remove ds from compute index in favor of using grad spacing directly

desc/compute/_curve.py

@@ -26,25 +26,6 @@ def _s(params, transforms, profiles, data, **kwargs):
     return data
 
 
-@register_compute_fun(
-    name="ds",
-    label="ds",
-    units="~",
-    units_long="None",
-    description="Spacing of curve parameter",
-    dim=1,
-    params=[],
-    transforms={"grid": []},
-    profiles=[],
-    coordinates="s",
-    data=[],
-    parameterization="desc.geometry.core.Curve",
-)
-def _ds(params, transforms, profiles, data, **kwargs):
-    data["ds"] = transforms["grid"].spacing[:, 2]
-    return data
-
-
 @register_compute_fun(
     name="X",
     label="X",
@@ -980,17 +961,17 @@ def _torsion(params, transforms, profiles, data, **kwargs):
     description="Length of the curve",
     dim=0,
     params=[],
-    transforms={},
+    transforms={"grid": []},
     profiles=[],
     coordinates="",
-    data=["ds", "x_s"],
+    data=["x_s"],
     parameterization=["desc.geometry.core.Curve"],
 )
 def _length(params, transforms, profiles, data, **kwargs):
     T = jnp.linalg.norm(data["x_s"], axis=-1)
     # this is equivalent to jnp.trapz(T, s) for a closed curve,
     # but also works if grid.endpoint is False
-    data["length"] = jnp.sum(T * data["ds"])
+    data["length"] = jnp.sum(T * transforms["grid"].spacing[:, 2])
     return data
 
 
@@ -1002,10 +983,10 @@ def _length(params, transforms, profiles, data, **kwargs):
     description="Length of the curve",
     dim=0,
     params=[],
-    transforms={},
+    transforms={"grid": []},
     profiles=[],
     coordinates="",
-    data=["ds", "x", "x_s"],
+    data=["x", "x_s"],
     parameterization="desc.geometry.curve.SplineXYZCurve",
     method="Interpolation type, Default 'cubic'. See SplineXYZCurve docs for options.",
 )
@@ -1015,7 +996,8 @@ def _length_SplineXYZCurve(params, transforms, profiles, data, **kwargs):
         if kwargs.get("basis", "rpz").lower() == "rpz":
             coords = rpz2xyz(coords)
         # ensure curve is closed
-        # if it's already closed this doesn't add any length since ds will be zero
+        # if it's already closed this doesn't add any length since
+        # grid spacing will be zero at the duplicate point
         coords = jnp.concatenate([coords, coords[:1]])
         X = coords[:, 0]
         Y = coords[:, 1]
@@ -1026,5 +1008,5 @@ def _length_SplineXYZCurve(params, transforms, profiles, data, **kwargs):
         T = jnp.linalg.norm(data["x_s"], axis=-1)
         # this is equivalent to jnp.trapz(T, s) for a closed curve
         # but also works if grid.endpoint is False
-        data["length"] = jnp.sum(T * data["ds"])
+        data["length"] = jnp.sum(T * transforms["grid"].spacing[:, 2])
     return data