[#88] refactor(Mesh): speed up world mesh parsing

This commit moves the allocation of many small vectors within `zenkit::Polygon` into a two larger vectors inside `zenkit::Mesh`, reducing the number of small allocation made.

Doing this leads to a 25% performance boost (from 193ms to 144ms) on my laptop running Fedora 40 on an Intel Core i7-11800H.

include/zenkit/Mesh.hh

@@ -81,8 +81,8 @@ namespace zenkit {
  int32_t lightmap;
  PolygonFlagSet flags;
 
- std::vector<uint32_t> vertices;
- std::vector<uint32_t> features;
+ size_t index_count;
+ size_t index_offset;
  };
 
  /// \brief Represents a *ZenGin* basic mesh.
@@ -161,6 +161,8 @@ namespace zenkit {
  std::vector<LightMap> lightmaps {};
 
  std::vector<Polygon> geometry {};
+ std::vector<uint32_t> polygon_vertex_indices;
+ std::vector<uint32_t> polygon_feature_indices;
 
  /// \brief A list of polygons of this mesh.
  PolygonList polygons {};

src/Mesh.cc

@@ -96,6 +96,11 @@ namespace zenkit {
  auto poly_count = c->read_uint();
  this->geometry.resize(poly_count);
 
+ // At least 3 indices per polygon (triangulated).
+ this->polygon_vertex_indices.reserve(poly_count * 3);
+ this->polygon_feature_indices.reserve(poly_count * 3);
+ uint32_t index_offset = 0;
+
  for (std::uint32_t i = 0; i < poly_count; ++i) {
  this->geometry[i].material = c->read_ushort();
  this->geometry[i].lightmap = c->read_short();
@@ -133,9 +138,14 @@ namespace zenkit {
  auto has_wide_indices = (version == MESH_VERSION_G2) || force_wide_indices;
 
  for (int32_t j = 0; j < vertex_count; ++j) {
- this->geometry[i].vertices.push_back(has_wide_indices ? c->read_uint() : c->read_ushort());
- this->geometry[i].features.push_back(c->read_uint());
+ this->polygon_vertex_indices.push_back(has_wide_indices ? c->read_uint()
+ : c->read_ushort());
+ this->polygon_feature_indices.push_back(c->read_uint());
  }
+
+ this->geometry[i].index_count = vertex_count;
+ this->geometry[i].index_offset = index_offset;
+ index_offset += vertex_count;
  }
 
  break;
@@ -205,22 +215,22 @@ namespace zenkit {
  }
 
  auto& polygon = this->geometry[i];
- if (polygon.vertices.size() < 3 || polygon.flags.is_portal || polygon.flags.is_ghost_occluder ||
+ if (polygon.index_count < 3 || polygon.flags.is_portal || polygon.flags.is_ghost_occluder ||
  polygon.flags.is_outdoor) {
  continue;
  }
 
  auto a = 1u;
 
  // NOTE(lmichaelis): This unpacks triangle fans
- for (auto b = 2u; b < polygon.vertices.size(); ++b) {
- auto root = 0u;
- this->polygons.vertex_indices.push_back(polygon.vertices[root]);
- this->polygons.vertex_indices.push_back(polygon.vertices[a]);
- this->polygons.vertex_indices.push_back(polygon.vertices[b]);
- this->polygons.feature_indices.push_back(polygon.features[root]);
- this->polygons.feature_indices.push_back(polygon.features[a]);
- this->polygons.feature_indices.push_back(polygon.features[b]);
+ for (auto b = 2u; b < polygon.index_count; ++b) {
+ auto root = polygon.index_offset;
+ this->polygons.vertex_indices.push_back(polygon_vertex_indices[root]);
+ this->polygons.vertex_indices.push_back(polygon_vertex_indices[root + a]);
+ this->polygons.vertex_indices.push_back(polygon_vertex_indices[root + b]);
+ this->polygons.feature_indices.push_back(polygon_feature_indices[root]);
+ this->polygons.feature_indices.push_back(polygon_feature_indices[root + a]);
+ this->polygons.feature_indices.push_back(polygon_feature_indices[root + b]);
  this->polygons.material_indices.push_back(polygon.material);
  this->polygons.lightmap_indices.push_back(polygon.lightmap);
  this->polygons.flags.push_back(polygon.flags);
@@ -297,8 +307,8 @@ namespace zenkit {
  c->write_short(flags.sector_index);
  }
 
- c->write_ubyte(poly.vertices.size());
- for (auto i = 0u; i < poly.vertices.size(); ++i) {
+ c->write_ubyte(poly.index_count);
+ for (auto i = 0u; i < poly.index_count; ++i) {
  if (version == GameVersion::GOTHIC_1) {
  c->write_ushort(this->polygons.vertex_indices[i]);
  c->write_uint(this->polygons.feature_indices[i]);