BUG: fix heuristic for detecting uniformly spaced grid axes

Author: neutrinoceros

src/gpgi/_data_types.py

@@ -112,6 +112,11 @@ def collect_exceptions(cls, data: Grid[FloatT]) -> list[Exception]:
         )
 
 
+def is_uniformly_spaced(arr: np.ndarray[tuple[int], np.dtype[FloatT]]) -> bool:
+    dbase = np.diff(arr)
+    return bool(dbase.std() / dbase.max() / dbase.size < 5 * np.finfo(arr.dtype).eps)
+
+
 @final
 class Grid(Generic[FloatT]):
     def __init__(
@@ -149,7 +154,7 @@ def __init__(
             (3,), -1, dtype=self.coordinates[self.axes[0]].dtype
         )
         for i, ax in enumerate(self.axes):
-            if self.size == 1 or np.diff(self.coordinates[ax]).std() < 1e-16:
+            if self.size == 1 or is_uniformly_spaced(self.coordinates[ax]):
                 # got a constant step in this direction, store it
                 self._dx[i] = self.coordinates[ax][1] - self.coordinates[ax][0]
 

tests/test_api.py

@@ -404,8 +404,8 @@ def test_inconsistent_grid_data(data, invalid_attr, expected):
             geometry="cartesian",
             grid={
                 "cell_edges": {
-                    "x": np.array([0.0, 0.0]),
-                    "y": np.array([0.0, 0.0, 0.0]),
+                    "x": np.array([0.0, 1.0]),
+                    "y": np.array([0.0, 2.0, 4.0]),
                 },
                 "fields": {
                     "density": data,
@@ -419,8 +419,8 @@ def test_validate_empty_fields():
         geometry="cartesian",
         grid={
             "cell_edges": {
-                "x": np.array([0.0, 0.0]),
-                "y": np.array([0.0, 0.0, 0.0]),
+                "x": np.array([0.0, 1.0]),
+                "y": np.array([0.0, 2.0, 4.0]),
             },
             "fields": {},
         },