src/chunk.cpp

   1 #include "chunk.hpp"
   2
   3 #include "generator.hpp"
   4
   5 #include <limits>
   6 #include <queue>
   7 #include <glm/gtx/transform.hpp>
   8
   9
  10 namespace blank {
  11
  12 Chunk::Chunk(const BlockTypeRegistry &types)
  13 : types(&types)
  14 , neighbor{ 0, 0, 0, 0, 0, 0 }
  15 , blocks()
  16 , model()
  17 , position(0, 0, 0)
  18 , dirty(false) {
  19
  20 }
  21
  22 Chunk::Chunk(Chunk &&other)
  23 : types(other.types)
  24 , blocks(std::move(other.blocks))
  25 , model(std::move(other.model))
  26 , dirty(other.dirty) {
  27         for (size_t i = 0; i < Block::FACE_COUNT; ++i) {
  28                 neighbor[i] = other.neighbor[i];
  29         }
  30 }
  31
  32 Chunk &Chunk::operator =(Chunk &&other) {
  33         types = other.types;
  34         for (size_t i = 0; i < Block::FACE_COUNT; ++i) {
  35                 neighbor[i] = other.neighbor[i];
  36         }
  37         blocks = std::move(other.blocks);
  38         model = std::move(other.model);
  39         dirty = other.dirty;
  40         return *this;
  41 }
  42
  43
  44 void Chunk::SetNeighbor(Chunk &other) {
  45         if (other.position == position - Pos(-1, 0, 0)) {
  46                 neighbor[Block::FACE_LEFT] = &other;
  47                 other.neighbor[Block::FACE_RIGHT] = this;
  48         } else if (other.position == position - Pos(1, 0, 0)) {
  49                 neighbor[Block::FACE_RIGHT] = &other;
  50                 other.neighbor[Block::FACE_LEFT] = this;
  51         } else if (other.position == position - Pos(0, -1, 0)) {
  52                 neighbor[Block::FACE_DOWN] = &other;
  53                 other.neighbor[Block::FACE_UP] = this;
  54         } else if (other.position == position - Pos(0, 1, 0)) {
  55                 neighbor[Block::FACE_UP] = &other;
  56                 other.neighbor[Block::FACE_DOWN] = this;
  57         } else if (other.position == position - Pos(0, 0, -1)) {
  58                 neighbor[Block::FACE_BACK] = &other;
  59                 other.neighbor[Block::FACE_FRONT] = this;
  60         } else if (other.position == position - Pos(0, 0, 1)) {
  61                 neighbor[Block::FACE_FRONT] = &other;
  62                 other.neighbor[Block::FACE_BACK] = this;
  63         }
  64 }
  65
  66 void Chunk::ClearNeighbors() {
  67         for (int i = 0; i < Block::FACE_COUNT; ++i) {
  68                 neighbor[i] = nullptr;
  69         }
  70 }
  71
  72 void Chunk::Unlink() {
  73         if (neighbor[Block::FACE_UP]) {
  74                 neighbor[Block::FACE_UP]->neighbor[Block::FACE_DOWN] = nullptr;
  75         }
  76         if (neighbor[Block::FACE_DOWN]) {
  77                 neighbor[Block::FACE_DOWN]->neighbor[Block::FACE_UP] = nullptr;
  78         }
  79         if (neighbor[Block::FACE_LEFT]) {
  80                 neighbor[Block::FACE_LEFT]->neighbor[Block::FACE_RIGHT] = nullptr;
  81         }
  82         if (neighbor[Block::FACE_RIGHT]) {
  83                 neighbor[Block::FACE_RIGHT]->neighbor[Block::FACE_LEFT] = nullptr;
  84         }
  85         if (neighbor[Block::FACE_FRONT]) {
  86                 neighbor[Block::FACE_FRONT]->neighbor[Block::FACE_BACK] = nullptr;
  87         }
  88         if (neighbor[Block::FACE_BACK]) {
  89                 neighbor[Block::FACE_BACK]->neighbor[Block::FACE_FRONT] = nullptr;
  90         }
  91 }
  92
  93 void Chunk::Relink() {
  94         if (neighbor[Block::FACE_UP]) {
  95                 neighbor[Block::FACE_UP]->neighbor[Block::FACE_DOWN] = this;
  96         }
  97         if (neighbor[Block::FACE_DOWN]) {
  98                 neighbor[Block::FACE_DOWN]->neighbor[Block::FACE_UP] = this;
  99         }
 100         if (neighbor[Block::FACE_LEFT]) {
 101                 neighbor[Block::FACE_LEFT]->neighbor[Block::FACE_RIGHT] = this;
 102         }
 103         if (neighbor[Block::FACE_RIGHT]) {
 104                 neighbor[Block::FACE_RIGHT]->neighbor[Block::FACE_LEFT] = this;
 105         }
 106         if (neighbor[Block::FACE_FRONT]) {
 107                 neighbor[Block::FACE_FRONT]->neighbor[Block::FACE_BACK] = this;
 108         }
 109         if (neighbor[Block::FACE_BACK]) {
 110                 neighbor[Block::FACE_BACK]->neighbor[Block::FACE_FRONT] = this;
