[libgee] Make ConcurrentSet implement SortedSet



commit f197d88265c1ba5adc30193902f065e4be74d0f3
Author: Maciej Piechotka <uzytkownik2 gmail com>
Date:   Sun Dec 2 10:58:09 2012 +0000

    Make ConcurrentSet implement SortedSet

 gee/concurrentset.vala       | 1002 +++++++++++++++++++++++++++++++++++++++---
 tests/testconcurrentset.vala |    4 +-
 tests/testsortedset.vala     |   66 ++--
 3 files changed, 990 insertions(+), 82 deletions(-)
---
diff --git a/gee/concurrentset.vala b/gee/concurrentset.vala
index 62d937d..854340f 100644
--- a/gee/concurrentset.vala
+++ b/gee/concurrentset.vala
@@ -28,7 +28,7 @@
  * of structure during iteration is allowed. However the change may not be immidiatly
  * visible to other threads.
  */
-public class Gee.ConcurrentSet<G> : AbstractSet<G> {
+public class Gee.ConcurrentSet<G> : AbstractSortedSet<G> {
 	public ConcurrentSet (owned CompareDataFunc<G>? compare_func = null) {
 		if (compare_func == null) {
 			compare_func = Functions.get_compare_func_for (typeof (G));
@@ -63,10 +63,14 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 		}
 		uint32 rand_int = rnd->int_range (0, int32.MAX);
 		uint8 height = 1 + (uint8)GLib.Bit.nth_lsf (~rand_int, -1);
-		Tower<G> prev = _head;
-		if (Tower.search<G> (_cmp, key, ref prev, null, height - 1)) {
+		TowerIter<G> prev = TowerIter<G>();
+		prev._iter[height - 1] = _head;
+		if (Tower.search<G> (_cmp, key, ref prev._iter[height - 1], null, height - 1)) {
 			return false;
 		}
+		for (int i = height - 2; i >= 0; i--) {
+			prev._iter[i] = prev._iter[height - 1];
+		}
 		Tower<G>? result = Tower.insert<G> (_cmp, ref prev, key, height - 1);
 		if (result != null) {
 			GLib.AtomicInt.inc (ref _size);
@@ -76,7 +80,11 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 
 	public override bool remove (G item) {
 		HazardPointer.Context ctx = new HazardPointer.Context ();
-		bool result = Tower.remove_key<G> (_cmp, _head, item);
+		TowerIter<G> prev = TowerIter<G>();
+		for (int i = 0; i < _MAX_HEIGHT; i++) {
+			prev._iter[i] = _head;
+		}
+		bool result = Tower.remove_key<G> (_cmp, ref prev, item);
 		if (result) {
 			GLib.AtomicInt.dec_and_test (ref _size);
 		}
@@ -91,6 +99,119 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 		}
 	}
 
+	public override G first () {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G>? prev = null;
+		Tower<G> curr = _head;
+		if (Tower.proceed<G> (_cmp, ref prev, ref curr, 0)) {
+			return curr._data;
+		} else {
+			return null;
+		}
+	}
+
+	public override G last () {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G>? prev = null;
+		Tower<G> curr = _head;
+		bool found = false;
+		for (int i = _MAX_HEIGHT; i >= 0; i--) {
+			while (Tower.proceed<G> (_cmp, ref prev, ref curr, 0)) {
+				found = true;
+			}
+		}
+		if (!found) {
+			return null;
+		}
+		return curr._data;
+	}
+
+	public override Gee.Iterator<G>? iterator_at (G element) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		TowerIter<G> prev = TowerIter<G> ();
+		TowerIter<G> curr;
+		for (int i = 0; i < _MAX_HEIGHT; i++) {
+			prev._iter[i] = _head;
+		}
+		if (!Tower.search_from_bookmark<G> (_cmp, element, ref prev, out curr)) {
+			return null;
+		}
+		return new Iterator<G>.point_at (this, ref prev, curr._iter[0]);
+	}
+
+	public override G? lower (G element) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G> prev = _head;
+		Tower.search<G> (_cmp, element, ref prev);
+		if (prev == _head) {
+			return null;
+		}
+		return prev._data;
+	}
+
+	public override G? higher (G element) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G> prev = _head;
+		Tower<G>? next;
+		if (Tower.search<G> (_cmp, element, ref prev, out next)) {
+			if (!Tower.proceed<G> (_cmp, ref prev, ref next, 0)) {
+				return null;
+			}
+		}
+		if (next == null) {
+			return null;
+		}
+		return next._data;
+	}
+
+	public override G? floor (G element) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G> prev = _head;
+		Tower<G>? next;
+		if (Tower.search<G> (_cmp, element, ref prev, out next)) {
+			return next._data;
+		} else if (prev != _head) {
+			return prev._data;
+		} else {
+			return null;
+		}
+	}
+
+	public override G? ceil (G element) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		Tower<G> prev = _head;
+		Tower<G>? next;
+		Tower.search<G> (_cmp, element, ref prev, out next);
+		if (next == null) {
+			return null;
+		}
+		return next._data;
+	}
+
+	public override SortedSet<G> head_set (G before) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		return new SubSet<G> (new Range<G>.head (this, before));
+	}
+
+	public override SortedSet<G> tail_set (G after) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		return new SubSet<G> (new Range<G>.tail (this, after));
+	}
+	public override SortedSet<G> sub_set (G from, G to) {
+		HazardPointer.Context ctx = new HazardPointer.Context ();
+		return new SubSet<G> (new Range<G> (this, from, to));
+	}
+
+	private unowned G max (G a, G b, out bool changed = null) {
+		changed = _cmp (a, b) < 0;
+		return changed ? b : a;
+	}
+
+	private unowned G min (G a, G b, out bool changed = null) {
+		changed = _cmp (a, b) > 0;
+		return changed ? b : a;
+	}
+
 #if DEBUG
 	public void dump () {
 		for (int i = _MAX_HEIGHT - 1; i >= 0; i--) {
@@ -103,6 +224,7 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 					printed = true;
 				}
 				stderr.printf("    Node %p%s - %s\n", curr, state == State.NONE ? "" : state == State.MARKED ? " (MARKED)" : " (FLAGGED)", (string)curr._data);
+				assert (curr._height > i);
 			}
 		}
 	}
@@ -124,34 +246,32 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 			assert (_curr != null);
 		}
 
