Make the builder write out filenames as they get compressed.

This saves yet more RAM in the builder; roughly halving the peak
usage, in fact. The layout of the file changes somewhat
(the filenames come first instead of last), but it shouldn't
matter for performance.

diff --git a/plocate-build.cpp b/plocate-build.cpp
index 37a3271c35efd2843c86a5d54d02246766524803..bde72b51fa3d6da02b59216e34d8dbfe695cca80 100644 (file)
--- a/plocate-build.cpp
+++ b/plocate-build.cpp
@@ -29,6 +29,8 @@ using namespace std::chrono;
 
 string zstd_compress(const string &src, string *tempbuf);
 
+constexpr unsigned num_overflow_slots = 16;
+
 static inline uint32_t read_unigram(const string_view s, size_t idx)
 {
        if (idx < s.size()) {
@@ -143,16 +145,17 @@ void PostingListBuilder::write_header(uint32_t docid)
 
 class Corpus {
 public:
-       Corpus(size_t block_size)
-               : block_size(block_size) {}
+       Corpus(FILE *outfp, size_t block_size)
+               : outfp(outfp), block_size(block_size) {}
        void add_file(string filename);
        void flush_block();
 
-       vector<string> filename_blocks;
+       vector<uint64_t> filename_blocks;
        unordered_map<uint32_t, PostingListBuilder> invindex;
        size_t num_files = 0, num_files_in_block = 0, num_blocks = 0;
 
 private:
+       FILE *outfp;
        string current_block;
        string tempbuf;
        const size_t block_size;
@@ -192,7 +195,12 @@ void Corpus::flush_block()
        }
 
        // Compress and add the filename block.
-       filename_blocks.push_back(zstd_compress(current_block, &tempbuf));
+       filename_blocks.push_back(ftell(outfp));
+       string compressed = zstd_compress(current_block, &tempbuf);
+       if (fwrite(compressed.data(), compressed.size(), 1, outfp) != 1) {
+               perror("fwrite()");
+               exit(1);
+       }
 
        current_block.clear();
        num_files_in_block = 0;
@@ -329,7 +337,20 @@ void do_build(const char *infile, const char *outfile, int block_size)
 {
        steady_clock::time_point start __attribute__((unused)) = steady_clock::now();
 
-       Corpus corpus(block_size);
+       umask(0027);
+       FILE *outfp = fopen(outfile, "wb");
+
+       // Write the header.
+       Header hdr;
+       memcpy(hdr.magic, "\0plocate", 8);
+       hdr.version = -1;  // Mark as broken.
+       hdr.hashtable_size = 0;  // Not known yet.
+       hdr.extra_ht_slots = num_overflow_slots;
+       hdr.hash_table_offset_bytes = -1;  // We don't know these offsets yet.
+       hdr.filename_index_offset_bytes = -1;
+       fwrite(&hdr, sizeof(hdr), 1, outfp);
+
+       Corpus corpus(outfp, block_size);
 
        read_mlocate(infile, &corpus);
        if (false) {  // To read a plain text file.
@@ -349,17 +370,29 @@ void do_build(const char *infile, const char *outfile, int block_size)
        corpus.flush_block();
        dprintf("Read %zu files from %s\n", corpus.num_files, infile);
 
+       // Stick an empty block at the end as sentinel.
+       corpus.filename_blocks.push_back(ftell(outfp));
+       const size_t bytes_for_filenames = corpus.filename_blocks.back() - corpus.filename_blocks.front();
+
+       // Write the offsets to the filenames.
+       hdr.filename_index_offset_bytes = ftell(outfp);
+       const size_t bytes_for_filename_index = corpus.filename_blocks.size() * sizeof(uint64_t);
+       fwrite(corpus.filename_blocks.data(), corpus.filename_blocks.size(), sizeof(uint64_t), outfp);
+       corpus.filename_blocks.clear();
+       corpus.filename_blocks.shrink_to_fit();
+
+       // Finish up encoding the posting lists.
        size_t trigrams = 0, longest_posting_list = 0;
-       size_t bytes_used = 0;
+       size_t bytes_for_posting_lists = 0;
        for (auto &[trigram, pl_builder] : corpus.invindex) {
                pl_builder.finish();
                longest_posting_list = max(longest_posting_list, pl_builder.num_docids);
                trigrams += pl_builder.num_docids;
-               bytes_used += pl_builder.encoded.size();
+               bytes_for_posting_lists += pl_builder.encoded.size();
        }
        dprintf("%zu files, %zu different trigrams, %zu entries, avg len %.2f, longest %zu\n",
                corpus.num_files, corpus.invindex.size(), trigrams, double(trigrams) / corpus.invindex.size(), longest_posting_list);
-       dprintf("%zu bytes used for posting lists (%.2f bits/entry)\n", bytes_used, 8 * bytes_used / double(trigrams));
+       dprintf("%zu bytes used for posting lists (%.2f bits/entry)\n", bytes_for_posting_lists, 8 * bytes_for_posting_lists / double(trigrams));
 
        dprintf("Building posting lists took %.1f ms.\n\n", 1e3 * duration<float>(steady_clock::now() - start).count());
 
