git.sesse.net Git - plocate/blob - database-builder.cpp

   1 #include "database-builder.h"
   2
   3 #include "dprintf.h"
   4 #include "turbopfor-encode.h"
   5
   6 #include <algorithm>
   7 #include <assert.h>
   8 #ifdef HAS_ENDIAN_H
   9 #include <endian.h>
  10 #endif
  11 #include <fcntl.h>
  12 #include <string.h>
  13 #include <string_view>
  14 #include <sys/stat.h>
  15 #include <sys/time.h>
  16 #include <sys/types.h>
  17 #include <unistd.h>
  18 #include <zdict.h>
  19 #include <zstd.h>
  20 #include <unicode/coll.h>
  21 #include <unicode/locid.h>
  22
  23 #define P4NENC_BOUND(n) ((n + 127) / 128 + (n + 32) * sizeof(uint32_t))
  24
  25 #define NUM_TRIGRAMS 16777216
  26
  27 using namespace std;
  28 using namespace std::chrono;
  29
  30 constexpr unsigned num_overflow_slots = 16;
  31
  32 string zstd_compress(const string &src, ZSTD_CDict *cdict, string *tempbuf);
  33
  34 class PostingListBuilder {
  35 public:
  36         inline void add_docid(uint32_t docid);
  37         inline void add_first_docid(uint32_t docid);
  38         void finish();
  39
  40         vector<unsigned char> encoded;
  41         size_t get_num_docids() const
  42         {
  43                 // Updated only when we flush, so check that we're finished.
  44                 assert(pending_deltas.empty());
  45                 return num_docids;
  46         }
  47
  48 private:
  49         void write_header(uint32_t docid);
  50         void append_block();
  51
  52         vector<uint32_t> pending_deltas;
  53
  54         uint32_t num_docids = 0;  // Should be size_t, except the format only supports 2^32 docids per posting list anyway.
  55         uint32_t last_docid = -1;
  56 };
  57
  58 void PostingListBuilder::add_docid(uint32_t docid)
  59 {
  60         // Deduplicate against the last inserted value, if any.
  61         if (docid == last_docid) {
  62                 return;
  63         }
  64
  65         pending_deltas.push_back(docid - last_docid - 1);
  66         last_docid = docid;
  67         if (pending_deltas.size() == 128) {
  68                 append_block();
  69                 pending_deltas.clear();
  70                 num_docids += 128;
  71         }
  72 }
  73
  74 void PostingListBuilder::add_first_docid(uint32_t docid)
  75 {
  76         write_header(docid);
  77         ++num_docids;
  78         last_docid = docid;
  79 }
  80
  81 void PostingListBuilder::finish()
  82 {
  83         if (pending_deltas.empty()) {
  84                 return;
  85         }
  86
  87         assert(!encoded.empty());  // write_header() should already have run.
  88
  89         // No interleaving for partial blocks.
  90         unsigned char buf[P4NENC_BOUND(128)];
  91         unsigned char *end = encode_pfor_single_block<128>(pending_deltas.data(), pending_deltas.size(), /*interleaved=*/false, buf);
  92         encoded.insert(encoded.end(), buf, end);
  93
  94         num_docids += pending_deltas.size();
  95         pending_deltas.clear();
  96 }
  97
  98 void PostingListBuilder::append_block()
  99 {
 100         unsigned char buf[P4NENC_BOUND(128)];
 101         assert(pending_deltas.size() == 128);
 102         unsigned char *end = encode_pfor_single_block<128>(pending_deltas.data(), 128, /*interleaved=*/true, buf);
 103         encoded.insert(encoded.end(), buf, end);
 104 }
 105
 106 void PostingListBuilder::write_header(uint32_t docid)
 107 {
 108         unsigned char buf[P4NENC_BOUND(1)];
 109         unsigned char *end = write_baseval(docid, buf);
 110         encoded.insert(encoded.end(), buf, end);
 111 }
 112
 113 void DictionaryBuilder::add_file(string filename, dir_time)
 114 {
 115         if (keep_current_block) {  // Only bother saving the filenames if we're actually keeping the block.
