Add a simplification step to cpp region parsing.

include/openmc/cell.h

@@ -106,6 +106,9 @@ class Region {
   //! Get a vector of the region expression in postfix notation
   vector<int32_t> generate_postfix(int32_t cell_id) const;
 
+  //! Get a vector of the region expression in infix notation
+  vector<int32_t> generate_infix(vector<int32_t> rpn) const;
+
   //! Determine if a particle is inside the cell for a simple cell (only
   //! intersection operators)
   bool contains_simple(Position r, Direction u, int32_t on_surface) const;

src/cell.cpp

@@ -673,6 +673,10 @@ Region::Region(std::string region_spec, int32_t cell_id)
           it--;
         }
       }
+    } else {
+      // Simplify expression by going to rpn and back
+      auto postfix = generate_postfix(cell_id);
+      expression_ = generate_infix(postfix);
     }
     expression_.shrink_to_fit();
 
@@ -879,6 +883,61 @@ vector<int32_t> Region::generate_postfix(int32_t cell_id) const
   return rpn;
 }
 
+//==============================================================================
+//! Convert Reverse Polish Notation (RPN) to infix using minimal parentheses.
+//==============================================================================
+
+vector<int32_t> Region::generate_infix(vector<int32_t> rpn) const
+{
+
+  vector<vector<int32_t>> stack;
+  vector<int32_t> pstack;
+  std::unordered_map<int32_t, int32_t> precedence = {
+    {OP_UNION, 1}, {OP_INTERSECTION, 2}, {OP_COMPLEMENT, 3}};
+
+  int HIGH = 4;
+
+  for (auto it = rpn.begin(); it != rpn.end(); ++it) {
+    auto token = *it;
+    if (precedence.find(token) != precedence.end()) {
+      auto p_c = precedence[token];
+
+      auto v_r = stack.back();
+      stack.pop_back();
+      auto p_r = pstack.back();
+      pstack.pop_back();
+
+      auto v_l = stack.back();
+      stack.pop_back();
+      auto p_l = pstack.back();
+      pstack.pop_back();
+
+      if (p_l != p_c && p_l != HIGH) {
+        v_l.emplace(v_l.begin(), OP_LEFT_PAREN);
+        v_l.emplace_back(OP_RIGHT_PAREN);
+      }
+      v_l.emplace_back(token);
+
+      if ((p_r != p_c && p_r != HIGH) ||
+          ((p_r == p_c) && (token == OP_COMPLEMENT))) {
+        v_r.emplace(v_r.begin(), OP_LEFT_PAREN);
+        v_r.emplace_back(OP_RIGHT_PAREN);
+      }
+
+      vector<int32_t> merged = v_l;
+      merged.reserve(v_l.size() + v_r.size());
+      merged.insert(merged.end(), v_r.begin(), v_r.end());
+
+      stack.emplace_back(merged);
+      pstack.emplace_back(p_c);
+    } else {
+      stack.emplace_back(vector<int32_t> {token});
+      pstack.emplace_back(HIGH);
+    }
+  }
+  return stack.back();
+}
+
 //==============================================================================
 
 std::string Region::str() const

tests/cpp_unit_tests/CMakeLists.txt

@@ -6,6 +6,7 @@ set(TEST_NAMES
   test_math
   test_mcpl_stat_sum
   test_mesh
+  test_region
   # Add additional unit test files here
 )
 

tests/cpp_unit_tests/test_region.cpp

@@ -0,0 +1,27 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "openmc/cell.h"
+#include "openmc/surface.h"
+
+#include <pugixml.hpp>
+
+TEST_CASE("Test region simplification")
+{
+  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 < 7; ++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;
+  }
+  auto region = openmc::Region("(-1 2 (-3 4) | (-5 6))", 0);
+  auto ref_val = " ( -1 2 -3 4 ) | ( -5 6 )";
+  auto test_val = region.str();
+  REQUIRE(test_val == ref_val);
+}