 111         }
 112 }
 113
 114
 115 namespace {
 116
 117 struct SetNode {
 118
 119         Chunk *chunk;
 120         Chunk::Pos pos;
 121
 122         SetNode(Chunk *chunk, Chunk::Pos pos)
 123         : chunk(chunk), pos(pos) { }
 124
 125         int Get() const { return chunk->GetLight(pos); }
 126         void Set(int level) { chunk->SetLight(pos, level); }
 127
 128         bool HasNext(Block::Face face) {
 129                 const Block *next = chunk->FindNext(pos, face);
 130                 return next && !chunk->Type(*next).block_light;
 131         }
 132         SetNode GetNext(Block::Face face) {
 133                 Chunk::Pos next_pos(pos + Block::FaceNormal(face));
 134                 if (Chunk::InBounds(next_pos)) {
 135                         return SetNode(chunk, next_pos);
 136                 } else {
 137                         return SetNode(&chunk->GetNeighbor(face), next_pos - (Block::FaceNormal(face) * Chunk::Extent()));
 138                 }
 139         }
 140
 141 };
 142
 143 struct UnsetNode
 144 : public SetNode {
 145
 146         int level;
 147
 148         UnsetNode(Chunk *chunk, Chunk::Pos pos)
 149         : SetNode(chunk, pos), level(Get()) { }
 150
 151         UnsetNode(const SetNode &set)
 152         : SetNode(set), level(Get()) { }
 153
 154         UnsetNode GetNext(Block::Face face) { return UnsetNode(SetNode::GetNext(face)); }
 155
 156 };
 157
 158 std::queue<SetNode> light_queue;
 159 std::queue<UnsetNode> dark_queue;
 160
 161 void work_light() {
 162         while (!light_queue.empty()) {
 163                 SetNode node = light_queue.front();
 164                 light_queue.pop();
 165
 166                 int level = node.Get() - 1;
 167                 for (int face = 0; face < Block::FACE_COUNT; ++face) {
 168                         if (node.HasNext(Block::Face(face))) {
 169                                 SetNode other = node.GetNext(Block::Face(face));
 170                                 if (other.Get() < level) {
 171                                         other.Set(level);
 172                                         light_queue.emplace(other);
 173                                 }
 174                         }
 175                 }
 176         }
 177 }
 178
 179 void work_dark() {
 180         while (!dark_queue.empty()) {
 181                 UnsetNode node = dark_queue.front();
 182                 dark_queue.pop();
 183
 184                 for (int face = 0; face < Block::FACE_COUNT; ++face) {
 185                         if (node.HasNext(Block::Face(face))) {
 186                                 UnsetNode other = node.GetNext(Block::Face(face));
 187                                 // TODO: if there a light source here with the same level this will err
 188                                 if (other.Get() != 0 && other.Get() < node.level) {
 189                                         other.Set(0);
 190                                         dark_queue.emplace(other);
 191                                 } else {
 192                                         light_queue.emplace(other);
 193                                 }
 194                         }
 195                 }
 196         }
 197 }
 198
 199 }
 200
 201 void Chunk::SetBlock(int index, const Block &block) {
 202         const BlockType &old_type = Type(blocks[index]);
 203         const BlockType &new_type = Type(block);
 204
 205         blocks[index] = block;
 206
 207         if (&old_type == &new_type) return;
 208
 209         if (new_type.luminosity > old_type.luminosity) {
 210                 // light added
 211                 SetLight(index, new_type.luminosity);
 212                 light_queue.emplace(this, ToPos(index));
 213                 work_light();
 214         } else if (new_type.luminosity < old_type.luminosity) {
 215                 // light removed
 216                 dark_queue.emplace(this, ToPos(index));
 217                 SetLight(index, 0);
 218                 work_dark();
 219                 SetLight(index, new_type.luminosity);
 220                 light_queue.emplace(this, ToPos(index));
 221                 work_light();
 222         } else if (new_type.block_light && !old_type.block_light) {