 116                 if (!current_block.empty()) {
 117                         current_block.push_back('\0');
 118                 }
 119                 current_block += filename;
 120         }
 121         if (++num_files_in_block == block_size) {
 122                 flush_block();
 123         }
 124 }
 125
 126 void DictionaryBuilder::flush_block()
 127 {
 128         if (keep_current_block) {
 129                 if (slot_for_current_block == -1) {
 130                         lengths.push_back(current_block.size());
 131                         sampled_blocks.push_back(move(current_block));
 132                 } else {
 133                         lengths[slot_for_current_block] = current_block.size();
 134                         sampled_blocks[slot_for_current_block] = move(current_block);
 135                 }
 136         }
 137         current_block.clear();
 138         num_files_in_block = 0;
 139         ++block_num;
 140
 141         if (block_num < blocks_to_keep) {
 142                 keep_current_block = true;
 143                 slot_for_current_block = -1;
 144         } else {
 145                 // Keep every block with equal probability (reservoir sampling).
 146                 uint64_t idx = uniform_int_distribution<uint64_t>(0, block_num)(reservoir_rand);
 147                 keep_current_block = (idx < blocks_to_keep);
 148                 slot_for_current_block = idx;
 149         }
 150 }
 151
 152 string DictionaryBuilder::train(size_t buf_size)
 153 {
 154         string dictionary_buf;
 155         sort(sampled_blocks.begin(), sampled_blocks.end());  // Seemingly important for decompression speed.
 156         for (const string &block : sampled_blocks) {
 157                 dictionary_buf += block;
 158         }
 159
 160         string buf;
 161         buf.resize(buf_size);
 162         size_t ret = ZDICT_trainFromBuffer(&buf[0], buf_size, dictionary_buf.data(), lengths.data(), lengths.size());
 163         if (ZDICT_isError(ret)) {
 164                 return "";
 165         }
 166         dprintf("Sampled %zu bytes in %zu blocks, built a dictionary of size %zu\n", dictionary_buf.size(), lengths.size(), ret);
 167         buf.resize(ret);
 168
 169         sampled_blocks.clear();
 170         lengths.clear();
 171
 172         return buf;
 173 }
 174
 175 class EncodingCorpus : public DatabaseReceiver {
 176 public:
 177         EncodingCorpus(FILE *outfp, size_t block_size, ZSTD_CDict *cdict, bool store_dir_times);
 178         ~EncodingCorpus();
 179
 180         void add_file(std::string filename, dir_time dt) override;
 181         void flush_block() override;
 182         void finish() override;
 183
 184         std::vector<uint64_t> filename_blocks;
 185         size_t num_files = 0, num_files_in_block = 0, num_blocks = 0;
 186         bool seen_trigram(uint32_t trgm)
 187         {
 188                 return invindex[trgm] != nullptr;
 189         }
 190         size_t num_files_seen() const override { return num_files; }
 191         PostingListBuilder &get_pl_builder(uint32_t trgm)
 192         {
 193                 return *invindex[trgm];
 194         }
 195
 196         void add_docid(uint32_t trgm, uint32_t docid)
 197         {
 198                 if (invindex[trgm] == nullptr) {
 199                         invindex[trgm] = new PostingListBuilder;
 200                         invindex[trgm]->add_first_docid(docid);
 201                 } else {
 202                         invindex[trgm]->add_docid(docid);
 203                 }
 204         }
 205
 206         size_t num_trigrams() const;
 207         std::string get_compressed_dir_times();
 208
 209 private:
 210         void compress_dir_times(size_t allowed_slop);
 211
 212         std::unique_ptr<PostingListBuilder *[]> invindex;
 213         FILE *outfp;
 214         off_t outfp_pos;  // Cheaper than calling ftell(outfp) all the time.
 215         std::string current_block;
 216         std::string tempbuf;
 217         const size_t block_size;
 218         const bool store_dir_times;
 219         ZSTD_CDict *cdict;
 220
 221         ZSTD_CStream *dir_time_ctx = nullptr;
 222         std::string dir_times;  // Buffer of still-uncompressed data.
