Fix potential edge case scoring in context search (#474)

* apply fast_sigmoid fn to context pair score

* remove redundant else statement

* better NaN and float32 handling

* remove unused import

qdrant_client/local/distances.py

@@ -147,12 +147,16 @@ def calculate_distance_core(
         return calculate_distance(query, vectors, distance_type)
 
 
+def fast_sigmoid(x: np.float32) -> np.float32:
+    if np.isnan(x):
+        # To avoid divisions on NaNs, which gets: RuntimeWarning: invalid value encountered in scalar divide
+        return x  # NaN
+
+    return x / np.add(1.0, abs(x))
+
+
 def scaled_fast_sigmoid(x: np.float32) -> np.float32:
-    if np.isfinite(x):
-        return 0.5 * (x / (1.0 + abs(x)) + 1.0)
-    else:
-        # To avoid NaNs, which gets: RuntimeWarning: invalid value encountered in scalar divide
-        return x
+    return 0.5 * (np.add(fast_sigmoid(x), 1.0))
 
 
 def calculate_recommend_best_scores(
@@ -234,7 +238,9 @@ def calculate_context_scores(
         neg = calculate_distance_core(pair.negative, vectors, distance_type)
 
         difference = pos - neg - EPSILON
-        pair_scores = np.minimum(difference, 0.0)
+        pair_scores = np.fromiter(
+            (fast_sigmoid(xi) for xi in np.minimum(difference, 0.0)), np.float32
+        )
         overall_scores += pair_scores
 
     return overall_scores