-		public Iterator.pointing (ConcurrentSet cset, Tower<G>[] prev, Tower<G> curr) {
-			for (int i = 0; i < _MAX_HEIGHT; i++) {
-				_prev[i] = prev[i];
-			}
+		public Iterator.point_at (ConcurrentSet cset, ref TowerIter<G> prev, Tower<G> curr) {
 			_curr = curr;
 			_set = cset;
+			_prev = prev;
 			assert (_curr != null);
 		}
 
 		public new bool foreach (ForallFunc<G> f) {
 			assert (_curr != null);
 			HazardPointer.Context ctx = new HazardPointer.Context ();
-			if (_prev[0] != null && !_removed) {
+			if (_prev._iter[0] != null && !_removed) {
 				if (!f (_curr._data)) {
 					assert (_curr != null);
 					return false;
 				}
 			}
-			Tower<G> new_prev = _prev[0];
+			Tower<G> new_prev = _prev._iter[0];
 			Tower<G>? new_curr = _curr;
 			while (Tower.proceed<G> (_set._cmp, ref new_prev, ref new_curr, 0)) {
 				assert (_curr != null);
 				if (!_removed) {
 					//FIXME: Help mark/delete on the way
-					_prev[0] = new_prev;
-					int prev_height = GLib.AtomicInt.get(ref _prev[0]._height);
+					_prev._iter[0] = new_prev;
+					int prev_height = _prev._iter[0].get_height();
 					for (int i = 1; i < prev_height; i++) {
-						_prev[i] = _prev[0];
+						_prev._iter[i] = _prev._iter[0];
 					}
 				}
 				_curr = new_curr;
@@ -167,16 +287,16 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 
 		public bool next () {
 			HazardPointer.Context ctx = new HazardPointer.Context ();
-			Tower<G> new_prev = _prev[0];
+			Tower<G>? new_prev = _prev._iter[0];
 			Tower<G>? new_curr = _curr;
 			bool success = Tower.proceed<G> (_set._cmp, ref new_prev, ref new_curr, 0);
 			if (success) {
 				if (!_removed) {
 					//FIXME: Help mark/delete on the way
-					_prev[0] = (owned)new_prev;
-					int prev_height = GLib.AtomicInt.get(ref _prev[0]._height);
+					_prev._iter[0] = (owned)new_prev;
+					int prev_height = _prev._iter[0].get_height();
 					for (int i = 1; i < prev_height; i++) {
-						_prev[i] = _prev[0];
+						_prev._iter[i] = _prev._iter[0];
 					}
 				}
 				_curr = (owned)new_curr;
@@ -188,7 +308,7 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 
 		public bool has_next () {
 			HazardPointer.Context ctx = new HazardPointer.Context ();
-			Tower<G> prev = _prev[0];
+			Tower<G> prev = _prev._iter[0];
 			Tower<G>? curr = _curr;
 			return Tower.proceed<G> (_set._cmp, ref prev, ref curr, 0);
 		}
@@ -201,22 +321,746 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 		public void remove () {
 			HazardPointer.Context ctx = new HazardPointer.Context ();
 			assert (valid);
-			if (Tower.remove<G> (_set._cmp, _prev, _curr)) {
+			if (Tower.remove<G> (_set._cmp, ref _prev, _curr)) {
 				AtomicInt.dec_and_test (ref _set._size);
 			}
 			_removed = true;
 		}
 
-		public bool valid { get { return _prev[0] != null && !_removed; } }
+		public bool valid { get { return _prev._iter[0] != null && !_removed; } }
 
 		public bool read_only { get { return true; } }
 
 		private bool _removed = false;
 		private ConcurrentSet<G> _set;
-		private Tower<G>? _prev[31 /*_MAX_HEIGHT*/];
+		private TowerIter<G> _prev;
 		private Tower<G> _curr;
 	}
 
