Fix reconstruct bug when by_residual is false (#2298)

Summary:
When I reconstruct with by_residual turned off, the distance was greatly increased.
This is because the reconstruct_from_offset function did not check if the by_residual option was off.
I fix this bug with simple if statement.
(like this https://github.com/facebookresearch/faiss/blob/main/faiss/IndexIVFPQ.cpp#L365)

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/2298

Reviewed By: alexanderguzhva

Differential Revision: D35746566

Pulled By: mdouze

fbshipit-source-id: 50f98c7cc97c7936507573fe41b65a79ecdbc4ca

faiss/IndexScalarQuantizer.cpp

@@ -251,13 +251,18 @@ void IndexIVFScalarQuantizer::reconstruct_from_offset(
         int64_t list_no,
         int64_t offset,
         float* recons) const {
-    std::vector<float> centroid(d);
-    quantizer->reconstruct(list_no, centroid.data());
-
     const uint8_t* code = invlists->get_single_code(list_no, offset);
-    sq.decode(code, recons, 1);
+
-    for (int i = 0; i < d; ++i) {
+    if (by_residual) {
-        recons[i] += centroid[i];
+        std::vector<float> centroid(d);
+        quantizer->reconstruct(list_no, centroid.data());
+
+        sq.decode(code, recons, 1);
+        for (int i = 0; i < d; ++i) {
+            recons[i] += centroid[i];
+        }
+    } else {
+        sq.decode(code, recons, 1);
     }
 }
 

tests/test_build_blocks.py

@@ -325,6 +325,31 @@ class TestScalarQuantizer(unittest.TestCase):
                     # print(dis, D[i, j])
                     assert abs(D[i, j] - dis) / dis < 1e-5
 
+    def test_reconstruct(self):
+        self.do_reconstruct(True)
+
+    def test_reconstruct_no_residual(self):
+        self.do_reconstruct(False)
+
+    def do_reconstruct(self, by_residual):
+        d = 32
+        xt, xb, xq = get_dataset_2(d, 100, 5, 5)
+
+        index = faiss.index_factory(d, "IVF10,SQ8")
+        index.by_residual = by_residual
+        index.train(xt)
+        index.add(xb)
+        index.nprobe = 10
+        D, I = index.search(xq, 4)
+        xb2 = index.reconstruct_n(0, index.ntotal)
+        for i in range(5):
+            for j in range(4):
+                self.assertAlmostEqual(
+                    ((xq[i] - xb2[I[i, j]]) ** 2).sum(),
+                    D[i, j],
+                    places=4
+                )
+
 
 class TestRandom(unittest.TestCase):