[graph] Add insert_node() to Graph #235 (#236)

* Update graph.rs

* Update graph.rs

src/graph.rs

@@ -2,15 +2,45 @@ mod graph_edge;
 mod graph_element;
 mod graph_node;
 
+pub(crate) struct Graph {
+    from: Vec<i64>,
+    to: Vec<i64>,
+    from_meta: Vec<i64>,
+    to_meta: Vec<i64>,
+    node_count: u64,
+}
+
 #[allow(dead_code)]
-pub(crate) struct Graph {}
+impl Graph {
+    pub(crate) fn new() -> Graph {
+        Graph {
+            from: vec![0],
+            to: vec![0],
+            from_meta: vec![0],
+            to_meta: vec![0],
+            node_count: 0,
+        }
+    }
+
+    pub(crate) fn insert_node(&mut self) -> i64 {
+        self.from.push(0);
+        self.to.push(0);
+        self.from_meta.push(0);
+        self.to_meta.push(0);
+        self.node_count += 1;
+        self.node_count as i64
+    }
+}
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn new() {
-        let _graph = Graph {};
+    fn insert_node() {
+        let mut graph = Graph::new();
+        let id = graph.insert_node();
+
+        assert_eq!(id, 1);
     }
 }