1 files changed, 95 insertions, 0 deletions
diff --git a/src/personalization/models/reranker/bge_reranker.py b/src/personalization/models/reranker/bge_reranker.py
new file mode 100644
index 0000000..a672f0a
--- /dev/null
+++ b/src/personalization/models/reranker/bge_reranker.py
@@ -0,0 +1,95 @@
+"""BGE Reranker - lightweight 278M parameter cross-encoder reranker."""
+
+from typing import List
+import torch
+from transformers import AutoModelForSequenceClassification, AutoTokenizer
+from .base import Reranker
+
+
+class BGEReranker(Reranker):
+    """
+    BGE Reranker using cross-encoder architecture.
+
+    Much lighter than Qwen3-Reranker-8B:
+    - bge-reranker-base: 278M params
+    - bge-reranker-large: 560M params
+    """
+
+    def __init__(
+        self,
+        model_path: str = "BAAI/bge-reranker-base",
+        device_map: str = "auto",
+        dtype: torch.dtype = torch.float16
+    ):
+        self.tokenizer = AutoTokenizer.from_pretrained(model_path)
+
+        # Handle specific device assignment
+        if device_map and device_map.startswith("cuda:"):
+            self.model = AutoModelForSequenceClassification.from_pretrained(
+                model_path,
+                torch_dtype=dtype,
+                device_map=None,
+            )
+            self.model = self.model.to(device_map)
+            self.device = device_map
+        else:
+            self.model = AutoModelForSequenceClassification.from_pretrained(
+                model_path,
+                torch_dtype=dtype,
+                device_map=device_map,
+            )
+            self.device = next(self.model.parameters()).device
+
+        self.model.eval()
+
+    def score(
+        self,
+        query: str,
+        docs: List[str],
+        batch_size: int = 32,
+        **kwargs,
+    ) -> List[float]:
+        """
+        Score documents using cross-encoder.
+
+        Args:
+            query: The query string
+            docs: List of document strings to score
+            batch_size: Batch size for processing
+
+        Returns:
+            List of relevance scores (higher = more relevant)
+        """
+        if not docs:
+            return []
+
+        # Create query-doc pairs
+        pairs = [[query, doc] for doc in docs]
+
+        all_scores = []
+
+        with torch.no_grad():
+            for i in range(0, len(pairs), batch_size):
+                batch = pairs[i:i + batch_size]
+
+                # Tokenize
+                inputs = self.tokenizer(
+                    batch,
+                    padding=True,
+                    truncation=True,
+                    max_length=512,
+                    return_tensors="pt"
+                )
+                inputs = {k: v.to(self.device) for k, v in inputs.items()}
+
+                # Get scores
+                outputs = self.model(**inputs)
+                scores = outputs.logits.squeeze(-1).float().cpu().tolist()
+
+                # Handle single item case
+                if isinstance(scores, float):
+                    scores = [scores]
+
+                all_scores.extend(scores)
+
+        return all_scores