+	private class SubSet<G> : AbstractSortedSet<G> {
+		public override int size {
+			get {
+				HazardPointer.Context ctx = new HazardPointer.Context ();
+				Tower<G>? curr;
+				Range.improve_bookmark<G> (_range, out curr);
+				if (curr != null) {
+					int acc = 1;
+					Tower<G>? prev = HazardPointer.get_pointer<Tower<G>> (&_range._bookmark._iter[0]);
+					while (Range.proceed<G> (_range, ref prev, ref curr, 0)) {
+						acc++;
+					}
+					return acc;
+				} else {
+					return 0;
+				}
+			}
+		}
+
+		public bool is_empty {
+			get {
+				HazardPointer.Context ctx = new HazardPointer.Context ();
+				Tower<G>? curr;
+				Range.improve_bookmark<G> (_range, out curr);
+				return curr != null;
+			}
+		}
+
+		public override bool read_only { get { return false; } }
+
+		public SubSet (Range<G> range) {
+			_range = range;
+		}
+
+		public override Gee.Iterator<G> iterator () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			return new SubIterator<G> (_range);
+		}
+
+		public override bool contains (G item) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (!Range.inside<G> (_range, item)) {
+				return false;
+			}
+			TowerIter<G> prev;
+			Range.improve_bookmark<G> (_range, null, out prev);
+			return Tower.search_from_bookmark<G> (_range._set._cmp, item, ref prev);
+		}
+
+		public override bool add (G key) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (!Range.inside<G> (_range, key)) {
+				return false;
+			}
+			TowerIter<G> prev;
+			Range.improve_bookmark<G> (_range, null, out prev);
+			Rand *rnd = ConcurrentSet.rand.get ();
+			if (rnd == null) {
+				rand.set (rnd = new Rand ());
+			}
+			uint32 rand_int = rnd->int_range (0, int32.MAX);
+			uint8 height = 1 + (uint8)GLib.Bit.nth_lsf (~rand_int, -1);
+			if (Tower.search_from_bookmark<G> (_range._set._cmp, key, ref prev, null, height - 1)) {
+				return false;
+			}
+			for (int i = height - 2; i >= 0; i--) {
+				prev._iter[i] = prev._iter[height - 1];
+			}
+			Tower<G>? result = Tower.insert<G> (_range._set._cmp, ref prev, key, height - 1);
+			if (result != null) {
+				GLib.AtomicInt.inc (ref _range._set._size);
+			}
+			return result != null;
+		}
+
+		public override bool remove (G key) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (!Range.inside<G> (_range, key)) {
+				return false;
+			}
+			TowerIter<G> prev;
+			Range.improve_bookmark<G> (_range, null, out prev);
+			// FIXME: Use full bookmark
+			bool result = Tower.remove_key<G> (_range._set._cmp, ref prev, key);
+			if (result) {
+				GLib.AtomicInt.dec_and_test (ref _range._set._size);
+			}
+			return result;
+		}
+
+		public override void clear () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			TowerIter<G> prev;
+			Tower<G>? first;
+			Range.improve_bookmark<G> (_range, out first, out prev);
+			while (first != null) {
+				Tower.remove<G> (_range._set._cmp, ref prev, first);
+				Range.improve_bookmark<G> (_range, out first, out prev);
+			}
+		}
+
+		public override G? first () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			Tower<G>? first;
+			Range.improve_bookmark<G> (_range, out first);
+			if (first == null) {
+				return null;
+			}
+			return first._data;
+		}
+
+		public override G? last () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			TowerIter<G> prev;
+			Range.improve_bookmark<G> (_range, null, out prev);
+			Tower<G>? curr = null;
+			for (int i = _MAX_HEIGHT - 1; i >= 0; i--) {
+				if (curr == null) {
+					curr = prev._iter[i].get_next ((uint8)i);
+					if (curr == null || !Range.inside<G> (_range, curr._data)) {
+						curr = null;
+						continue;
+					}
+				}
+				bool improved = false;
+				while (Range.proceed<G> (_range, ref prev._iter[i], ref curr, (uint8)i)) {
+					improved = true;
+				}
+				if (improved && i > 0) {
+					prev._iter[i - 1] = prev._iter[i];
+				}
+			}
+			if (curr == null) {
+				return null;
+			}
+			return curr._data;
+		}
+
+		public override Gee.Iterator<G>? iterator_at (G element) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (!Range.inside<G> (_range, element)) {
+				return null;
+			}
+			TowerIter<G> prev;
+			TowerIter<G> next;
+			Range.improve_bookmark<G> (_range, null, out prev);
+			if (!Tower.search_from_bookmark<G> (_range._set._cmp, element, ref prev, out next)) {
+				return null;
+			}
+			return new SubIterator<G>.point_at (_range, ref prev, next._iter[0]);
+		}
+
+		public override G? lower (G element) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			switch (Range.cmp<G> (_range, element)) {
+			case Range.Position.BEFORE:
+			case Range.Position.EMPTY:
+				return null;
+			case Range.Position.INSIDE:
+				TowerIter<G> prev;
+				Range.improve_bookmark<G> (_range, null, out prev);
+				Tower.search_from_bookmark<G> (_range._set._cmp, element, ref prev);
+				if (prev._iter[0] == _range._set._head || !Range.inside (_range, prev._iter[0]._data)) {
+					return null;
+				}
+				return prev._iter[0]._data;
+			case Range.Position.AFTER:
+				return last ();
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public override G? higher (G element) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			switch (Range.cmp<G> (_range, element)) {
+			case Range.Position.BEFORE:
+				return first ();
+			case Range.Position.INSIDE:
+				TowerIter<G> prev;
+				TowerIter<G> curr;
+				Range.improve_bookmark<G> (_range, null, out prev);
+				if (Tower.search_from_bookmark<G> (_range._set._cmp, element, ref prev, out curr)) {
+					if (!Tower.proceed<G> (_range._set._cmp, ref prev._iter[0], ref curr._iter[0], 0)) {
+						return null;
+					}
+				}
+				if (curr._iter[0] == null || !Range.inside(_range, curr._iter[0]._data)) {
+					return null;
+				}
+				return curr._iter[0]._data;
+			case Range.Position.AFTER:
+			case Range.Position.EMPTY:
+				return null;
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public override G? floor (G element) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			switch (Range.cmp<G> (_range, element)) {
+			case Range.Position.BEFORE:
+			case Range.Position.EMPTY:
+				return null;
+			case Range.Position.INSIDE:
+				TowerIter<G> curr;
+				TowerIter<G> prev;
+				Range.improve_bookmark<G> (_range, null, out prev);
+				if (!Tower.search_from_bookmark<G> (_range._set._cmp, element, ref prev, out curr)) {
+					curr._iter[0] = (owned)prev._iter[0];
+				}
+				if (curr._iter[0] == null || curr._iter[0].is_head () || !Range.inside(_range, curr._iter[0]._data)) {
+					return null;
+				}
+				return curr._iter[0]._data;
+			case Range.Position.AFTER:
+				return last ();
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public override G? ceil (G element) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			switch (Range.cmp<G> (_range, element)) {
+			case Range.Position.BEFORE:
+				return first ();
+			case Range.Position.INSIDE:
+				TowerIter<G> curr;
+				TowerIter<G> prev;
+				Range.improve_bookmark<G> (_range, null, out prev);
+				Tower.search_from_bookmark<G> (_range._set._cmp, element, ref prev, out curr);
+				if (curr._iter[0] == null || !Range.inside(_range, curr._iter[0]._data)) {
