From b2cfc8b6f2fbcd26cbe99f98b4a03679dab8671b Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Thu, 21 Aug 2025 22:52:25 -0700
Subject: [PATCH] fix cyclic detection of partial strings in
 StackfulPreOrderHeapIter (#3050)

---
 src/heap_iter.rs  | 43 +++++++++++++++++--------------------------
 src/heap_print.rs | 16 ++++++++++------
 2 files changed, 27 insertions(+), 32 deletions(-)

diff --git a/src/heap_iter.rs b/src/heap_iter.rs
index 97489a6b..583d297d 100644
--- a/src/heap_iter.rs
+++ b/src/heap_iter.rs
@@ -445,24 +445,17 @@ impl<'a, ElideLists: ListElisionPolicy> StackfulPreOrderHeapIter<'a, ElideLists>
                    let cell = *cell;
                    let tail_idx = self.heap.scan_slice_to_str(vh).tail_idx;
 
-                   // forward the current PStrLoc cell if the zero
-                   // byte at the end of the string buffer
-                   // is marked
-                   let buf_bytes = self.heap[tail_idx - 1].into_bytes();
-
-                   if buf_bytes[7] != 0u8 {
-                       let cell = self.read_cell_mut(h);
-                       cell.set_forwarding_bit(true);
-                   }
+                   if self.heap[tail_idx - 1].get_mark_bit() {
+                       self.read_cell_mut(h).set_forwarding_bit(true);
+                       self.stack.push(h);
 
-                   // now mark it as if were a HeapCellValue, even
-                   // though it's not! this is fine as long as its tag
-                   // is never inspected, which it isn't.
+                       continue;
+                   } else {
+                       self.heap[tail_idx - 1].set_mark_bit(true);
+                   }
 
-                   self.push_if_unmarked(
-                       IterStackLoc::iterable_loc(tail_idx - 1, HeapOrStackTag::Heap),
-                   );
-                   self.stack.push(IterStackLoc::mark_loc(tail_idx, HeapOrStackTag::Heap));
+                   self.stack.push(IterStackLoc::iterable_loc(tail_idx - 1, HeapOrStackTag::Heap));
+                   self.stack.push(IterStackLoc::pending_mark_loc(tail_idx, HeapOrStackTag::Heap));
 
                    return Some(cell);
                }
@@ -1959,10 +1952,6 @@ mod tests {
                 unmark_cell_bits!(iter.next().unwrap()),
                 pstr_loc_as_cell!(heap_index!(3) + 2)
             );
-            assert_eq!(
-                unmark_cell_bits!(iter.next().unwrap()),
-                pstr_loc_as_cell!(heap_index!(3) + 2)
-            );
             assert_eq!(iter.next(), None);
         }
 
@@ -2177,8 +2166,14 @@ mod tests {
             );
 
             assert_eq!(iter.heap.slice_to_str(0, "a string".len()), "a string");
-            assert_eq!(iter.next().unwrap(), pstr_loc_as_cell!(0));
-            assert_eq!(iter.next().unwrap(), empty_list_as_cell!());
+            assert_eq!(
+                unmark_cell_bits!(iter.next().unwrap()),
+                pstr_loc_as_cell!(0)
+            );
+            assert_eq!(
+                unmark_cell_bits!(iter.next().unwrap()),
+                empty_list_as_cell!()
+            );
             assert_eq!(iter.next(), None);
         }
 
@@ -2554,10 +2549,6 @@ mod tests {
                 unmark_cell_bits!(iter.next().unwrap()),
                 pstr_loc_as_cell!(heap_index!(3) + 2)
             );
-            assert_eq!(
-                unmark_cell_bits!(iter.next().unwrap()),
-                pstr_loc_as_cell!(heap_index!(3) + 2)
-            );
             assert_eq!(
                 unmark_cell_bits!(iter.next().unwrap()),
                 pstr_loc_as_cell!(heap_index!(3))
diff --git a/src/heap_print.rs b/src/heap_print.rs
index 2186c8ed..53dbbe63 100644
--- a/src/heap_print.rs
+++ b/src/heap_print.rs
@@ -904,16 +904,13 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
                                         debug_assert!(cell.is_ref());
 
                                         let h = if cell.get_tag() == HeapCellValueTag::PStrLoc {
-                                            self.iter.focus().value()
+                                            self.state_stack
+                                                .push(TokenOrRedirect::Atom(atom!("...")));
+                                            return None;
                                         } else {
                                             cell.get_value()
                                         } as usize;
 
-                                        self.iter.push_stack(IterStackLoc::iterable_loc(
-                                            h,
-                                            HeapOrStackTag::Heap,
-                                        ));
-
                                         // as usual, the WAM's
                                         // optimization of the Lis tag
                                         // (conflating the location of
@@ -934,10 +931,17 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
                                             }
                                         }
 
+                                        self.iter.push_stack(IterStackLoc::iterable_loc(
+                                            h,
+                                            HeapOrStackTag::Heap,
+                                        ));
+
                                         if let Some(cell) = self.iter.next() {
                                             orig_cell = cell;
                                             continue;
                                         }
+
+                                        return Some(cell);
                                     };
                                 }
                             }
-- 
2.54.0