Python C glue for B1 index

Closes #2255

python/_tskitmodule.c

@@ -10802,6 +10802,27 @@ Tree_get_colless_index(Tree *self)
     return ret;
 }
 
+static PyObject *
+Tree_get_b1_index(Tree *self)
+{
+    PyObject *ret = NULL;
+    int err;
+    double result;
+
+    if (Tree_check_state(self) != 0) {
+        goto out;
+    }
+
+    err = tsk_tree_b1_index(self->tree, &result);
+    if (err != 0) {
+        handle_library_error(err);
+        goto out;
+    }
+    ret = Py_BuildValue("d", result);
+out:
+    return ret;
+}
+
 static PyObject *
 Tree_get_root_threshold(Tree *self)
 {
@@ -11221,6 +11242,10 @@ static PyMethodDef Tree_methods[] = {
         .ml_meth = (PyCFunction) Tree_get_colless_index,
         .ml_flags = METH_NOARGS,
         .ml_doc = "Returns the Colless index for this tree." },
+    { .ml_name = "get_b1_index",
+        .ml_meth = (PyCFunction) Tree_get_b1_index,
+        .ml_flags = METH_NOARGS,
+        .ml_doc = "Returns the B1 index for this tree." },
     { NULL } /* Sentinel */
 };
 

python/tskit/trees.py

@@ -2793,16 +2793,7 @@ def b1_index(self):
         :return: The B1 balance index.
         :rtype: float
         """
-        # TODO implement in C
-        max_path_length = np.zeros(self.tree_sequence.num_nodes, dtype=int)
-        total = 0.0
-        for u in self.postorder():
-            if self.parent(u) != tskit.NULL and self.is_internal(u):
-                max_path_length[u] = 1 + max(
-                    max_path_length[v] for v in self.children(u)
-                )
-                total += 1 / max_path_length[u]
-        return total
+        return self._ll_tree.get_b1_index()
 
     def colless_index(self):
         """