+					return null;
+				}
+				return curr._iter[0]._data;
+			case Range.Position.AFTER:
+			case Range.Position.EMPTY:
+				return null;
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public override SortedSet<G> head_set (G before) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			return new SubSet<G> (Range.cut_tail (_range, before));
+		}
+
+		public override SortedSet<G> tail_set (G after) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			return new SubSet<G> (Range.cut_head (_range, after));
+		}
+
+		public override SortedSet<G> sub_set (G from, G to) {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			return new SubSet<G> (Range.cut (_range, from, to));
+		}
+
+		private Range<G> _range;
+	}
+
+	private class SubIterator<G> : Object, Traversable<G>, Gee.Iterator<G> {
+		public SubIterator (Range<G> range) {
+			Range.improve_bookmark<G> (range);
+			_range = range;
+		}
+
+		public SubIterator.point_at (Range<G> range, ref TowerIter<G> prev, owned Tower<G> curr) {
+			Range.improve_bookmark<G> (range);
+			_range = range;
+			_prev = prev;
+			_curr = curr;
+		}
+
+		public new bool foreach (ForallFunc<G> f) {
+			assert (_curr != null);
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (!begin ()) {
+				return true;
+			}
+			if (_prev._iter[0] != null && !_removed) {
+				if (!f (_curr._data)) {
+					assert (_curr != null);
+					return false;
+				}
+			}
+			Tower<G> new_prev = _prev._iter[0];
+			Tower<G>? new_curr = _curr;
+			while (Range.proceed<G> (_range, ref new_prev, ref new_curr, 0)) {
+				assert (_curr != null);
+				if (!f (_curr._data)) {
+					assert (_curr != null);
+					return false;
+				}
+				if (!_removed) {
+					//FIXME: Help mark/delete on the way
+					_prev._iter[0] = (owned)new_prev;
+					int prev_height = _prev._iter[0].get_height();
+					for (int i = 1; i < prev_height; i++) {
+						_prev._iter[i] = _prev._iter[0];
+					}
+				}
+				_curr = (owned)new_curr;
+				_removed = false;
+			}
+			assert (_curr != null);
+			return true;
+		}
+
+		public bool next () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (_prev._iter[0] == null) {
+				return begin ();
+			} else {
+				Tower<G> new_prev = _prev._iter[0];
+				Tower<G> new_curr = _curr;
+				if (Range.proceed<G> (_range, ref new_prev, ref new_curr, 0)) {
+					if (!_removed) {
+						//FIXME: Help mark/delete on the way
+						_prev._iter[0] = (owned)new_prev;
+						int prev_height = _prev._iter[0].get_height();
+						for (int i = 1; i < prev_height; i++) {
+							_prev._iter[i] = _prev._iter[0];
+						}
+					}
+					_curr = (owned)new_curr;
+					_removed = false;
+					return true;
+				} else {
+					return false;
+				}
+			}
+		}
+
+		public bool has_next () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			if (_prev._iter[0] == null) {
+				Tower<G> next;
+				Range.improve_bookmark<G> (_range, out next);
+				if (next != null && Range.beyond<G> (_range, next)) {
+					next = null;
+				}
+				return next != null;
+			} else {
+				Tower<G> new_prev = _prev._iter[0];
+				Tower<G> new_curr = _curr;
+				return Range.proceed<G> (_range, ref new_prev, ref new_curr, 0);
+			}
+		}
+
+		public new G get () {
+			assert (valid);
+			return _curr._data;
+		}
+
+		public void remove () {
+			HazardPointer.Context ctx = new HazardPointer.Context ();
+			assert (valid);
+			if (Tower.remove<G> (_range._set._cmp, ref _prev, _curr)) {
+				AtomicInt.dec_and_test (ref _range._set._size);
+			}
+			_removed = true;
+		}
+
+		public bool valid {
+			get {
+				bool is_valid = _prev._iter[0] != null && !_removed;
+				assert (!is_valid || _curr != null);
+				return is_valid;
+			}
+		}
+
+		public bool read_only { get { return false; } }
+
+		private bool begin () {
+			if (_prev._iter[0] != null) {
+				return true;
+			}
+			Range.improve_bookmark<G> (_range, out _curr, out _prev);
+			if (_curr == null) {
+				for (int i = 0; i < _MAX_HEIGHT; i++) {
+					_prev._iter[i] = null;
+				}
+			}
+			return _curr != null;
+		}
+
+		private Range<G> _range;
+		private TowerIter<G> _prev;
+		private Tower<G>? _curr = null;
+		private bool _removed = false;
+	}
+
+	private class Range<G> {
+		public G? _start;
+		public G? _end;
+		public RangeType _type;
+		public TowerIter<G> _bookmark;
+		public ConcurrentSet<G> _set;
+
+		public Range (ConcurrentSet<G> cset, G start, G end) {
+			_start = start;
+			_end = end;
+			if (cset._cmp(start, end) < 0) {
+				_type = RangeType.BOUNDED;
+				for (int i = 0; i < _MAX_HEIGHT; i++) {
+					_bookmark._iter[i] = cset._head;
+				}
+			} else {
+				_type = RangeType.EMPTY;
+			}
+			_set = cset;
+		}
+
+		public Range.head (ConcurrentSet<G> cset, G end) {
+			_end = end;
+			_type = RangeType.HEAD;
+			for (int i = 0; i < _MAX_HEIGHT; i++) {
+				_bookmark._iter[i] = cset._head;
+			}
+			_set = cset;
+		}
+
+		public Range.tail (ConcurrentSet<G> cset, G start) {
+			_start = start;
+			_type = RangeType.TAIL;
+			for (int i = 0; i < _MAX_HEIGHT; i++) {
+				_bookmark._iter[i] = cset._head;
+			}
+			_set = cset;
+		}
+
+		public Range.empty (ConcurrentSet<G> cset) {
+			_type = RangeType.EMPTY;
+			_set = cset;
+		}
+
+		private void copy_bookmark (Range<G> range) {
+			for (int i = 0; i < _MAX_HEIGHT; i++) {
+				_bookmark._iter[i] = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[i]);
+			}
+		}
+
+		public static Range<G> cut_head<G> (Range<G> from, G start) {
+			Range<G> result = new Range<G>.empty (from._set);
+			switch (from._type) {
+			case RangeType.HEAD:
+				if (from._set._cmp (start, from._end) < 0) {
+					result._start = start;
+					result._end = from._end;
+					result._type = RangeType.BOUNDED;
+				} else {
+					result._type = RangeType.EMPTY;
+				}
+				break;
+			case RangeType.TAIL:
+				result._start = from._set.max (from._start, start);
+				result._type = RangeType.TAIL;
+				break;
+			case RangeType.BOUNDED:
+				if (from._set._cmp (from._start, start) < 0) {
+					result._start = from._set.max (from._start, start);
+					result._end = from._end;
+					result._type = RangeType.BOUNDED;
+				} else {
+					result._type = RangeType.EMPTY;
+				}
+				break;
+			case RangeType.EMPTY:
+				result._type = RangeType.EMPTY;
+				break;
+			default:
+				assert_not_reached ();
+			}
+			if (result._type != RangeType.EMPTY) {
+				improve_bookmark<G> (from);
+				result.copy_bookmark (from);
+				improve_bookmark<G> (result);
+			}
+			return result;
+		}
+
+		public static Range<G> cut_tail<G> (Range<G> from, G end) {
+			Range<G> result = new Range<G>.empty (from._set);
+			switch (from._type) {
+			case RangeType.HEAD:
+				result._end = from._set.min (from._end, end);
+				result._type = RangeType.HEAD;
+				break;
+			case RangeType.TAIL:
+				if (from._set._cmp (from._start, end) < 0) {
+					result._start = from._start;
+					result._end = end;
+					result._type = RangeType.BOUNDED;
+				} else {
+					result._type = RangeType.EMPTY;
+				}
+				break;
+			case RangeType.BOUNDED:
+				if (from._set._cmp (from._start, end) < 0) {
+					result._start = from._start;
