add DLLInsert

util.d

@@ -215,6 +215,36 @@ DLLPush(T, U)(T* list, U* node, U* nil)
 	}
 }
 
+void
+DLLInsert(T, U)(T* list, U* node, U* prev, U* nil)
+{
+	node.next = node.prev = nil;
+
+	if(CheckNil(nil, list.first) && CheckNil(nil, list.last))
+	{
+		assert(CheckNil(nil, prev));
+		list.first = list.last = node;
+	}
+	else if(list.first == prev && list.last == prev)
+	{
+		list.last = node;
+		node.prev = prev;
+		prev.next = node;
+	}
+	else if(list.last == prev)
+	{
+		prev.next = node;
+		node.prev = prev;
+		list.last = node;
+	}
+	else
+	{
+		node.next = prev.next;
+		node.prev = prev;
+		prev.next = node;
+	}
+}
+
 struct Stack(T)
 {
 	Node!(T)* top;
@@ -986,6 +1016,8 @@ unittest
 		DLLRemove(&list, &nodes[1], null);
 
 		TestDLList(&list, res1);
+
+		DLLInsert(&list, &nodes[1], &nodes[0], null);
 	}
 
 	{ // MemCpy