")
+  {
+    // The (2 3) intersection should be grouped
+    auto region = openmc::Region("1 | 2 3", 0);
+    REQUIRE(region.str() == " 1 | ( 2 3 )");
+  }
+
+  SECTION("Nested parentheses preserved")
+  {
+    // These parentheses are meaningful and should be preserved
+    auto region = openmc::Region("(1 | 2) (3 | 4)", 0);
+    REQUIRE(region.str() == " ( 1 | 2 ) ( 3 | 4 )");
+  }
+
+  SECTION("Deep nesting")
+  {
+    auto region = openmc::Region("((1 2) | (3 4)) 5", 0);
+    REQUIRE(region.str() == " ( ( 1 2 ) | ( 3 4 ) ) 5");
+  }
+
+  SECTION("Multiple unions")
+  {
+    auto region = openmc::Region("1 | 2 | 3", 0);
+    REQUIRE(region.str() == " 1 | 2 | 3");
+  }
+
+  SECTION("Multiple intersections")
+  {
+    auto region = openmc::Region("1 2 3", 0);
+    // Simple cell - no operators in output
+    REQUIRE(region.str() == " 1 2 3");
+  }
+
+  SECTION("Complex mixed expression")
+  {
+    auto region = openmc::Region("1 2 | 3 4 | 5 6", 0);
+    REQUIRE(region.str() == " ( 1 2 ) | ( 3 4 ) | ( 5 6 )");
+  }
+}