+					result._end = from._set.min (from._end, end);
+					result._type = RangeType.BOUNDED;
+				} else {
+					result._type = RangeType.EMPTY;
+				}
+				break;
+			case RangeType.EMPTY:
+				result._type = RangeType.EMPTY;
+				break;
+			default:
+				assert_not_reached ();
+			}
+			if (result._type != RangeType.EMPTY) {
+				improve_bookmark<G> (from);
+				result.copy_bookmark (from);
+				improve_bookmark<G> (result);
+			}
+			return result;
+		}
+
+		public static Range<G> cut<G> (Range<G> from, G start, G end) {
+			Range<G> result = new Range<G>.empty (from._set);
+			result._type = RangeType.BOUNDED;
+			switch (from._type) {
+			case RangeType.HEAD:
+				end = from._set.min (from._end, end);
+				break;
+			case RangeType.TAIL:
+				start = from._set.max (from._start, start);
+				break;
+			case RangeType.BOUNDED:
+				start = from._set.max (from._start, start);
+				end = from._set.min (from._end, end);
+				break;
+			case RangeType.EMPTY:
+				result._type = RangeType.EMPTY;
+				break;
+			default:
+				assert_not_reached ();
+			}
+			if (result._type != RangeType.EMPTY && from._set._cmp (start, end) < 0) {
+				result._start = start;
+				result._end = end;
+				result._type = RangeType.BOUNDED;
+			} else {
+				result._type = RangeType.EMPTY;
+			}
+			if (result._type != RangeType.EMPTY) {
+				improve_bookmark<G> (from);
+				result.copy_bookmark (from);
+				improve_bookmark<G> (result);
+			}
+			return result;
+		}
+
+		public static void improve_bookmark<G> (Range<G> range, out Tower<G>? out_curr = null, out TowerIter<G> prev = null) {
+			prev = TowerIter<G>();
+			switch (range._type) {
+			case RangeType.HEAD:
+				if (&out_curr != null) {
+					out_curr = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[0]);
+					if (&prev != null) {
+						prev._iter[0] = (owned)out_curr;
+						out_curr = prev._iter[0].get_next (0);
+					} else {
+						out_curr = out_curr.get_next (0);
+					}
+				}
+				if (&prev != null) {
+					for (int i = &out_curr != null ? 1 : 0; i < _MAX_HEIGHT; i++) {
+						prev._iter[i] = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[i]);
+					}
+				}
+				break;
+			case RangeType.EMPTY:
+				out_curr = null;
+				break;
+			case RangeType.TAIL:
+			case RangeType.BOUNDED:
+				Tower<G>? last_best = null;
+				for (int i = _MAX_HEIGHT - 1; i >= 0; i--) {
+					Tower<G> curr = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[i]);
+					Tower<G> curr_old = curr;
+					assert (curr != null);
+					Tower.backtrace<G> (ref curr, (uint8)i);
+					if (last_best != null && last_best != curr && Tower.compare<G> (range._set._cmp, curr, last_best) < 0) {
+						curr = last_best;
+					}
+					if (curr != curr_old) {
+						if (!HazardPointer.compare_and_exchange_pointer<Tower<G>> (&range._bookmark._iter[i], curr_old, curr)) {
+							curr = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[i]);
+						}
+					}
+					Tower<G> next = curr.get_next ((uint8)i);
+					if (&out_curr != null && i == 0) {
+						out_curr = next;
+					}
+					while (next != null && Tower.compare_data<G> (range._set._cmp, next, range._start) < 0) {
+						Tower.proceed<G> (range._set._cmp, ref curr, ref next, (uint8)i, true);
+						if (&curr != null && i == 0 && next != null) {
+							out_curr = next;
+						}
+						if (Tower.compare_data<G> (range._set._cmp, curr, range._start) < 0) {
+							if (!HazardPointer.compare_and_exchange_pointer<Tower<G>> (&range._bookmark._iter[i], curr_old, curr)) {
+								curr = HazardPointer.get_pointer<Tower<G>> (&range._bookmark._iter[i]);
+							}
+							curr_old = curr;
+						} else {
+							break;
+						}
+					}
+					if (&prev != null) {
+						prev._iter[i] = curr;
+					}
+					last_best = (owned)curr;
+				}
+				break;
+			default:
+				assert_not_reached ();
+			}
+			if (&out_curr != null && out_curr != null && Range.beyond<G> (range, out_curr)) {
+				out_curr = null;
+			}
+#if DEBUG
+			stderr.printf("Bookmark after:\n");
+			for (int i = _MAX_HEIGHT - 1; i >= 0; i--) {
+				stderr.printf("    Level %d:\n", i);
+				Tower<string>? t = HazardPointer.get_pointer<Tower<string>> (&range._bookmark._iter[i]);
+				assert (t == null || t.get_height () > i);
+				while (t != null) {
+					stderr.printf("      %s\n", t.is_head () ? "<<head>>" : t._data);
+					t = t.get_next ((uint8)i);
+					assert (t == null || t.get_height () > i);
+				}
+			}
+#endif
+		}
+
+		public static bool proceed<G> (Range<G> range, ref Tower<G>? prev, ref Tower<G> curr, uint8 level) {
+			switch (range._type) {
+			case RangeType.EMPTY:
+				return false;
+			case RangeType.TAIL:
+				return Tower.proceed<G> (range._set._cmp, ref prev, ref curr, level);
+			case RangeType.HEAD:
+			case RangeType.BOUNDED:
+				Tower<G>? tmp_prev = prev;
+				Tower<G>? tmp_curr = curr;
+				if (!Tower.proceed<G> (range._set._cmp, ref tmp_prev, ref tmp_curr, level)) {
+					return false;
+				} else if (Tower.compare_data<G> (range._set._cmp, tmp_curr, range._end) >= 0) {
+					return false;
+				} else {
+					prev = (owned)tmp_prev;
+					curr = (owned)tmp_curr;
+					return true;
+				}
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public static bool inside<G> (Range<G> range, G val) {
+			switch (range._type) {
+			case RangeType.HEAD:
+				return range._set._cmp (val, range._end) < 0;
+			case RangeType.TAIL:
+				return range._set._cmp (val, range._start) >= 0;
+			case RangeType.BOUNDED:
+				return range._set._cmp (val, range._start) >= 0 && range._set._cmp (val, range._end) < 0;
+			case RangeType.EMPTY:
+				return false;
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public static bool beyond<G> (Range<G> range, Tower<G> tw) {
+			switch (range._type) {
+			case RangeType.EMPTY:
+				return true;
+			case RangeType.TAIL:
+				return false;
+			case RangeType.HEAD:
+			case RangeType.BOUNDED:
+				return Tower.compare_data<G> (range._set._cmp, tw, range._end) >= 0;
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		public static int cmp<G> (Range<G> range, G val) {
+			switch (range._type) {
+			case RangeType.HEAD:
+				return range._set._cmp (val, range._end) < 0 ? Position.INSIDE : Position.AFTER;
+			case RangeType.TAIL:
+				return range._set._cmp (val, range._start) >= 0 ? Position.INSIDE : Position.BEFORE;
+			case RangeType.BOUNDED:
+				return range._set._cmp (val, range._start) >= 0 ? (range._set._cmp (val, range._end) < 0 ? Position.INSIDE : Position.AFTER) : Position.BEFORE;
+			case RangeType.EMPTY:
+				return Position.EMPTY;
+			default:
+				assert_not_reached ();
+			}
+		}
+
+		enum Position {
+			BEFORE = -1,
+			INSIDE = 0,
+			AFTER = 1,
+			EMPTY
+		}
+	}
+
+	public enum RangeType {
+		HEAD,
+		TAIL,
+		BOUNDED,
+		EMPTY
+	}
+
 	private class Tower<G> {
 		public inline Tower (G data, uint8 height) {
 			_nodes = new TowerNode<G>[height];
@@ -227,18 +1071,22 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 
 		public inline Tower.head () {
 			_nodes = new TowerNode<G>[_MAX_HEIGHT];
-			_height = _MAX_HEIGHT;
+			_height = -1;
+#if DEBUG
+			_data = (G)"<<head>>";
+#endif
 		}
 