 223         std::string dir_times_compressed;
 224 };
 225
 226 EncodingCorpus::EncodingCorpus(FILE *outfp, size_t block_size, ZSTD_CDict *cdict, bool store_dir_times)
 227         : invindex(new PostingListBuilder *[NUM_TRIGRAMS]), outfp(outfp), outfp_pos(ftell(outfp)), block_size(block_size), store_dir_times(store_dir_times), cdict(cdict)
 228 {
 229         fill(invindex.get(), invindex.get() + NUM_TRIGRAMS, nullptr);
 230         if (store_dir_times) {
 231                 dir_time_ctx = ZSTD_createCStream();
 232                 ZSTD_initCStream(dir_time_ctx, /*level=*/6);
 233         }
 234 }
 235
 236 EncodingCorpus::~EncodingCorpus()
 237 {
 238         for (unsigned i = 0; i < NUM_TRIGRAMS; ++i) {
 239                 delete invindex[i];
 240         }
 241 }
 242
 243 void EncodingCorpus::add_file(string filename, dir_time dt)
 244 {
 245         ++num_files;
 246         if (!current_block.empty()) {
 247                 current_block.push_back('\0');
 248         }
 249         current_block += filename;
 250         if (++num_files_in_block == block_size) {
 251                 flush_block();
 252         }
 253
 254         if (store_dir_times) {
 255                 if (dt.sec == -1) {
 256                         // Not a directory.
 257                         dir_times.push_back('\0');
 258                 } else {
 259                         dir_times.push_back('\1');
 260                         dir_times.append(reinterpret_cast<char *>(&dt.sec), sizeof(dt.sec));
 261                         dir_times.append(reinterpret_cast<char *>(&dt.nsec), sizeof(dt.nsec));
 262                 }
 263                 compress_dir_times(/*allowed_slop=*/4096);
 264         }
 265 }
 266
 267 void EncodingCorpus::compress_dir_times(size_t allowed_slop)
 268 {
 269         while (dir_times.size() >= allowed_slop) {
 270                 size_t old_size = dir_times_compressed.size();
 271                 dir_times_compressed.resize(old_size + 4096);
 272
 273                 ZSTD_outBuffer outbuf;
 274                 outbuf.dst = dir_times_compressed.data() + old_size;
 275                 outbuf.size = 4096;
 276                 outbuf.pos = 0;
 277
 278                 ZSTD_inBuffer inbuf;
 279                 inbuf.src = dir_times.data();
 280                 inbuf.size = dir_times.size();
 281                 inbuf.pos = 0;
 282
 283                 int ret = ZSTD_compressStream(dir_time_ctx, &outbuf, &inbuf);
 284                 if (ret < 0) {
 285                         fprintf(stderr, "ZSTD_compressStream() failed\n");
 286                         exit(1);
 287                 }
 288
 289                 dir_times_compressed.resize(old_size + outbuf.pos);
 290                 dir_times.erase(dir_times.begin(), dir_times.begin() + inbuf.pos);
 291
 292                 if (outbuf.pos == 0 && inbuf.pos == 0) {
 293                         // Nothing happened (not enough data?), try again later.
 294                         return;
 295                 }
 296         }
 297 }
 298
 299 void EncodingCorpus::flush_block()
 300 {
 301         if (current_block.empty()) {
 302                 return;
 303         }
 304
 305         uint32_t docid = num_blocks;
 306
 307         // Oh, ICU...
 308         vector<uint8_t> sort_key;
 309         sort_key.resize(32);
 310         int32_t num_locales;
 311         const icu::Locale* locales = icu::Collator::getAvailableLocales(num_locales);
 312         for (int i = 0; i < num_locales; ++i) {
 313                 const icu::Locale &loc = locales[i];
 314                 if (strcmp(loc.getName(), "en_US_POSIX") == 0) {
 315                         continue;  // Too weird.
