diff --git a/fastcore/utils.py b/fastcore/utils.py
new file mode 100644
index 0000000..47e4c22
--- /dev/null
+++ b/fastcore/utils.py
@@ -0,0 +1,15 @@
+import numpy as np
+
+
+def node_indices(A, B):
+    """For each node ID in A find its index in B.
+
+    Typically `A` will be parent IDs and `B` will be node IDs.
+    Negative IDs (= parents of root nodes) will be passed through.
+
+    Note that there is no check whether all IDs in A actually exist in B. If
+    an ID in A does not exist in B it gets a negative index (i.e. like roots).
+    """
+    ix_dict = dict(zip(B, np.arange(len(B))))
+
+    return np.array([ix_dict.get(p, -1) for p in A])