itree.[ch]: Add sanity checks, comments, and minor tweaks

* src/alloc.c (mark_overlay): Add sanity check.

* src/buffer.c (next_overlay_change, previous_overlay_change):
Tweak code to keep the same vars for the bounds.

* src/itree.c (interval_tree_clear, interval_tree_insert)
(interval_tree_remove, interval_tree_insert_fix, interval_tree_remove_fix):
Adjust to the `color` -> `red` change.
(interval_tree_clear): Prefer `true/false` for booleans.
(interval_generator_create): Use an actual `interval_tree_order` value
rather than 0.
(interval_generator_next): Simplify a tiny bit.  Add comment.
(interval_generator_narrow): Add sanity check.

* src/itree.h (struct interval_node): Replace `color` field with
boolean `red` field.
(enum interval_tree_order): Remove unused `ITREE_DEFLT_ORDER` value.

* src/pdumper.c (dump_interval_node): Adjust to the
`color` -> `red` change.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 20b8981bd66..be55dcf8dfd 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6505,6 +6505,9 @@ mark_char_table (struct Lisp_Vector *ptr, enum pvec_type pvectype)
 static void
 mark_overlay (struct Lisp_Overlay *ov)
 {
+  /* We don't mark the `interval_node` object, because it is managed manually
+     rather than by the GC.  */
+  eassert (BASE_EQ (ov->interval->data, make_lisp_ptr (ov, Lisp_Vectorlike)));
   set_vectorlike_marked (&ov->header);
   mark_object (ov->plist);
 }