 316                 }
 317                 UErrorCode status = U_ZERO_ERROR;
 318                 icu::Collator *coll = icu::Collator::createInstance(loc, status);
 319                 if (U_FAILURE(status)) {
 320                         fprintf(stderr, "ERROR: Failed to create collator\n");
 321                         exit(1);
 322                 }
 323                 coll->setStrength(icu::Collator::PRIMARY);
 324                 const char *ptr = current_block.c_str();
 325                 const char *end = ptr + current_block.size();
 326                 while (ptr < end) {
 327                         size_t len = strlen(ptr);
 328                         int32_t sortkey_len;
 329                         for ( ;; ) {
 330                                 sortkey_len = coll->getSortKey(icu::UnicodeString::fromUTF8(icu::StringPiece(ptr, len)), sort_key.data(), sort_key.size());
 331                                 if (sortkey_len < sort_key.size()) {  // Note <, not <=; we need to keep a slop byte.
 332                                         break;
 333                                 }
 334                                 sort_key.resize(sortkey_len * 3 / 2);
 335                         }
 336
 337                         const uint8_t *keyptr = &sort_key[0];
 338                         const uint8_t *keyend = keyptr + sortkey_len;
 339                         while (keyptr < keyend - 3) {
 340                                 // NOTE: Will read one byte past the end of the trigram, but it's OK,
 341                                 // since we always call it from contexts where there's a terminating zero byte.
 342                                 uint32_t trgm;
 343                                 memcpy(&trgm, keyptr, sizeof(trgm));
 344                                 ++keyptr;
 345                                 trgm = le32toh(trgm);
 346                                 add_docid(trgm & 0xffffff, docid);
 347                         }
 348
 349                         ptr += len + 1;
 350                 }
 351         }
 352 #if 0
 353         // Create trigrams.
 354         const char *ptr = current_block.c_str();
 355         const char *end = ptr + current_block.size();
 356         while (ptr < end - 3) {  // Must be at least one filename left, that's at least three bytes.
 357                 if (ptr[0] == '\0') {
 358                         // This filename is zero bytes, so skip it (and the zero terminator).
 359                         ++ptr;
 360                         continue;
 361                 } else if (ptr[1] == '\0') {
 362                         // This filename is one byte, so skip it (and the zero terminator).
 363                         ptr += 2;
 364                         continue;
 365                 } else if (ptr[2] == '\0') {
 366                         // This filename is two bytes, so skip it (and the zero terminator).
 367                         ptr += 3;
 368                         continue;
 369                 }
 370                 for (;;) {
 371                         // NOTE: Will read one byte past the end of the trigram, but it's OK,
 372                         // since we always call it from contexts where there's a terminating zero byte.
 373                         uint32_t trgm;
 374                         memcpy(&trgm, ptr, sizeof(trgm));
 375                         ++ptr;
 376                         trgm = le32toh(trgm);
 377                         add_docid(trgm & 0xffffff, docid);
 378                         if (trgm <= 0xffffff) {
 379                                 // Terminating zero byte, so we're done with this filename.
 380                                 // Skip the remaining two bytes, and the zero terminator.
 381                                 ptr += 3;
 382                                 break;
 383                         }
 384                 }
 385         }
 386 #endif
 387
 388         // Compress and add the filename block.
 389         filename_blocks.push_back(outfp_pos);
 390         string compressed = zstd_compress(current_block, cdict, &tempbuf);
 391         if (fwrite(compressed.data(), compressed.size(), 1, outfp) != 1) {
 392                 perror("fwrite()");
 393                 exit(1);
 394         }
 395         outfp_pos += compressed.size();
 396
 397         current_block.clear();
 398         num_files_in_block = 0;
 399         ++num_blocks;
 400 }
 401
 402 void EncodingCorpus::finish()
 403 {
 404         flush_block();
 405 }
 406
 407 size_t EncodingCorpus::num_trigrams() const
 408 {
 409         size_t num = 0;
 410         for (unsigned trgm = 0; trgm < NUM_TRIGRAMS; ++trgm) {
 411                 if (invindex[trgm] != nullptr) {
 412                         ++num;
 413                 }
 414         }
 415         return num;
 416 }
 417
 418 string EncodingCorpus::get_compressed_dir_times()
 419 {
 420         if (!store_dir_times) {
 421                 return "";
 422         }
 423         compress_dir_times(/*allowed_slop=*/0);
 424         assert(dir_times.empty());
 425
 426         for (;;) {
 427                 size_t old_size = dir_times_compressed.size();
 428                 dir_times_compressed.resize(old_size + 4096);
 429
 430                 ZSTD_outBuffer outbuf;
 431                 outbuf.dst = dir_times_compressed.data() + old_size;
 432                 outbuf.size = 4096;
 433                 outbuf.pos = 0;
 434
 435                 int ret = ZSTD_endStream(dir_time_ctx, &outbuf);
 436                 if (ret < 0) {
 437                         fprintf(stderr, "ZSTD_compressStream() failed\n");
 438                         exit(1);
 439                 }
 440
 441                 dir_times_compressed.resize(old_size + outbuf.pos);
 442
 443                 if (ret == 0) {
 444                         // All done.