 		inline ~Tower () {
-			for (uint i = 0; i < _height; i++) {
-				HazardPointer<Tower<G>>.set_pointer (&_nodes[i]._succ, null);
-				HazardPointer<Tower<G>>.set_pointer (&_nodes[i]._backlink, null);
+			uint8 height = get_height();
+			for (uint8 i = 0; i < height; i++) {
+				set_succ (null, State.NONE, i);
+				set_backlink (null, i);
 			}
 			_nodes = null;
 		}
 
-		public static inline bool search<G> (CompareDataFunc<G>? cmp, G key, ref Tower<G> prev, out Tower<G>? next, uint8 to_level = 0, uint8 from_level = (uint8)_MAX_HEIGHT - 1) {
+		public static inline bool search<G> (CompareDataFunc<G>? cmp, G key, ref Tower<G> prev, out Tower<G>? next = null, uint8 to_level = 0, uint8 from_level = (uint8)_MAX_HEIGHT - 1) {
 			assert (from_level >= to_level);
 			bool res = false;
 			next = null;
@@ -248,25 +1096,47 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 			return res;
 		}
 
+		public static inline bool search_from_bookmark<G> (CompareDataFunc<G>? cmp, G key, ref TowerIter<G> prev, out TowerIter<G> next = null, uint8 to_level = 0, uint8 from_level = (uint8)_MAX_HEIGHT - 1) {
+			assert (from_level >= to_level);
+			bool res = false;
+			for (int i = from_level; i >= to_level; i--) {
+				unowned Tower<G> tmp_prev = prev._iter[i]; // Should be treated as NULL-like value
+				Tower<G> tmp_next;
+				res = search_helper<G> (cmp, key, ref prev._iter[i], out tmp_next, (uint8)i);
+				if (&next != null) {
+					next._iter[i] = (owned)tmp_next;
+				}
+				if (prev._iter[i] != tmp_prev) {
+					prev._iter[i] = prev._iter[i];
+					if (i > to_level && compare<G> (cmp, prev._iter[i - 1], prev._iter[i]) <= 0) {
+						prev._iter[i - 1] = prev._iter[i];
+					}
+				}
+			}
+			return res;
+		}
+
 		private static inline bool search_helper<G> (CompareDataFunc<G>? cmp, G key, ref Tower<G>? prev, out Tower<G>? next, uint8 level) {
 			next = prev.get_next (level);
-			while (next != null && cmp(key, next._data) < 0 && proceed<G> (cmp, ref prev, ref next, level, true));
+			while (next != null && compare_data<G> (cmp, next, key) < 0 && proceed<G> (cmp, ref prev, ref next, level, true));
 			return next != null && cmp(key, next._data) == 0;
 		}
 