 223                 // obstacle added
 224                 dark_queue.emplace(this, ToPos(index));
 225                 SetLight(index, 0);
 226                 work_dark();
 227                 work_light();
 228         } else if (!new_type.block_light && old_type.block_light) {
 229                 // obstacle removed
 230                 int level = 0;
 231                 for (int face = 0; face < Block::FACE_COUNT; ++face) {
 232                         Pos next_pos(ToPos(index) + Block::FaceNormal(Block::Face(face)));
 233                         int next_level = 0;
 234                         if (InBounds(next_pos)) {
 235                                 next_level = GetLight(next_pos);
 236                         } else {
 237                                 if (HasNeighbor(Block::Face(face))) {
 238                                         next_pos -= (Block::FaceNormal(Block::Face(face)) * Chunk::Extent());
 239                                         next_level = GetNeighbor(Block::Face(face)).GetLight(next_pos);
 240                                 }
 241                         }
 242                         if (level < next_level) {
 243                                 level = next_level;
 244                         }
 245                 }
 246                 if (level > 1) {
 247                         SetLight(index, level - 1);
 248                         light_queue.emplace(this, ToPos(index));
 249                         work_light();
 250                 }
 251         }
 252 }
 253
 254 const Block *Chunk::FindNext(const Pos &pos, Block::Face face) const {
 255         Pos next_pos(pos + Block::FaceNormal(face));
 256         if (InBounds(next_pos)) {
 257                 return &BlockAt(pos + Block::FaceNormal(face));
 258         } else if (HasNeighbor(face)) {
 259                 return &GetNeighbor(face).BlockAt(next_pos - (Block::FaceNormal(face) * Extent()));
 260         } else {
 261                 return nullptr;
 262         }
 263 }
 264
 265
 266 void Chunk::SetLight(int index, int level) {
 267         light[index] = level;
 268 }
 269
 270 int Chunk::GetLight(int index) const {
 271         return light[index];
 272 }
 273
 274
 275 bool Chunk::IsSurface(const Pos &pos) const {
 276         const Block &block = BlockAt(pos);
 277         if (!Type(block).visible) {
 278                 return false;
 279         }
 280         for (int face = 0; face < Block::FACE_COUNT; ++face) {
 281                 const Block *next = FindNext(pos, Block::Face(face));
 282                 if (!next || !Type(*next).visible) {
 283                         return true;
 284                 }
 285         }
 286         return false;
 287 }
 288
 289
 290 void Chunk::Allocate() {
 291         blocks.resize(Size(), Block(0));
 292         light.resize(Size(), 0);
 293 }
 294
 295
 296 void Chunk::Draw() {
 297         if (dirty) {
 298                 Update();
 299         }
 300         model.Draw();
 301 }
 302
 303
 304 bool Chunk::Intersection(
 305         const Ray &ray,
 306         const glm::mat4 &M,
 307         int &blkid,
 308         float &dist,
 309         glm::vec3 &normal
 310 ) const {
 311         // TODO: should be possible to heavily optimize this
 312         int id = 0;
 313         blkid = -1;
 314         dist = std::numeric_limits<float>::infinity();
 315         for (int z = 0; z < Depth(); ++z) {
 316                 for (int y = 0; y < Height(); ++y) {
 317                         for (int x = 0; x < Width(); ++x, ++id) {
 318                                 if (!Type(blocks[id]).visible) {
 319                                         continue;
 320                                 }
 321                                 float cur_dist;
 322                                 glm::vec3 cur_norm;
 323                                 if (Type(blocks[id]).shape->Intersects(ray, M * ToTransform(id), cur_dist, cur_norm)) {
 324                                         if (cur_dist < dist) {
 325                                                 blkid = id;
 326                                                 dist = cur_dist;
 327                                                 normal = cur_norm;
 328                                         }