 445                         break;
 446                 }
 447         }
 448
 449         return dir_times_compressed;
 450 }
 451
 452 string zstd_compress(const string &src, ZSTD_CDict *cdict, string *tempbuf)
 453 {
 454         static ZSTD_CCtx *ctx = nullptr;
 455         if (ctx == nullptr) {
 456                 ctx = ZSTD_createCCtx();
 457         }
 458
 459         size_t max_size = ZSTD_compressBound(src.size());
 460         if (tempbuf->size() < max_size) {
 461                 tempbuf->resize(max_size);
 462         }
 463         size_t size;
 464         if (cdict == nullptr) {
 465                 size = ZSTD_compressCCtx(ctx, &(*tempbuf)[0], max_size, src.data(), src.size(), /*level=*/6);
 466         } else {
 467                 size = ZSTD_compress_usingCDict(ctx, &(*tempbuf)[0], max_size, src.data(), src.size(), cdict);
 468         }
 469         return string(tempbuf->data(), size);
 470 }
 471
 472 bool is_prime(uint32_t x)
 473 {
 474         if ((x % 2) == 0 || (x % 3) == 0) {
 475                 return false;
 476         }
 477         uint32_t limit = ceil(sqrt(x));
 478         for (uint32_t factor = 5; factor <= limit; ++factor) {
 479                 if ((x % factor) == 0) {
 480                         return false;
 481                 }
 482         }
 483         return true;
 484 }
 485
 486 uint32_t next_prime(uint32_t x)
 487 {
 488         if ((x % 2) == 0) {
 489                 ++x;
 490         }
 491         while (!is_prime(x)) {
 492                 x += 2;
 493         }
 494         return x;
 495 }
 496
 497 unique_ptr<Trigram[]> create_hashtable(EncodingCorpus &corpus, const vector<uint32_t> &all_trigrams, uint32_t ht_size, uint32_t num_overflow_slots)
 498 {
 499         unique_ptr<Trigram[]> ht(new Trigram[ht_size + num_overflow_slots + 1]);  // 1 for the sentinel element at the end.
 500         for (unsigned i = 0; i < ht_size + num_overflow_slots + 1; ++i) {
 501                 ht[i].trgm = uint32_t(-1);
 502                 ht[i].num_docids = 0;
 503                 ht[i].offset = 0;
 504         }
 505         for (uint32_t trgm : all_trigrams) {
 506                 // We don't know offset yet, so set it to zero.
 507                 Trigram to_insert{ trgm, uint32_t(corpus.get_pl_builder(trgm).get_num_docids()), 0 };
 508
 509                 uint32_t bucket = hash_trigram(trgm, ht_size);
 510                 unsigned distance = 0;
 511                 while (ht[bucket].num_docids != 0) {
 512                         // Robin Hood hashing; reduces the longest distance by a lot.
