fix: cursor behavior when using gc-snapshot

crates/loro-internal/src/loro.rs

@@ -1360,6 +1360,10 @@ impl LoroDoc {
                     .ok_or(CannotFindRelativePosition::ContainerDeleted)?;
                 // We know where the target id is when we trace back to the delete_op_id.
                 let Some(delete_op_id) = find_last_delete_op(&oplog, id, idx) else {
+                    if oplog.trimmed_vv().includes_id(id) {
+                        return Err(CannotFindRelativePosition::HistoryCleared);
+                    }
+
                     tracing::error!("Cannot find id {}", id);
                     return Err(CannotFindRelativePosition::IdNotFound);
                 };
@@ -1557,8 +1561,12 @@ impl LoroDoc {
     }
 }
 
+// FIXME: PERF: This method is quite slow because it iterates all the changes
 fn find_last_delete_op(oplog: &OpLog, id: ID, idx: ContainerIdx) -> Option<ID> {
-    let start_vv = oplog.dag.frontiers_to_vv(&id.into())?;
+    let start_vv = oplog
+        .dag
+        .frontiers_to_vv(&id.into())
+        .unwrap_or_else(|| oplog.trimmed_vv().to_vv());
     for change in oplog.iter_changes_causally_rev(&start_vv, oplog.vv()) {
         for op in change.ops.iter().rev() {
             if op.container != idx {

crates/loro/tests/integration_test/gc_test.rs

@@ -4,7 +4,7 @@ use std::{
 };
 
 use super::gen_action;
-use loro::{ExportMode, Frontiers, LoroDoc, ID};
+use loro::{cursor::CannotFindRelativePosition, ExportMode, Frontiers, LoroDoc, ID};
 
 #[test]
 fn test_gc() -> anyhow::Result<()> {
@@ -240,3 +240,60 @@ fn the_vv_on_gc_doc() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_cursor_that_cannot_be_found_when_exporting_gc_snapshot() -> anyhow::Result<()> {
+    let doc = LoroDoc::new();
+    doc.set_peer_id(1)?;
+    doc.get_text("text").insert(0, "Hello world")?;
+    let c = doc
+        .get_text("text")
+        .get_cursor(3, loro::cursor::Side::Left)
+        .unwrap();
+    doc.get_text("text").delete(0, 5)?;
+    doc.commit();
+    let snapshot = doc.export(loro::ExportMode::gc_snapshot(&doc.oplog_frontiers()));
+    let new_doc = LoroDoc::new();
+    new_doc.import(&snapshot)?;
+    let result = new_doc.get_cursor_pos(&c);
+    match result {
+        Ok(v) => {
+            dbg!(v);
+            unreachable!()
+        }
+        Err(CannotFindRelativePosition::HistoryCleared) => {}
+        Err(x) => {
+            dbg!(x);
+            unreachable!()
+        }
+    }
+    Ok(())
+}
+
+#[test]
+fn test_cursor_that_can_be_found_when_exporting_gc_snapshot() -> anyhow::Result<()> {
+    let doc = LoroDoc::new();
+    doc.set_peer_id(1)?;
+    doc.get_text("text").insert(0, "Hello world")?;
+    doc.commit();
+    let c = doc
+        .get_text("text")
+        .get_cursor(3, loro::cursor::Side::Left)
+        .unwrap();
+    doc.get_text("text").delete(0, 5)?;
+    doc.commit();
+    let snapshot = doc.export(loro::ExportMode::gc_snapshot_from_id(ID::new(1, 10)));
+    let new_doc = LoroDoc::new();
+    new_doc.import(&snapshot)?;
+    let result = new_doc.get_cursor_pos(&c);
+    match result {
+        Ok(v) => {
+            assert_eq!(v.current.pos, 0);
+        }
+        Err(x) => {
+            dbg!(x);
+            unreachable!()
+        }
+    }
+    Ok(())
+}