test: improve reranker test robustness

tests/test_rerank.py

@@ -3,11 +3,17 @@
 import pytest
 from rerankers.models.flashrank_ranker import FlashRankRanker
 from rerankers.models.ranker import BaseRanker
+from scipy.stats import kendalltau
 
 from raglite import RAGLiteConfig, hybrid_search, rerank_chunks, retrieve_chunks
 from raglite._database import Chunk
 
 
+def kendall_tau(a: list[Chunk], b: list[Chunk]) -> float:
+    """Measure the Kendall rank correlation coefficient between two lists."""
+    return kendalltau(range(len(a)), [a.index(c) for c in b])[0]
+
+
 @pytest.fixture(
     params=[
         pytest.param(None, id="no_reranker"),
@@ -47,9 +53,9 @@ def test_reranker(
     assert all(chunk_id == chunk.id for chunk_id, chunk in zip(chunk_ids, chunks, strict=True))
     # Rerank the chunks given an inverted chunk order.
     reranked_chunks = rerank_chunks(query, chunks[::-1], config=raglite_test_config)
-    if reranker is not None and "text-embedding-3-small" not in raglite_test_config.embedder:
-        assert reranked_chunks[0] in chunks[:3]
+    if reranker:
+        assert kendall_tau(chunks, reranked_chunks) >= kendall_tau(chunks[::-1], reranked_chunks)
     # Test that we can also rerank given the chunk_ids only.
     reranked_chunks = rerank_chunks(query, chunk_ids[::-1], config=raglite_test_config)
-    if reranker is not None and "text-embedding-3-small" not in raglite_test_config.embedder:
-        assert reranked_chunks[0] in chunks[:3]
+    if reranker:
+        assert kendall_tau(chunks, reranked_chunks) >= kendall_tau(chunks[::-1], reranked_chunks)