diff --git a/src/lib.rs b/src/lib.rs
index f7afa462d..1511ea7b1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,27 +6,11 @@
 #[cfg(test)] #[macro_use] extern crate quickcheck;
 extern crate derive_more;
 
-
 mod tree;
+mod node_data;
 
 pub use tree::Tree;
-
-/// Node metadata.
-#[repr(C)]
-#[derive(Debug)]
-pub struct NodeData {
-    subtree_commitment: [u8; 32],
-    start_time: u32,
-    end_time: u32,
-    start_target: u32,
-    end_target: u32,
-    start_sapling_root: [u8; 32],
-    end_sapling_root: [u8; 32],
-    subtree_total_work: u64,
-    start_height: u32,
-    end_height: u32,
-    shielded_tx: u64,
-}
+pub use node_data::NodeData;
 
 #[derive(Debug, derive_more::Display)]
 pub enum Error {
diff --git a/src/node_data.rs b/src/node_data.rs
new file mode 100644
index 000000000..8d81c71b3
--- /dev/null
+++ b/src/node_data.rs
@@ -0,0 +1,37 @@
+/// Node metadata.
+#[repr(C)]
+#[derive(Debug)]
+pub struct NodeData {
+    pub subtree_commitment: [u8; 32],
+    pub start_time: u32,
+    pub end_time: u32,
+    pub start_target: u32,
+    pub end_target: u32,
+    pub start_sapling_root: [u8; 32],
+    pub end_sapling_root: [u8; 32],
+    pub subtree_total_work: u64,
+    pub start_height: u32,
+    pub end_height: u32,
+    pub shielded_tx: u64,
+}
+
+impl NodeData {
+    pub fn combine(left: &NodeData, right: &NodeData) -> NodeData {
+        NodeData {
+            // TODO: hash children
+            subtree_commitment: [0u8; 32],
+            start_time: left.start_time,
+            end_time: right.end_time,
+            start_target: left.start_target,
+            end_target: right.end_target,
+            start_sapling_root: left.start_sapling_root,
+            end_sapling_root: right.end_sapling_root,
+
+            // TODO: sum work?
+            subtree_total_work: 0,
+            start_height: left.start_height,
+            end_height: right.end_height,
+            shielded_tx: left.shielded_tx + right.shielded_tx,
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/tree.rs b/src/tree.rs
index 2faebb60e..c5fc08f84 100644
--- a/src/tree.rs
+++ b/src/tree.rs
@@ -272,29 +272,10 @@ impl<'a> IndexedNode<'a> {
 
 }
 
-fn combine_data(left: &NodeData, right: &NodeData) -> NodeData {
-    NodeData {
-        // TODO: hash children
-        subtree_commitment: [0u8; 32],
-        start_time: left.start_time,
-        end_time: right.end_time,
-        start_target: left.start_target,
-        end_target: right.end_target,
-        start_sapling_root: left.start_sapling_root,
-        end_sapling_root: right.end_sapling_root,
-
-        // TODO: sum work?
-        subtree_total_work: 0,
-        start_height: left.start_height,
-        end_height: right.end_height,
-        shielded_tx: left.shielded_tx + right.shielded_tx,
-    }
-}
-
 fn combine_nodes<'a>(left: IndexedNode<'a>, right: IndexedNode<'a>) -> Entry {
     Entry {
         kind: EntryKind::Node(left.link, right.link),
-        data: combine_data(&left.node.data, &right.node.data),
+        data: NodeData::combine(&left.node.data, &right.node.data),
     }
 }