fix: resolve 6 failing tests in test_backends.py

- Skip GPU index tests when FAISS is not installed instead of failing
  with RuntimeError (add @pytest.mark.skipif decorator)
- Fix ADC broadcasting bug: remove erroneous .T transpose on
  distance_table indexing which produced shape (100,10) instead of
  (10,100)
- Fix distributed index test: assert valid shard distribution instead
  of checking for impossible shard key 4 (valid range is 0-3)

Signed-off-by: Maxime Grenu <maxime.grenu@gmail.com>

Author: cluster2600

python/tests/test_backends.py

@@ -35,6 +35,10 @@ def test_get_backend_info(self):
         assert "selected" in info
 
 
+@pytest.mark.skipif(
+    not detect.FAISS_AVAILABLE,
+    reason="FAISS not installed",
+)
 class TestGPUIndex:
     """Tests for GPU index."""
 
@@ -251,7 +255,10 @@ def test_distributed_index(self):
         vector_ids = [f"v_{i}" for i in range(100)]
 
         index.add(vectors, vector_ids)
-        assert 4 in index._local_indexes
+        # With 4 shards using hash strategy, 100 vectors should distribute
+        # across multiple shards (shard indices are 0-3, never 4)
+        assert len(index._local_indexes) > 0
+        assert all(0 <= s < 4 for s in index._local_indexes)
 
     def test_result_merger(self):
         """Test result merging."""

python/zvec/backends/search.py

@@ -33,7 +33,7 @@ def asymmetric_distance_computation(
     distances = np.zeros((n_queries, n_codes), dtype=np.float32)
 
     for i in range(codes.shape[1]):  # m sub-vectors
-        distances += distance_table[:, i, codes[:, i]].T
+        distances += distance_table[:, i, codes[:, i]]
 
     return distances