hnswlib.v0.8.0: `checkIntegrity` fail on `tests/cpp/sift_1b.cpp`

[lyj-fdu]

My branch is at v0.8.0. When building hnsw index in tests/cpp/sift_1b.cpp, I add appr_alg->checkIntegrity(); on line 346 to see if the graph built by the algorithm has in-edge for every node during report_every. However, the assertion fail!

the code fragment I modified is as follows

                if (j1 % report_every == 0) {
                    cout << j1 / (0.01 * vecsize) << " %, "
                         << report_every / (1000.0 * 1e-6 * stopw.getElapsedTimeMicro()) << " kips " << " Mem: "
                         << getCurrentRSS() / 1000000 << " Mb \n";
                    stopw.reset();
                    appr_alg->checkIntegrity(); // add this at line 346
                }

the output of command line like this

~/code/hnswlib/tests/cpp/downloads$ ../../../build/main 
Loading GT:
Loading queries:
Building index:
0.05 %, 40.9574 kips  Mem: 40877 Mb 
main: ~/code/hnswlib/tests/cpp/../../hnswlib/hnswalg.h:1403: void hnswlib::HierarchicalNSW<dist_t>::checkIntegrity() [with dist_t = int]: Assertion `inbound_connections_num[i] > 0' failed.
Aborted (core dumped)

If some node has no in-edge, then it's totally isolated, which means that search on the graph will never visisted it, even it has out-edge.