 329                                 }
 330                         }
 331                 }
 332         }
 333
 334         if (blkid < 0) {
 335                 return false;
 336         } else {
 337                 normal = glm::vec3(BlockAt(blkid).Transform() * glm::vec4(normal, 0.0f));
 338                 return true;
 339         }
 340 }
 341
 342 void Chunk::Position(const Pos &pos) {
 343         position = pos;
 344 }
 345
 346 glm::mat4 Chunk::Transform(const Pos &offset) const {
 347         return glm::translate((position - offset) * Extent());
 348 }
 349
 350
 351 namespace {
 352
 353 Model::Buffer buf;
 354
 355 }
 356
 357 void Chunk::CheckUpdate() {
 358         if (dirty) {
 359                 Update();
 360         }
 361 }
 362
 363 void Chunk::Update() {
 364         int vtx_count = 0, idx_count = 0;
 365         for (const auto &block : blocks) {
 366                 const Shape *shape = Type(block).shape;
 367                 vtx_count += shape->VertexCount();
 368                 idx_count += shape->VertexIndexCount();
 369         }
 370         buf.Clear();
 371         buf.Reserve(vtx_count, idx_count);
 372
 373         Model::Index vtx_counter = 0;
 374         for (size_t i = 0; i < Size(); ++i) {
 375                 const BlockType &type = Type(blocks[i]);
 376
 377                 if (!type.visible || Obstructed(i)) continue;
 378
 379                 type.FillModel(buf, ToTransform(i), vtx_counter);
 380                 vtx_counter += type.shape->VertexCount();
 381         }
 382
 383         model.Update(buf);
 384         dirty = false;
 385 }
 386
 387 bool Chunk::Obstructed(int idx) const {
 388         Chunk::Pos pos(ToPos(idx));
 389
 390         Chunk::Pos left_pos(pos + Chunk::Pos(-1, 0, 0));
 391         const Block *left_block = nullptr;
 392         if (InBounds(left_pos)) {
 393                 left_block = &BlockAt(left_pos);
 394         } else if (HasNeighbor(Block::FACE_LEFT)) {
 395                 left_pos += Chunk::Pos(Width(), 0, 0);
 396                 left_block = &GetNeighbor(Block::FACE_LEFT).BlockAt(left_pos);
 397         } else {
 398                 return false;
 399         }
 400         if (!Type(*left_block).FaceFilled(*left_block, Block::FACE_RIGHT)) {
 401                 return false;
 402         }
 403
 404         Chunk::Pos right_pos(pos + Chunk::Pos(1, 0, 0));
 405         const Block *right_block = nullptr;
 406         if (InBounds(right_pos)) {
 407                 right_block = &BlockAt(right_pos);
 408         } else if (HasNeighbor(Block::FACE_RIGHT)) {
 409                 right_pos += Chunk::Pos(-Width(), 0, 0);
 410                 right_block = &GetNeighbor(Block::FACE_RIGHT).BlockAt(right_pos);
 411         } else {
 412                 return false;
 413         }
 414         if (!Type(*right_block).FaceFilled(*right_block, Block::FACE_LEFT)) {
 415                 return false;
 416         }
 417
 418         Chunk::Pos down_pos(pos + Chunk::Pos(0, -1, 0));
 419         const Block *down_block = nullptr;
 420         if (InBounds(down_pos)) {
 421                 down_block = &BlockAt(down_pos);
 422         } else if (HasNeighbor(Block::FACE_DOWN)) {
 423                 down_pos += Chunk::Pos(0, Height(), 0);
 424                 down_block = &GetNeighbor(Block::FACE_DOWN).BlockAt(down_pos);
 425         } else {
 426                 return false;
 427         }
 428         if (!Type(*down_block).FaceFilled(*down_block, Block::FACE_UP)) {
 429                 return false;
 430         }
 431
 432         Chunk::Pos up_pos(pos + Chunk::Pos(0, 1, 0));
 433         const Block *up_block = nullptr;
 434         if (InBounds(up_pos)) {
 435                 up_block = &BlockAt(up_pos);
 436         } else if (HasNeighbor(Block::FACE_UP)) {
 437                 up_pos += Chunk::Pos(0, -Height(), 0);
 438                 up_block = &GetNeighbor(Block::FACE_UP).BlockAt(up_pos);
 439         } else {
 440                 return false;
 441         }
 442         if (!Type(*up_block).FaceFilled(*up_block, Block::FACE_DOWN)) {
 443                 return false;
 444         }
 445
 446         Chunk::Pos back_pos(pos + Chunk::Pos(0, 0, -1));
 447         const Block *back_block = nullptr;
 448         if (InBounds(back_pos)) {
