From f8bc074d82c3ed56ffef1c1df728af6258e0797f Mon Sep 17 00:00:00 2001
From: Zach Schuermann <zachary.zvs@gmail.com>
Date: Wed, 27 Nov 2024 12:49:50 -0800
Subject: [PATCH] new Snapshot::new_from() API

---
 kernel/src/snapshot.rs          | 42 ++++++++++++++++++++++++++++++++-
 kernel/src/table_changes/mod.rs |  2 +-
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/kernel/src/snapshot.rs b/kernel/src/snapshot.rs
index 30063da3e..852bba40f 100644
--- a/kernel/src/snapshot.rs
+++ b/kernel/src/snapshot.rs
@@ -53,7 +53,8 @@ impl Snapshot {
     ///
     /// - `table_root`: url pointing at the table root (where `_delta_log` folder is located)
     /// - `engine`: Implementation of [`Engine`] apis.
-    /// - `version`: target version of the [`Snapshot`]
+    /// - `version`: target version of the [`Snapshot`]. None will create a snapshot at the latest
+    ///   version of the table.
     pub fn try_new(
         table_root: Url,
         engine: &dyn Engine,
@@ -71,6 +72,26 @@ impl Snapshot {
         Self::try_new_from_log_segment(table_root, log_segment, engine)
     }
 
+    /// Create a new [`Snapshot`] instance from an existing [`Snapshot`]. This is useful when you
+    /// already have a [`Snapshot`] lying around and want to do the minimal work to 'update' the
+    /// snapshot to a later version.
+    ///
+    /// # Parameters
+    ///
+    /// - `existing_snapshot`: reference to an existing [`Snapshot`]
+    /// - `engine`: Implementation of [`Engine`] apis.
+    /// - `version`: target version of the [`Snapshot`]. None will create a snapshot at the latest
+    ///   version of the table.
+    pub fn new_from(
+        existing_snapshot: &Snapshot,
+        engine: &dyn Engine,
+        version: Option<Version>,
+    ) -> DeltaResult<Self> {
+        // TODO(zach): for now we just pass through to the old API. We should instead optimize this
+        // to avoid replaying overlapping LogSegments.
+        Self::try_new(existing_snapshot.table_root.clone(), engine, version)
+    }
+
     /// Create a new [`Snapshot`] instance.
     pub(crate) fn try_new_from_log_segment(
         location: Url,
@@ -250,6 +271,25 @@ mod tests {
         assert_eq!(snapshot.schema(), &expected);
     }
 
+    #[test]
+    fn test_snapshot_new_from() {
+        let path =
+            std::fs::canonicalize(PathBuf::from("./tests/data/table-with-dv-small/")).unwrap();
+        let url = url::Url::from_directory_path(path).unwrap();
+
+        let engine = SyncEngine::new();
+        let old_snapshot = Snapshot::try_new(url, &engine, Some(0)).unwrap();
+        let snapshot = Snapshot::new_from(&old_snapshot, &engine, Some(0)).unwrap();
+
+        let expected =
+            Protocol::try_new(3, 7, Some(["deletionVectors"]), Some(["deletionVectors"])).unwrap();
+        assert_eq!(snapshot.protocol(), &expected);
+
+        let schema_string = r#"{"type":"struct","fields":[{"name":"value","type":"integer","nullable":true,"metadata":{}}]}"#;
+        let expected: StructType = serde_json::from_str(schema_string).unwrap();
+        assert_eq!(snapshot.schema(), &expected);
+    }
+
     #[test]
     fn test_read_table_with_last_checkpoint() {
         let path = std::fs::canonicalize(PathBuf::from(
diff --git a/kernel/src/table_changes/mod.rs b/kernel/src/table_changes/mod.rs
index 2005762f9..3cfa6c726 100644
--- a/kernel/src/table_changes/mod.rs
+++ b/kernel/src/table_changes/mod.rs
@@ -90,7 +90,7 @@ impl TableChanges {
         // supported for every protocol action in the CDF range.
         let start_snapshot =
             Snapshot::try_new(table_root.as_url().clone(), engine, Some(start_version))?;
-        let end_snapshot = Snapshot::try_new(table_root.as_url().clone(), engine, end_version)?;
+        let end_snapshot = Snapshot::new_from(&start_snapshot, engine, end_version)?;
 
         // Verify CDF is enabled at the beginning and end of the interval to fail early. We must
         // still check that CDF is enabled for every metadata action in the CDF range.