-		public static inline Tower<G>? insert<G> (CompareDataFunc<G>? cmp, ref Tower<G> prev, G key, uint8 chosen_level) {
+		public static inline Tower<G>? insert<G> (CompareDataFunc<G>? cmp, ref TowerIter<G> prev, G key, uint8 chosen_level) {
 			return insert_helper<G> (cmp, ref prev, key, chosen_level, chosen_level);
 		}
 
-		private static inline Tower<G>? insert_helper<G> (CompareDataFunc<G>? cmp, ref Tower<G> prev, G key, uint8 chosen_level, uint8 level) {
+		private static inline Tower<G>? insert_helper<G> (CompareDataFunc<G>? cmp, ref TowerIter<G> prev, G key, uint8 chosen_level, uint8 level) {
 			Tower<G>? new_tower;
 			Tower<G>? next;
-			if (search_helper (cmp, key, ref prev, out next, level)) {
+			if (search_helper (cmp, key, ref prev._iter[level], out next, level)) {
 				return null;
 			}
 			if (level > 0) {
-				Tower<G> prev_down = prev;
-				new_tower = insert_helper<G> (cmp, ref prev_down, key, chosen_level, level - 1);
+				if (compare<G> (cmp, prev._iter[level], prev._iter[level - 1]) >= 0) {
+					prev._iter[level - 1] = prev._iter[level];
+				}
+				new_tower = insert_helper<G> (cmp, ref prev, key, chosen_level, level - 1);
 			} else {
 				new_tower = new Tower<G> (key, chosen_level + 1);
 			}
@@ -275,53 +1145,52 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 			}
 			while (true) {
 				State prev_state;
-				Tower<G>? prev_next = prev.get_succ (out prev_state, level);
+				Tower<G>? prev_next = prev._iter[level].get_succ (out prev_state, level);
 				if (prev_state == State.FLAGGED) {
-					prev_next.help_flagged (prev, level);
+					prev_next.help_flagged (prev._iter[level], level);
 				} else {
 					new_tower.set_succ (next, State.NONE, level);
-					bool result = prev.compare_and_exchange (next, State.NONE, new_tower, State.NONE, level);
+					bool result = prev._iter[level].compare_and_exchange (next, State.NONE, new_tower, State.NONE, level);
 					if (result)
 						break;
-					prev_next = prev.get_succ (out prev_state, level);
+					prev_next = prev._iter[level].get_succ (out prev_state, level);
 					if (prev_state == State.FLAGGED) {
-						prev_next.help_flagged (prev, level);
+						prev_next.help_flagged (prev._iter[level], level);
 					}
-					backtrace<G> (ref prev, level);
+					backtrace<G> (ref prev._iter[level], level);
 				}
-				if (search_helper (cmp, key, ref prev, null, level)) {
+				if (search_helper (cmp, key, ref prev._iter[level], null, level)) {
 					return null;
 				}
 			}
 			GLib.AtomicInt.inc (ref new_tower._height);
 			if (new_tower.get_state (0) == State.MARKED) {
-				remove_level (cmp, ref prev, new_tower, level);
+				remove_level (cmp, ref prev._iter[level], new_tower, level);
 				return null;
 			}
 			return new_tower;
 		}
 
-		public static inline bool remove_key<G> (CompareDataFunc<G>? cmp, Tower<G> arg_prev, G key, uint8 from_level = (uint8)_MAX_HEIGHT - 1) {
-			Tower<G> prev[31 /*_MAX_HEIGHT*/];
-			prev[from_level] = arg_prev;
+		public static inline bool remove_key<G> (CompareDataFunc<G>? cmp, ref TowerIter<G> prev, G key, uint8 from_level = (uint8)_MAX_HEIGHT - 1) {
 			for (int i = from_level; i >= 1; i--) {
 				Tower<G> next;
-				search_helper<G> (cmp, key, ref prev[i], out next, (uint8)i);
-				prev[i - 1] = prev[i];
+				search_helper<G> (cmp, key, ref prev._iter[i], out next, (uint8)i);
+				if (compare<G> (cmp, prev._iter[i - 1], prev._iter[i]) < 0) {
+					prev._iter[i - 1] = prev._iter[i];
+				}
 			}
 			Tower<G>? curr;
-			if (search_helper<G> (cmp, key, ref prev[0], out curr, 0)) {
-				return remove<G> (cmp, prev, curr);
+			if (search_helper<G> (cmp, key, ref prev._iter[0], out curr, 0)) {
+				return remove<G> (cmp, ref prev, curr);
 			} else {
 				return false;
 			}
 		}
 
-		public static inline bool remove<G> (CompareDataFunc<G>? cmp, Tower<G>[] prev, Tower<G> curr) {
-			assert (prev.length >= AtomicInt.get (ref curr._height));
-			bool removed = remove_level (cmp, ref prev[0], curr, 0);
-			for (int i = 1; i < prev.length; i++) {
-				remove_level (cmp, ref prev[i], curr, (uint8)i);
+		public static inline bool remove<G> (CompareDataFunc<G>? cmp, ref TowerIter<G> prev, Tower<G> curr) {
+			bool removed = remove_level (cmp, ref prev._iter[0], curr, 0);
+			for (int i = 1; i < _MAX_HEIGHT; i++) {
+				remove_level (cmp, ref prev._iter[i], curr, (uint8)i);
 			}
 			return removed;
 		}
@@ -339,7 +1208,7 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 			Tower<G> curr = arg_curr;
 			Tower<G>? next = curr.get_next (level);
 			if (next != null) {
-				while (next.get_state (0) == State.MARKED) {
+				while (next != null && next.get_state (0) == State.MARKED) {
 					bool status;
 					next.try_flag (cmp, ref curr, out status, level);
 					if (status) {
@@ -446,6 +1315,27 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 			HazardPointer.compare_and_exchange_pointer<Tower<G>?> (&_nodes[level]._backlink, null, backlink);
 		}
 
