From fa2fef9283f79e9f6806340a9169244b249a8ed4 Mon Sep 17 00:00:00 2001
From: tamasmeszaros <meszaros.q@gmail.com>
Date: Thu, 15 Dec 2022 12:27:53 +0100
Subject: [PATCH] Optimize csg slicing with large number of positive parts

---
 src/libslic3r/CSGMesh/SliceCSGMesh.hpp | 36 ++++++++++++++++++--------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/src/libslic3r/CSGMesh/SliceCSGMesh.hpp b/src/libslic3r/CSGMesh/SliceCSGMesh.hpp
index f8a7354204..76ca22ec45 100644
--- a/src/libslic3r/CSGMesh/SliceCSGMesh.hpp
+++ b/src/libslic3r/CSGMesh/SliceCSGMesh.hpp
@@ -4,6 +4,7 @@
 #include "CSGMesh.hpp"
 #include "libslic3r/TriangleMeshSlicer.hpp"
 #include "libslic3r/ClipperUtils.hpp"
+#include "libslic3r/Execution/ExecutionTBB.hpp"
 
 namespace Slic3r { namespace csg {
 
@@ -18,6 +19,8 @@ std::vector<ExPolygons> slice_csgmesh_ex(
 
     MeshSlicingParamsEx params_cpy = params;
     auto trafo = params.trafo;
+    auto nonempty_indices = reserve_vector<size_t>(slicegrid.size());
+
     for (const auto &m : csg) {
         const indexed_triangle_set *its = csg::get_mesh(m);
         if (!its)
@@ -30,13 +33,18 @@ std::vector<ExPolygons> slice_csgmesh_ex(
 
         assert(slices.size() == slicegrid.size());
 
+        nonempty_indices.clear();
         for (size_t i = 0; i < slicegrid.size(); ++i) {
+            if (get_operation(m) == CSGType::Intersection || !slices[i].empty())
+                nonempty_indices.emplace_back(i);
+        }
+
+        auto mergefn = [&m, &slices, &ret](size_t i){
             switch(get_operation(m)) {
             case CSGType::Union:
                 for (ExPolygon &expoly : slices[i])
                     ret[i].emplace_back(std::move(expoly));
 
-                ret[i] = union_ex(ret[i]);
                 break;
             case CSGType::Difference:
                 ret[i] = diff_ex(ret[i], slices[i]);
@@ -45,19 +53,25 @@ std::vector<ExPolygons> slice_csgmesh_ex(
                 ret[i] = intersection_ex(ret[i], slices[i]);
                 break;
             }
-        }
+        };
 
-        for (ExPolygons &slice : ret) {
-            auto it = std::remove_if(slice.begin(), slice.end(), [](const ExPolygon &p){
-                return p.area() < double(SCALED_EPSILON) * double(SCALED_EPSILON);
-            });
-
-            // Hopefully, ExPolygons are moved, not copied to new positions
-            // and that is cheap for expolygons
-            slice.erase(it, slice.end());
-        }
+        execution::for_each(ex_tbb,
+                            nonempty_indices.begin(), nonempty_indices.end(),
+                            mergefn,
+                            execution::max_concurrency(ex_tbb));
     }
 
+    execution::for_each(ex_tbb, ret.begin(), ret.end(), [](ExPolygons &slice) {
+        auto it = std::remove_if(slice.begin(), slice.end(), [](const ExPolygon &p){
+            return p.area() < double(SCALED_EPSILON) * double(SCALED_EPSILON);
+        });
+
+        // Hopefully, ExPolygons are moved, not copied to new positions
+        // and that is cheap for expolygons
+        slice.erase(it, slice.end());
+        slice = union_ex(slice);
+    }, execution::max_concurrency(ex_tbb));
+
     return ret;
 }