 449                 back_block = &BlockAt(back_pos);
 450         } else if (HasNeighbor(Block::FACE_BACK)) {
 451                 back_pos += Chunk::Pos(0, 0, Depth());
 452                 back_block = &GetNeighbor(Block::FACE_BACK).BlockAt(back_pos);
 453         } else {
 454                 return false;
 455         }
 456         if (!Type(*back_block).FaceFilled(*back_block, Block::FACE_FRONT)) {
 457                 return false;
 458         }
 459
 460         Chunk::Pos front_pos(pos + Chunk::Pos(0, 0, 1));
 461         const Block *front_block = nullptr;
 462         if (InBounds(front_pos)) {
 463                 front_block = &BlockAt(front_pos);
 464         } else if (HasNeighbor(Block::FACE_FRONT)) {
 465                 front_pos += Chunk::Pos(0, 0, -Depth());
 466                 front_block = &GetNeighbor(Block::FACE_FRONT).BlockAt(front_pos);
 467         } else {
 468                 return false;
 469         }
 470         if (!Type(*front_block).FaceFilled(*front_block, Block::FACE_BACK)) {
 471                 return false;
 472         }
 473
 474         return true;
 475 }
 476
 477 glm::mat4 Chunk::ToTransform(int idx) const {
 478         return glm::translate(glm::mat4(1.0f), ToCoords(idx)) * blocks[idx].Transform();
 479 }
 480
 481
 482 ChunkLoader::ChunkLoader(const Config &config, const BlockTypeRegistry &reg, const Generator &gen)
 483 : base(0, 0, 0)
 484 , reg(reg)
 485 , gen(gen)
 486 , loaded()
 487 , to_generate()
 488 , to_free()
 489 , load_dist(config.load_dist)
 490 , unload_dist(config.unload_dist) {
 491
 492 }
 493
 494 namespace {
 495
 496 struct ChunkLess {
 497
 498         explicit ChunkLess(const Chunk::Pos &base)
 499         : base(base) { }
 500
 501         bool operator ()(const Chunk::Pos &a, const Chunk::Pos &b) const {
 502                 Chunk::Pos da(base - a);
 503                 Chunk::Pos db(base - b);
 504                 return
 505                         da.x * da.x + da.y * da.y + da.z * da.z <
 506                         db.x * db.x + db.y * db.y + db.z * db.z;
 507         }
 508
 509         Chunk::Pos base;
 510
 511 };
 512
 513 }
 514
 515 void ChunkLoader::Generate(const Chunk::Pos &from, const Chunk::Pos &to) {
 516         for (int z = from.z; z < to.z; ++z) {
 517                 for (int y = from.y; y < to.y; ++y) {
 518                         for (int x = from.x; x < to.x; ++x) {
 519                                 Chunk::Pos pos(x, y, z);
 520                                 if (Known(pos)) {
 521                                         continue;
 522                                 } else if (pos == base) {
 523                                         Generate(pos);
 524
 525                                 //      orientation testing
 526                                 //      for (int i = 0; i < Block::FACE_COUNT; ++i) {
 527                                 //              for (int j = 0; j < Block::TURN_COUNT; ++j) {
 528                                 //                      loaded.back().BlockAt(512 * j + 2 * i) = Block(3 * (j + 1), Block::Face(i), Block::Turn(j));
 529                                 //              }
 530                                 //      }
 531                                 //      loaded.back().Invalidate();
 532                                 //      loaded.back().CheckUpdate();
 533                                 } else {
 534                                         to_generate.emplace_back(pos);
 535                                 }
 536                         }
 537                 }
 538         }
 539         to_generate.sort(ChunkLess(base));
 540 }
 541
 542 Chunk &ChunkLoader::Generate(const Chunk::Pos &pos) {
 543         loaded.emplace_back(reg);
 544         Chunk &chunk = loaded.back();
 545         chunk.Position(pos);
 546         Insert(chunk);
 547         gen(chunk);
 548         return chunk;
 549 }
 550
 551 void ChunkLoader::Insert(Chunk &chunk) {
 552         for (Chunk &other : loaded) {
 553                 chunk.SetNeighbor(other);
 554         }
 555 }
 556
 557 void ChunkLoader::Remove(Chunk &chunk) {
 558         chunk.Unlink();
 559 }
 560
 561 Chunk *ChunkLoader::Loaded(const Chunk::Pos &pos) {