 513                         unsigned other_distance = bucket - hash_trigram(ht[bucket].trgm, ht_size);
 514                         if (distance > other_distance) {
 515                                 swap(to_insert, ht[bucket]);
 516                                 distance = other_distance;
 517                         }
 518
 519                         ++bucket, ++distance;
 520                         if (distance > num_overflow_slots) {
 521                                 return nullptr;
 522                         }
 523                 }
 524                 ht[bucket] = to_insert;
 525         }
 526         return ht;
 527 }
 528
 529 DatabaseBuilder::DatabaseBuilder(const char *outfile, gid_t owner, int block_size, string dictionary, bool check_visibility)
 530         : outfile(outfile), block_size(block_size)
 531 {
 532         umask(0027);
 533
 534         string path = outfile;
 535         path.resize(path.find_last_of('/') + 1);
 536         if (path.empty()) {
 537                 path = ".";
 538         }
 539         int fd = -1;
 540 #ifdef O_TMPFILE
 541         fd = open(path.c_str(), O_WRONLY | O_TMPFILE, 0640);
 542         if (fd == -1 && errno != EOPNOTSUPP && errno != EISDIR) {
 543                 perror(path.c_str());
 544                 exit(1);
 545         }
 546 #endif
 547         if (fd == -1) {
 548                 temp_filename = string(outfile) + ".XXXXXX";
 549                 fd = mkstemp(&temp_filename[0]);
 550                 if (fd == -1) {
 551                         perror(temp_filename.c_str());
 552                         exit(1);
 553                 }
 554                 if (fchmod(fd, 0640) == -1) {
 555                         perror("fchmod");
 556                         exit(1);
 557                 }
 558         }
 559
 560         if (owner != (gid_t)-1) {
 561                 if (fchown(fd, (uid_t)-1, owner) == -1) {
 562                         perror("fchown");
 563                         exit(1);
 564                 }
 565         }
 566
 567         outfp = fdopen(fd, "wb");
 568         if (outfp == nullptr) {
 569                 perror(outfile);
 570                 exit(1);
 571         }
 572
 573         // Write the header.
 574         memcpy(hdr.magic, "\0plocate", 8);
 575         hdr.version = -1;  // Mark as broken.
 576         hdr.hashtable_size = 0;  // Not known yet.
 577         hdr.extra_ht_slots = num_overflow_slots;
 578         hdr.num_docids = 0;
 579         hdr.hash_table_offset_bytes = -1;  // We don't know these offsets yet.
 580         hdr.max_version = 2;
 581         hdr.filename_index_offset_bytes = -1;
 582         hdr.zstd_dictionary_length_bytes = -1;
 583         hdr.check_visibility = check_visibility;
 584         fwrite(&hdr, sizeof(hdr), 1, outfp);
 585
 586         if (dictionary.empty()) {
 587                 hdr.zstd_dictionary_offset_bytes = 0;
 588                 hdr.zstd_dictionary_length_bytes = 0;
 589         } else {
 590                 hdr.zstd_dictionary_offset_bytes = ftell(outfp);
 591                 fwrite(dictionary.data(), dictionary.size(), 1, outfp);
 592                 hdr.zstd_dictionary_length_bytes = dictionary.size();
 593                 cdict = ZSTD_createCDict(dictionary.data(), dictionary.size(), /*level=*/6);
 594         }
 595
 596         hdr.directory_data_length_bytes = 0;
 597         hdr.directory_data_offset_bytes = 0;
 598         hdr.next_zstd_dictionary_length_bytes = 0;
 599         hdr.next_zstd_dictionary_offset_bytes = 0;
 600         hdr.conf_block_length_bytes = 0;
 601         hdr.conf_block_offset_bytes = 0;
 602 }
 603
 604 DatabaseReceiver *DatabaseBuilder::start_corpus(bool store_dir_times)
 605 {
 606         corpus_start = steady_clock::now();
 607         corpus = new EncodingCorpus(outfp, block_size, cdict, store_dir_times);
 608         return corpus;
 609 }
 610
 611 void DatabaseBuilder::set_next_dictionary(std::string next_dictionary)
 612 {
 613         this->next_dictionary = move(next_dictionary);
 614 }
 615
 616 void DatabaseBuilder::set_conf_block(std::string conf_block)
 617 {
 618         this->conf_block = move(conf_block);
 619 }
 620
 621 void DatabaseBuilder::finish_corpus()
 622 {
 623         corpus->finish();
 624         hdr.num_docids = corpus->filename_blocks.size();
 625
 626         // Stick an empty block at the end as sentinel.
