Fix IndexIVFFastScan reconstruct_from_offset method (facebookresearch#4095)

Summary:
Resolves issue facebookresearch#4089 - IndexIVFPQFastScan crashes with certain nlist values

The `reconstruct_from_offset` method in `IndexIVFFastScan` was incorrectly reconstructing vectors, causing crashes when the `nlist` parameter was not byte-aligned (e.g. 100 instead of 256).

The root cause was that the `list_no` (Voronoi cell number) was not being properly encoded into the `code` vector before passing it to the `sa_decode` function. This resulted in invalid `list_no` values being read in `sa_decode`, triggering the assertion failure `'list_no >= 0 && list_no < nlist'` when `nlist` in some cases.

This PR fixes the issue with the following changes to `reconstruct_from_offset`:

1. Encode the `list_no` into the beginning of the `code` vector using the existing `encode_listno` method
2. Start the `BitstringWriter` after the coarse code portion of `code` (shifted by `coarse_code_size()` bytes)
3. Remove the residual centroid addition logic, as it is already handled in `sa_decode`

After these changes:
- Crashes no longer occur for any `nlist` value
- Reconstruction is now correct, matching the output of `IndexIVFPQ`

Fixes facebookresearch#4089

Please review and let me know if any changes are needed. Thanks!

Pull Request resolved: facebookresearch#4095

Reviewed By: asadoughi

Differential Revision: D67937160

Pulled By: mdouze

fbshipit-source-id: 4705106ba49c01c788b3c75c39c2260615f45764

Author: alisafaya

faiss/IndexIVFFastScan.cpp

@@ -1353,34 +1353,30 @@ void IndexIVFFastScan::reconstruct_from_offset(
         int64_t offset,
         float* recons) const {
     // unpack codes
+    size_t coarse_size = coarse_code_size();
+    std::vector<uint8_t> code(coarse_size + code_size, 0);
+    encode_listno(list_no, code.data());
     InvertedLists::ScopedCodes list_codes(invlists, list_no);
-    std::vector<uint8_t> code(code_size, 0);
-    BitstringWriter bsw(code.data(), code_size);
+    BitstringWriter bsw(code.data() + coarse_size, code_size);
+
     for (size_t m = 0; m < M; m++) {
         uint8_t c =
                 pq4_get_packed_element(list_codes.get(), bbs, M2, offset, m);
         bsw.write(c, nbits);
     }
-    sa_decode(1, code.data(), recons);
 
-    // add centroid to it
-    if (by_residual) {
-        std::vector<float> centroid(d);
-        quantizer->reconstruct(list_no, centroid.data());
-        for (int i = 0; i < d; ++i) {
-            recons[i] += centroid[i];
-        }
-    }
+    sa_decode(1, code.data(), recons);
 }
 
 void IndexIVFFastScan::reconstruct_orig_invlists() {
     FAISS_THROW_IF_NOT(orig_invlists != nullptr);
     FAISS_THROW_IF_NOT(orig_invlists->list_size(0) == 0);
 
+#pragma omp parallel for if (nlist > 100)
     for (size_t list_no = 0; list_no < nlist; list_no++) {
         InvertedLists::ScopedCodes codes(invlists, list_no);
         InvertedLists::ScopedIds ids(invlists, list_no);
-        size_t list_size = orig_invlists->list_size(list_no);
+        size_t list_size = invlists->list_size(list_no);
         std::vector<uint8_t> code(code_size, 0);
 
         for (size_t offset = 0; offset < list_size; offset++) {

faiss/IndexIVFPQFastScan.cpp

@@ -76,6 +76,7 @@ IndexIVFPQFastScan::IndexIVFPQFastScan(const IndexIVFPQ& orig, int bbs)
                precomputed_table.nbytes());
     }
 
+#pragma omp parallel for if (nlist > 100)
     for (size_t i = 0; i < nlist; i++) {
         size_t nb = orig.invlists->list_size(i);
         size_t nb2 = roundup(nb, bbs);

tests/test_fast_scan_ivf.py

@@ -543,6 +543,37 @@ def test_by_residual_odd_dim(self):
         self.do_test(by_residual=True, d=30)
 
 
+class TestReconstruct(unittest.TestCase):
+
+    def do_test(self, by_residual=False):
+        d = 32
+        metric = faiss.METRIC_L2
+
+        ds = datasets.SyntheticDataset(d, 2000, 5000, 200)
+
+        index = faiss.IndexIVFPQFastScan(faiss.IndexFlatL2(d), d, 50, d // 2, 4, metric)
+        index.by_residual = by_residual
+        index.make_direct_map(True)
+        index.train(ds.get_train())
+        index.add(ds.get_database())
+
+        # Test reconstruction
+        index.reconstruct(123) # single id
+        index.reconstruct_n(123, 10) # single id
+        index.reconstruct_batch(np.arange(10))
+
+        # Test original list reconstruction
+        index.orig_invlists = faiss.ArrayInvertedLists(index.nlist, index.code_size)
+        index.reconstruct_orig_invlists()
+        assert index.orig_invlists.compute_ntotal() == index.ntotal
+
+    def test_no_residual(self):
+        self.do_test(by_residual=False)
+
+    def test_by_residual(self):
+        self.do_test(by_residual=True)
+
+
 class TestIsTrained(unittest.TestCase):
 
     def test_issue_2019(self):