Update _FM_utils.py

mikeio/spatial/_FM_utils.py

@@ -644,7 +644,23 @@ def _to_polygons(node_coordinates: np.ndarray, element_table: np.ndarray) -> lis
     return polygons
 
 
-def _create_node_element_matrix(element_table, num_nodes):
+def _create_node_element_matrix(element_table: np.ndarray, num_nodes: int) -> csr_matrix:
+    """Creates a sparse node-element connectivity matrix from a given element table.
+
+    Parameters
+    ----------
+    element_table : np.array
+        The element table (A 2D array where each row represents an element and each
+        column corresponds to a node index involved in the element.)
+    num_nodes : int
+        The total number of nodes in the mesh.
+    
+    Returns
+    -------
+    scipy.sparse.csr_matrix
+        A sparse matrix of shape (num_nodes, number of elements), where the entry
+        (i, j) is 1 if node i is part of element j, and 0 otherwise.
+    """
     row_ind = element_table.ravel()
     col_ind = np.repeat(np.arange(element_table.shape[0]), element_table.shape[1])
     data = np.ones(len(row_ind), dtype=int)