Fix sparse slicing

phiml/math/_sparse.py

@@ -424,6 +424,8 @@ def _getitem(self, selection: dict) -> 'Tensor':
                 assert sel.step in (None, 1), f"Only step size 1 supported for sparse indexing but got {sel.step}"
                 start = sel.start or 0
                 stop = self._dense_shape[dim].size if sel.stop is None else sel.stop
+                if stop < 0:
+                    stop += self._shape.get_size(dim)
                 keep &= (start <= dim_indices) & (dim_indices < stop)
                 from . import vec
                 indices -= vec('sparse_idx', **{d: start if d == dim else 0 for d in indices.sparse_idx.item_names})
@@ -621,6 +623,8 @@ def _getitem(self, selection: dict) -> 'Tensor':
                     raise NotImplementedError("Slicing not yet supported for batched sparse tensors")
                 start = ptr_sel.start or 0
                 stop = compressed.volume if ptr_sel.stop is None else ptr_sel.stop
+                if stop < 0:
+                    stop += compressed.size
                 pointers = pointers[start:stop+1]
                 indices = indices[{instance(indices).name: slice(int(pointers[0]), int(pointers[-1]))}]
                 values = values[{instance(values).name: slice(int(pointers[0]), int(pointers[-1]))}]
@@ -639,6 +643,8 @@ def _getitem(self, selection: dict) -> 'Tensor':
                 assert ind_sel.step in (None, 1), f"Only step size 1 supported for sparse indexing but got {ind_sel.step}"
                 start = ind_sel.start or 0
                 stop = uncompressed.volume if ind_sel.stop is None else ind_sel.stop
+                if stop < 0:
+                    stop += uncompressed.size
                 keep = (start <= indices) & (indices < stop)
                 from ._ops import where
                 values = where(keep, values, 0)