 627         corpus->filename_blocks.push_back(ftell(outfp));
 628         const size_t bytes_for_filenames = corpus->filename_blocks.back() - corpus->filename_blocks.front();
 629
 630         // Write the offsets to the filenames.
 631         hdr.filename_index_offset_bytes = ftell(outfp);
 632         const size_t bytes_for_filename_index = corpus->filename_blocks.size() * sizeof(uint64_t);
 633         fwrite(corpus->filename_blocks.data(), corpus->filename_blocks.size(), sizeof(uint64_t), outfp);
 634         corpus->filename_blocks.clear();
 635         corpus->filename_blocks.shrink_to_fit();
 636
 637         // Finish up encoding the posting lists.
 638         size_t trigrams = 0, longest_posting_list = 0;
 639         size_t bytes_for_posting_lists = 0;
 640         for (unsigned trgm = 0; trgm < NUM_TRIGRAMS; ++trgm) {
 641                 if (!corpus->seen_trigram(trgm))
 642                         continue;
 643                 PostingListBuilder &pl_builder = corpus->get_pl_builder(trgm);
 644                 pl_builder.finish();
 645                 longest_posting_list = max(longest_posting_list, pl_builder.get_num_docids());
 646                 trigrams += pl_builder.get_num_docids();
 647                 bytes_for_posting_lists += pl_builder.encoded.size();
 648         }
 649         size_t num_trigrams = corpus->num_trigrams();
 650         dprintf("%zu files, %zu different trigrams, %zu entries, avg len %.2f, longest %zu\n",
 651                 corpus->num_files, num_trigrams, trigrams, double(trigrams) / num_trigrams, longest_posting_list);
 652         dprintf("%zu bytes used for posting lists (%.2f bits/entry)\n", bytes_for_posting_lists, 8 * bytes_for_posting_lists / double(trigrams));
 653
 654         dprintf("Building posting lists took %.1f ms.\n\n", 1e3 * duration<float>(steady_clock::now() - corpus_start).count());
 655
 656         // Find the used trigrams.
 657         vector<uint32_t> all_trigrams;
 658         for (unsigned trgm = 0; trgm < NUM_TRIGRAMS; ++trgm) {
 659                 if (corpus->seen_trigram(trgm)) {
 660                         all_trigrams.push_back(trgm);
 661                 }
 662         }
 663
 664         // Create the hash table.
 665         unique_ptr<Trigram[]> hashtable;
 666         uint32_t ht_size = next_prime(all_trigrams.size());
 667         for (;;) {
 668                 hashtable = create_hashtable(*corpus, all_trigrams, ht_size, num_overflow_slots);
 669                 if (hashtable == nullptr) {
 670                         dprintf("Failed creating hash table of size %u, increasing by 5%% and trying again.\n", ht_size);
 671                         ht_size = next_prime(ht_size * 1.05);
 672                 } else {
 673                         dprintf("Created hash table of size %u.\n\n", ht_size);
 674                         break;
 675                 }
 676         }
 677
 678         // Find the offsets for each posting list.
 679         size_t bytes_for_hashtable = (ht_size + num_overflow_slots + 1) * sizeof(Trigram);
 680         uint64_t offset = ftell(outfp) + bytes_for_hashtable;
 681         for (unsigned i = 0; i < ht_size + num_overflow_slots + 1; ++i) {
 682                 hashtable[i].offset = offset;  // Needs to be there even for empty slots.
 683                 if (hashtable[i].num_docids == 0) {
 684                         continue;
 685                 }
 686
 687                 const vector<unsigned char> &encoded = corpus->get_pl_builder(hashtable[i].trgm).encoded;
 688                 offset += encoded.size();
 689         }
 690
 691         // Write the hash table.
 692         hdr.hash_table_offset_bytes = ftell(outfp);
 693         hdr.hashtable_size = ht_size;
 694         fwrite(hashtable.get(), ht_size + num_overflow_slots + 1, sizeof(Trigram), outfp);
 695
 696         // Write the actual posting lists.
