From 3625b529da81fb8ad4182477ef62f2e3321ff929 Mon Sep 17 00:00:00 2001
From: Sergei Zharinov <zharinov@users.noreply.github.com>
Date: Sat, 3 Jan 2026 14:57:47 -0300
Subject: [PATCH] fix(engine): restore cursor position after Call returns

---
 crates/plotnik-lib/src/engine/frame.rs | 11 +++++++----
 crates/plotnik-lib/src/engine/vm.rs    | 13 ++++++++++---
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/crates/plotnik-lib/src/engine/frame.rs b/crates/plotnik-lib/src/engine/frame.rs
index a0632da0..78773410 100644
--- a/crates/plotnik-lib/src/engine/frame.rs
+++ b/crates/plotnik-lib/src/engine/frame.rs
@@ -10,6 +10,8 @@ pub struct Frame {
     pub return_addr: u16,
     /// Parent frame index (for cactus stack).
     pub parent: Option<u32>,
+    /// Cursor position before Call navigation (for restoration on Return).
+    pub cursor_position: u32,
 }
 
 /// Append-only arena for frames (cactus stack implementation).
@@ -33,24 +35,25 @@ impl FrameArena {
     }
 
     /// Push a new frame, returns its index.
-    pub fn push(&mut self, return_addr: u16) -> u32 {
+    pub fn push(&mut self, return_addr: u16, cursor_position: u32) -> u32 {
         let idx = self.frames.len() as u32;
         self.frames.push(Frame {
             return_addr,
             parent: self.current,
+            cursor_position,
         });
         self.current = Some(idx);
         idx
     }
 
-    /// Pop the current frame, returning its return address.
+    /// Pop the current frame, returning its return address and cursor position.
     ///
     /// Panics if the stack is empty.
-    pub fn pop(&mut self) -> u16 {
+    pub fn pop(&mut self) -> (u16, u32) {
         let current_idx = self.current.expect("pop on empty frame stack");
         let frame = self.frames[current_idx as usize];
         self.current = frame.parent;
-        frame.return_addr
+        (frame.return_addr, frame.cursor_position)
     }
 
     /// Restore frame state for backtracking.
diff --git a/crates/plotnik-lib/src/engine/vm.rs b/crates/plotnik-lib/src/engine/vm.rs
index e1c36143..c1043148 100644
--- a/crates/plotnik-lib/src/engine/vm.rs
+++ b/crates/plotnik-lib/src/engine/vm.rs
@@ -231,8 +231,12 @@ impl<'t> VM<'t> {
 
         self.navigate_to_field(c.nav, c.node_field, tracer)?;
 
+        // Save cursor position AFTER navigation - this is where the callee starts matching.
+        // On Return, we restore to this position so Nav::Next in quantifier loops works correctly.
+        let saved_cursor = self.cursor.descendant_index();
+
         tracer.trace_call(c.target.get());
-        self.frames.push(c.next.get());
+        self.frames.push(c.next.get(), saved_cursor);
         self.recursion_depth += 1;
         self.ip = c.target.get();
         Ok(())
@@ -292,16 +296,19 @@ impl<'t> VM<'t> {
             return Err(RuntimeError::Accept);
         }
 
-        let return_addr = self.frames.pop();
+        let (return_addr, saved_cursor) = self.frames.pop();
         self.recursion_depth -= 1;
 
         // Prune frames (O(1) amortized)
         self.frames.prune(self.checkpoints.max_frame_ref());
 
-        // Set matched_node to current cursor position so effects after
+        // Set matched_node BEFORE restoring cursor so effects after
         // a Call can capture the node that the callee matched.
         self.matched_node = Some(self.cursor.node());
 
+        // Restore cursor position for subsequent navigation (e.g., Nav::Next in quantifier loop)
+        self.cursor.goto_descendant(saved_cursor);
+
         self.ip = return_addr;
         Ok(())
     }