 562         for (Chunk &chunk : loaded) {
 563                 if (chunk.Position() == pos) {
 564                         return &chunk;
 565                 }
 566         }
 567         return nullptr;
 568 }
 569
 570 bool ChunkLoader::Queued(const Chunk::Pos &pos) {
 571         for (const Chunk::Pos &chunk : to_generate) {
 572                 if (chunk == pos) {
 573                         return true;
 574                 }
 575         }
 576         return nullptr;
 577 }
 578
 579 bool ChunkLoader::Known(const Chunk::Pos &pos) {
 580         if (Loaded(pos)) return true;
 581         return Queued(pos);
 582 }
 583
 584 Chunk &ChunkLoader::ForceLoad(const Chunk::Pos &pos) {
 585         Chunk *chunk = Loaded(pos);
 586         if (chunk) {
 587                 return *chunk;
 588         }
 589
 590         for (auto iter(to_generate.begin()), end(to_generate.end()); iter != end; ++iter) {
 591                 if (*iter == pos) {
 592                         to_generate.erase(iter);
 593                         break;
 594                 }
 595         }
 596
 597         return Generate(pos);
 598 }
 599
 600 void ChunkLoader::Rebase(const Chunk::Pos &new_base) {
 601         if (new_base == base) {
 602                 return;
 603         }
 604         base = new_base;
 605
 606         // unload far away chunks
 607         for (auto iter(loaded.begin()), end(loaded.end()); iter != end;) {
 608                 if (std::abs(base.x - iter->Position().x) > unload_dist
 609                                 || std::abs(base.y - iter->Position().y) > unload_dist
 610                                 || std::abs(base.z - iter->Position().z) > unload_dist) {
 611                         auto saved = iter;
 612                         Remove(*saved);
 613                         ++iter;
 614                         to_free.splice(to_free.end(), loaded, saved);
 615                 } else {
 616                         ++iter;
 617                 }
 618         }
 619         // abort far away queued chunks
 620         for (auto iter(to_generate.begin()), end(to_generate.end()); iter != end;) {
 621                 if (std::abs(base.x - iter->x) > unload_dist
 622                                 || std::abs(base.y - iter->y) > unload_dist
 623                                 || std::abs(base.z - iter->z) > unload_dist) {
 624                         iter = to_generate.erase(iter);
 625                 } else {
 626                         ++iter;
 627                 }
 628         }
 629         // add missing new chunks
 630         GenerateSurrounding(base);
 631 }
 632
 633 void ChunkLoader::GenerateSurrounding(const Chunk::Pos &pos) {
 634         const Chunk::Pos offset(load_dist, load_dist, load_dist);
 635         Generate(pos - offset, pos + offset);
 636 }
 637
 638 void ChunkLoader::Update() {
 639         bool reused = false;
 640         if (!to_generate.empty()) {
 641                 Chunk::Pos pos(to_generate.front());
 642
 643                 for (auto iter(to_free.begin()), end(to_free.end()); iter != end; ++iter) {
 644                         if (iter->Position() == pos) {
 645                                 iter->Relink();
 646                                 loaded.splice(loaded.end(), to_free, iter);
 647                                 reused = true;
 648                                 break;
 649                         }
 650                 }
 651
 652                 if (!reused) {
 653                         if (to_free.empty()) {
 654                                 loaded.emplace_back(reg);
 655                         } else {
 656                                 to_free.front().ClearNeighbors();
 657                                 loaded.splice(loaded.end(), to_free, to_free.begin());
 658                                 reused = true;
 659                         }
 660                         Chunk &chunk = loaded.back();
 661                         chunk.Position(pos);
 662                         Insert(chunk);
 663                         gen(chunk);
 664                 }
 665                 to_generate.pop_front();
 666         }
 667
 668         if (!reused && !to_free.empty()) {
 669                 to_free.pop_front();
 670         }
 671 }
 672
 673 }