perf: reduce scalar distance validation overhead

src/variopt/distance.py

@@ -1,6 +1,6 @@
 """Internal helpers for validating distance-like quantities."""
 
-import numpy as np
+from math import isfinite
 
 
 def require_valid_distance(distance: float) -> float:
@@ -22,7 +22,7 @@ def require_valid_distance(distance: float) -> float:
         If ``distance`` is non-finite or negative.
     """
     normalized_distance = float(distance)
-    if not np.isfinite(normalized_distance):
+    if not isfinite(normalized_distance):
         msg = "distance must be finite"
         raise ValueError(msg)
 

tests/core/test_distance.py

@@ -0,0 +1,40 @@
+"""Tests for internal distance normalization."""
+
+from math import copysign
+
+import numpy as np
+import pytest
+
+from variopt.distance import require_valid_distance
+
+
+@pytest.mark.parametrize(
+    ("distance", "expected"),
+    (
+        (0.0, 0.0),
+        (1, 1.0),
+        (np.float64(1.5), 1.5),
+    ),
+)
+def test_require_valid_distance_normalizes_finite_non_negative_values(
+    distance: float,
+    expected: float,
+) -> None:
+    assert require_valid_distance(distance) == expected
+
+
+def test_require_valid_distance_preserves_negative_zero() -> None:
+    normalized_distance = require_valid_distance(-0.0)
+
+    assert copysign(1.0, normalized_distance) == -1.0
+
+
+@pytest.mark.parametrize("distance", (float("nan"), float("inf"), float("-inf")))
+def test_require_valid_distance_rejects_non_finite_values(distance: float) -> None:
+    with pytest.raises(ValueError, match="distance must be finite"):
+        _ = require_valid_distance(distance)
+
+
+def test_require_valid_distance_rejects_negative_values() -> None:
+    with pytest.raises(ValueError, match="distance must be non-negative"):
+        _ = require_valid_distance(-1.0)