 697         for (unsigned i = 0; i < ht_size + num_overflow_slots + 1; ++i) {
 698                 if (hashtable[i].num_docids == 0) {
 699                         continue;
 700                 }
 701                 const vector<unsigned char> &encoded = corpus->get_pl_builder(hashtable[i].trgm).encoded;
 702                 fwrite(encoded.data(), encoded.size(), 1, outfp);
 703         }
 704
 705         // Finally, write the directory times (for updatedb).
 706         string compressed_dir_times = corpus->get_compressed_dir_times();
 707         size_t bytes_for_compressed_dir_times = 0;
 708         if (!compressed_dir_times.empty()) {
 709                 hdr.directory_data_offset_bytes = ftell(outfp);
 710                 hdr.directory_data_length_bytes = compressed_dir_times.size();
 711                 fwrite(compressed_dir_times.data(), compressed_dir_times.size(), 1, outfp);
 712                 bytes_for_compressed_dir_times = compressed_dir_times.size();
 713                 compressed_dir_times.clear();
 714         }
 715
 716         // Write the recommended dictionary for next update.
 717         if (!next_dictionary.empty()) {
 718                 hdr.next_zstd_dictionary_offset_bytes = ftell(outfp);
 719                 hdr.next_zstd_dictionary_length_bytes = next_dictionary.size();
 720                 fwrite(next_dictionary.data(), next_dictionary.size(), 1, outfp);
 721         }
 722
 723         // And the configuration block.
 724         if (!conf_block.empty()) {
 725                 hdr.conf_block_offset_bytes = ftell(outfp);
 726                 hdr.conf_block_length_bytes = conf_block.size();
 727                 fwrite(conf_block.data(), conf_block.size(), 1, outfp);
 728         }
 729
 730         // Rewind, and write the updated header.
 731         hdr.version = 1;
 732         fseek(outfp, 0, SEEK_SET);
 733         fwrite(&hdr, sizeof(hdr), 1, outfp);
 734
 735         if (!temp_filename.empty()) {
 736                 if (rename(temp_filename.c_str(), outfile.c_str()) == -1) {
 737                         perror("rename");
 738                         exit(1);
 739                 }
 740         } else {
 741 #ifdef O_TMPFILE
 742                 // Give the file a proper name, making it visible in the file system.
 743                 // TODO: It would be nice to be able to do this atomically, like with rename.
 744                 unlink(outfile.c_str());
 745                 char procpath[256];
 746                 snprintf(procpath, sizeof(procpath), "/proc/self/fd/%d", fileno(outfp));
 747                 if (linkat(AT_FDCWD, procpath, AT_FDCWD, outfile.c_str(), AT_SYMLINK_FOLLOW) == -1) {
 748                         perror("linkat");
 749                         exit(1);
 750                 }
 751 #endif
 752         }
 753
 754         fclose(outfp);
 755
 756         size_t total_bytes = (bytes_for_hashtable + bytes_for_posting_lists + bytes_for_filename_index + bytes_for_filenames + bytes_for_compressed_dir_times);
 757
 758         dprintf("Block size:     %7d files\n", block_size);
 759         dprintf("Dictionary:     %'7.1f MB\n", hdr.zstd_dictionary_length_bytes / 1048576.0);
 760         dprintf("Hash table:     %'7.1f MB\n", bytes_for_hashtable / 1048576.0);
 761         dprintf("Posting lists:  %'7.1f MB\n", bytes_for_posting_lists / 1048576.0);
 762         dprintf("Filename index: %'7.1f MB\n", bytes_for_filename_index / 1048576.0);
 763         dprintf("Filenames:      %'7.1f MB\n", bytes_for_filenames / 1048576.0);
 764         if (bytes_for_compressed_dir_times != 0) {
 765                 dprintf("Modify times:   %'7.1f MB\n", bytes_for_compressed_dir_times / 1048576.0);
 766         }
 767         dprintf("Total:          %'7.1f MB\n", total_bytes / 1048576.0);
 768         dprintf("\n");
 769 }