Make sure n_steps_default >= 3

gcm_filters/filter.py

@@ -71,6 +71,24 @@ def _taper_target(target_spec: TargetSpec):
 }
 
 
+def _compute_n_steps_default(
+    ndim, filter_shape, filter_scale, dx_min, transition_width
+):
+    """Compute the default number of steps for 1D or 2D filters based on provided parameters."""
+
+    n_steps_factor = filter_params[filter_shape][ndim]["offset"] + filter_params[
+        filter_shape
+    ][ndim]["factor"] * (
+        (np.pi / transition_width) ** filter_params[filter_shape][ndim]["exponent"]
+    )
+
+    filter_factor = filter_scale / dx_min
+
+    n_steps_default = max(np.ceil(n_steps_factor * filter_factor).astype(int), 3)
+
+    return n_steps_default
+
+
 class FilterSpec(NamedTuple):
     n_steps: int
     s_max: float
@@ -332,20 +350,19 @@ def __post_init__(self):
             raise ValueError(f"Transition width must be > 1.")
 
         # Get default number of steps
-        filter_factor = self.filter_scale / self.dx_min
         if self.ndim > 2:
             if self.n_steps < 3:
                 raise ValueError(f"When ndim > 2, you must set n_steps manually")
             else:
                 n_steps_default = self.n_steps  # For ndim>2 we don't have a default
         else:
-            n_steps_factor = filter_params[self.filter_shape][self.ndim][
-                "offset"
-            ] + filter_params[self.filter_shape][self.ndim]["factor"] * (
-                (np.pi / self.transition_width)
-                ** filter_params[self.filter_shape][self.ndim]["exponent"]
+            n_steps_default = _compute_n_steps_default(
+                self.ndim,
+                self.filter_shape,
+                self.filter_scale,
+                self.dx_min,
+                self.transition_width,
             )
-            n_steps_default = np.ceil(n_steps_factor * filter_factor).astype(int)
 
         # Set n_steps if needed and issue n_step warning, if needed
         if self.n_steps < 3: