feat: Add NamespaceIdent.parent() (apache#641)

* Add NamespaceIdent.parent()

* Use split_last

crates/iceberg/src/catalog/mod.rs

@@ -133,6 +133,18 @@ impl NamespaceIdent {
     pub fn inner(self) -> Vec<String> {
         self.0
     }
+
+    /// Get the parent of this namespace.
+    /// Returns None if this namespace only has a single element and thus has no parent.
+    pub fn parent(&self) -> Option<Self> {
+        self.0.split_last().and_then(|(_, parent)| {
+            if parent.is_empty() {
+                None
+            } else {
+                Some(Self(parent.to_vec()))
+            }
+        })
+    }
 }
 
 impl AsRef<Vec<String>> for NamespaceIdent {
@@ -480,6 +492,17 @@ mod tests {
     };
     use crate::{NamespaceIdent, TableCreation, TableIdent, TableRequirement, TableUpdate};
 
+    #[test]
+    fn test_parent_namespace() {
+        let ns1 = NamespaceIdent::from_strs(vec!["ns1"]).unwrap();
+        let ns2 = NamespaceIdent::from_strs(vec!["ns1", "ns2"]).unwrap();
+        let ns3 = NamespaceIdent::from_strs(vec!["ns1", "ns2", "ns3"]).unwrap();
+
+        assert_eq!(ns1.parent(), None);
+        assert_eq!(ns2.parent(), Some(ns1.clone()));
+        assert_eq!(ns3.parent(), Some(ns2.clone()));
+    }
+
     #[test]
     fn test_create_table_id() {
         let table_id = TableIdent {