util/IntrusiveList: add method splice()

src/util/IntrusiveList.hxx

@@ -526,4 +526,40 @@ public:
 
 		++counter;
 	}
+
+	/**
+	 * Move the given range of items of the given list to this one
+	 * before the given position.
+	 */
+	void splice(iterator position, IntrusiveList &from,
+		    iterator _begin, iterator _end, size_type n) noexcept {
+		if (_begin == _end)
+			return;
+
+		auto &next_node = ToNode(*position);
+		auto &prev_node = ToNode(*std::prev(position));
+
+		auto &first_node = ToNode(*_begin);
+		auto &before_first_node = ToNode(*std::prev(_begin));
+		auto &last_node = ToNode(*std::prev(_end));
+		auto &after_last_node = ToNode(*_end);
+
+		/* remove from the other list */
+		IntrusiveListNode::Connect(before_first_node, after_last_node);
+		from.counter -= n;
+
+		/* insert into this list */
+		IntrusiveListNode::Connect(prev_node, first_node);
+		IntrusiveListNode::Connect(last_node, next_node);
+		counter += n;
+	}
+
+	/**
+	 * Move all items of the given list to this one before the
+	 * given position.
+	 */
+	void splice(iterator position, IntrusiveList &from) noexcept {
+		spice(position, from, from.begin(), from.end(),
+		      constant_time_size ? size() : 1);
+	}
 };

test/util/TestIntrusiveList.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2021 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2020-2022 Max Kellermann <max.kellermann@gmail.com>
  * All rights reserved.
  *
  * author: Max Kellermann <mk@cm4all.com>
@@ -89,6 +89,60 @@ TEST(IntrusiveList, Basic)
 	ASSERT_EQ(i, list.begin());
 	--i;
 	ASSERT_EQ(i, list.end());
+
+	IntrusiveList<Item> other_list;
+	Item d, e, f, g;
+	other_list.push_back(d);
+	other_list.push_back(e);
+	other_list.push_back(f);
+	other_list.push_back(g);
+
+	list.splice(std::next(list.begin()), other_list,
+		    other_list.iterator_to(e),
+		    other_list.iterator_to(g), 2);
+
+	i = other_list.begin();
+	ASSERT_EQ(&*i, &d);
+	++i;
+	ASSERT_EQ(&*i, &g);
+	++i;
+	ASSERT_EQ(i, other_list.end());
+	++i;
+	ASSERT_EQ(&*i, &d);
+	--i;
+	ASSERT_EQ(i, other_list.end());
+	--i;
+	ASSERT_EQ(&*i, &g);
+	--i;
+	ASSERT_EQ(&*i, &d);
+	ASSERT_EQ(i, other_list.begin());
+
+	i = list.begin();
+	ASSERT_EQ(&*i, &a);
+	++i;
+	ASSERT_EQ(&*i, &e);
+	++i;
+	ASSERT_EQ(&*i, &f);
+	++i;
+	ASSERT_EQ(&*i, &c);
+	++i;
+	ASSERT_EQ(i, list.end());
+	++i;
+	ASSERT_EQ(&*i, &a);
+
+	--i;
+	ASSERT_EQ(i, list.end());
+	--i;
+	ASSERT_EQ(&*i, &c);
+	--i;
+	ASSERT_EQ(&*i, &f);
+	--i;
+	ASSERT_EQ(&*i, &e);
+	--i;
+	ASSERT_EQ(&*i, &a);
+	ASSERT_EQ(i, list.begin());
+	--i;
+	ASSERT_EQ(i, list.end());
 }
 
 TEST(IntrusiveList, SafeLink)