@@ -371,9 +404,9 @@ void do_build(const char *infile, const char *outfile, int block_size)
        }
        sort(all_trigrams.begin(), all_trigrams.end());
 
+       // Create the hash table.
        unique_ptr<Trigram[]> hashtable;
        uint32_t ht_size = next_prime(all_trigrams.size());
-       constexpr unsigned num_overflow_slots = 16;
        for (;;) {
                hashtable = create_hashtable(corpus, all_trigrams, ht_size, num_overflow_slots);
                if (hashtable == nullptr) {
@@ -385,64 +418,37 @@ void do_build(const char *infile, const char *outfile, int block_size)
                }
        }
 
-       umask(0027);
-       FILE *outfp = fopen(outfile, "wb");
-
        // Find the offsets for each posting list.
        size_t bytes_for_hashtable = (ht_size + num_overflow_slots + 1) * sizeof(Trigram);
-       uint64_t offset = sizeof(Header) + bytes_for_hashtable;
+       uint64_t offset = ftell(outfp) + bytes_for_hashtable;
        for (unsigned i = 0; i < ht_size + num_overflow_slots + 1; ++i) {
                hashtable[i].offset = offset;  // Needs to be there even for empty slots.
                if (hashtable[i].num_docids == 0) {
                        continue;
                }
 
-               const PostingListBuilder &pl_builder = corpus.invindex[hashtable[i].trgm];
-               offset += pl_builder.encoded.size();
+               const string &encoded = corpus.invindex[hashtable[i].trgm].encoded;
+               offset += encoded.size();
        }
 
-       // Write the header.
-       Header hdr;
-       memcpy(hdr.magic, "\0plocate", 8);
-       hdr.version = 0;
-       hdr.hashtable_size = ht_size;
-       hdr.extra_ht_slots = num_overflow_slots;
-       hdr.hash_table_offset_bytes = sizeof(hdr);  // This member is just there for flexibility.
-       hdr.filename_index_offset_bytes = offset;
-       fwrite(&hdr, sizeof(hdr), 1, outfp);
-
        // Write the hash table.
+       hdr.hash_table_offset_bytes = ftell(outfp);
+       hdr.hashtable_size = ht_size;
        fwrite(hashtable.get(), ht_size + num_overflow_slots + 1, sizeof(Trigram), outfp);
 
        // Write the actual posting lists.
-       size_t bytes_for_posting_lists = 0;
        for (unsigned i = 0; i < ht_size + num_overflow_slots + 1; ++i) {
                if (hashtable[i].num_docids == 0) {
                        continue;
                }
                const string &encoded = corpus.invindex[hashtable[i].trgm].encoded;
                fwrite(encoded.data(), encoded.size(), 1, outfp);
-               bytes_for_posting_lists += encoded.size();
-       }
-
-       // Stick an empty block at the end as sentinel.
-       corpus.filename_blocks.push_back("");
-
-       // Write the offsets to the filenames.
-       size_t bytes_for_filename_index = 0, bytes_for_filenames = 0;
-       offset = hdr.filename_index_offset_bytes + corpus.filename_blocks.size() * sizeof(offset);
-       for (const string &filename : corpus.filename_blocks) {
-               fwrite(&offset, sizeof(offset), 1, outfp);
-               offset += filename.size();
-               bytes_for_filename_index += sizeof(offset);
-               bytes_for_filenames += filename.size();
-       }
-
-       // Write the actual filenames.
-       for (const string &filename : corpus.filename_blocks) {
-               fwrite(filename.data(), filename.size(), 1, outfp);
        }
 
+       // Rewind, and write the updated header.
+       hdr.version = 0;
+       fseek(outfp, 0, SEEK_SET);
+       fwrite(&hdr, sizeof(hdr), 1, outfp);
        fclose(outfp);
 
        size_t total_bytes __attribute__((unused)) = (bytes_for_hashtable + bytes_for_posting_lists + bytes_for_filename_index + bytes_for_filenames);