+		public inline int get_height () {
+			int height = GLib.AtomicInt.get (ref _height);
+			return height != -1 ? height : _MAX_HEIGHT;
+		}
+
+		public inline bool is_head () {
+			int height = GLib.AtomicInt.get (ref _height);
+			return height == -1;
+		}
+
+		public inline static int compare<G> (CompareDataFunc<G>? cmp, Tower<G> a, Tower<G> b) {
+			bool ah = GLib.AtomicInt.get (ref a._height) == -1;
+			bool bh = GLib.AtomicInt.get (ref b._height) == -1;
+			return ah ? (bh ? 0 : -1) : (bh ? 1 : cmp(a._data, b._data));
+		}
+
+		public inline static int compare_data<G> (CompareDataFunc<G>? cmp, Tower<G> a, G b) {
+			bool ah = GLib.AtomicInt.get (ref a._height) == -1;
+			return ah ? -1 : cmp(a._data, b);
+		}
+
 		[NoArrayLength]
 		public TowerNode<G>[] _nodes;
 		public G _data;
@@ -457,6 +1347,12 @@ public class Gee.ConcurrentSet<G> : AbstractSet<G> {
 		public Tower<G> *_backlink;
 	}
 
+	private struct TowerIter<G> {
+		[NoArrayLength]
+		public Tower<G>? _iter[31 /*_MAX_HEIGHT*/];
+	}
+
+
 	private enum State {
 		NONE = 0,
 		MARKED = 1,
diff --git a/tests/testconcurrentset.vala b/tests/testconcurrentset.vala
index 39fcfcf..0b23c4e 100644
--- a/tests/testconcurrentset.vala
+++ b/tests/testconcurrentset.vala
@@ -22,9 +22,9 @@
 
 using Gee;
 
-public class ConcurrentSetTests : SetTests {
+public class ConcurrentSetTests : SortedSetTests {
 	public ConcurrentSetTests () {
-		base ("ConcurrentSet");
+		base ("ConcurrentSet", false);
 	}
 
 	public override void set_up () {
diff --git a/tests/testsortedset.vala b/tests/testsortedset.vala
index fdd6e44..00c9bd2 100644
--- a/tests/testsortedset.vala
+++ b/tests/testsortedset.vala
@@ -24,8 +24,9 @@ using GLib;
 using Gee;
 
 public abstract class SortedSetTests : SetTests {
-	public SortedSetTests (string name) {
+	public SortedSetTests (string name, bool strict = true) {
 		base (name);
+		this.strict = strict;
 		add_test ("[SortedSet] first", test_first);
 		add_test ("[SortedSet] last", test_last);
 		add_test ("[SortedSet] ordering", test_ordering);
@@ -34,10 +35,10 @@ public abstract class SortedSetTests : SetTests {
 		add_test ("[SortedSet] higher", test_higher);
 		add_test ("[SortedSet] floor", test_floor);
 		add_test ("[SortedSet] ceil", test_ceil);
-		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.HEAD).get_suite ());
-		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.TAIL).get_suite ());
-		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.SUB).get_suite ());
-		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.EMPTY).get_suite ());
+		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.HEAD, strict).get_suite ());
+		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.TAIL, strict).get_suite ());
+		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.SUB, strict).get_suite ());
+		get_suite ().add_suite (new SubSetTests (this, SubSetTests.Type.EMPTY, strict).get_suite ());
 	}
 
 	public void test_ordering () {
@@ -90,12 +91,14 @@ public abstract class SortedSetTests : SetTests {
 	public void test_first () {
 		var test_set = test_collection as SortedSet<string>;
 
-		if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
-		                       TestTrapFlags.SILENCE_STDERR)) {
-			test_set.first ();
-			Posix.exit (0);
+		if (strict) {
+			if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
+			                       TestTrapFlags.SILENCE_STDERR)) {
+				test_set.first ();
+				Posix.exit (0);
+			}
+			Test.trap_assert_failed ();
 		}
-		Test.trap_assert_failed ();
 
 		assert (test_set.add ("one"));
 		assert (test_set.add ("two"));
@@ -110,12 +113,14 @@ public abstract class SortedSetTests : SetTests {
 	public void test_last () {
 		var test_set = test_collection as SortedSet<string>;
 
-		if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
-		                       TestTrapFlags.SILENCE_STDERR)) {
-			test_set.last ();
-			Posix.exit (0);
+		if (strict) {
+			if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
+			                       TestTrapFlags.SILENCE_STDERR)) {
+				test_set.last ();
+				Posix.exit (0);
+			}
+			Test.trap_assert_failed ();
 		}
-		Test.trap_assert_failed ();
 
 		assert (test_set.add ("one"));
 		assert (test_set.add ("two"));
@@ -267,10 +272,11 @@ public abstract class SortedSetTests : SetTests {
 		}
 		private Type type;
 
-		public SubSetTests (SortedSetTests test, Type type) {
+		public SubSetTests (SortedSetTests test, Type type, bool strict) {
 			base ("%s Subset".printf (type.to_string ()));
 			this.test = test;
 			this.type = type;
+			this.strict = strict;
 			add_test ("[Collection] size", test_size);
 			add_test ("[Collection] contains", test_contains);
 			add_test ("[Collection] add", test_add);
@@ -580,18 +586,20 @@ public abstract class SortedSetTests : SetTests {
 				assert (subset.last () == "six");
 				break;
 			case Type.EMPTY:
-				if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
-				                       TestTrapFlags.SILENCE_STDERR)) {
-					subset.first ();
-					Posix.exit (0);
+				if (strict) {
+					if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
+					                       TestTrapFlags.SILENCE_STDERR)) {
+						subset.first ();
+						Posix.exit (0);
+					}
+					Test.trap_assert_failed ();
+					if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
+					                       TestTrapFlags.SILENCE_STDERR)) {
+						subset.last ();
+						Posix.exit (0);
+					}
+					Test.trap_assert_failed ();
 				}
-				Test.trap_assert_failed ();
-				if (Test.trap_fork (0, TestTrapFlags.SILENCE_STDOUT |
-				                       TestTrapFlags.SILENCE_STDERR)) {
-					subset.last ();
-					Posix.exit (0);
-				}
-				Test.trap_assert_failed ();
 				break;
 			default:
 				assert_not_reached ();
@@ -842,6 +850,10 @@ public abstract class SortedSetTests : SetTests {
 				assert_not_reached ();
 			}
 		}
+
+		private bool strict;
 	}
+
+	private bool strict;
 }
 



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