Sentinel node

In computer programming, a sentinel node is a specifically designated node used with linked lists and trees as a traversal path terminator. This type of node does not hold or reference any data managed by the data structure.

This article is about the computer programming construct. For the body part, see Sentinel lymph node.
Not to be confused with sentinel value.

Benefits

Sentinels are used as an alternative over using NULL as the path terminator in order to get one or more of the following benefits:

  • Marginally increased speed of operations
  • Increased data structure robustness (arguably)

Drawbacks
  • Marginally increased algorithmic complexity and code size.
  • If the data structure is accessed concurrently (which means that all nodes being accessed have to be protected at least for “read-only”), for a sentinel-based implementation the sentinel node has to be protected for “read-write” by a mutex. This extra mutex in quite a few use scenarios can cause severe performance degradation.[1] One way to avoid it is to protect the list structure as a whole for “read-write”, whereas in the version with NULL it suffices to protect the data structure as a whole for “read-only” (if an update operation will not follow).
  • The sentinel concept is not useful for the recording of the data structure on disk.

Examples

Below are two versions of a subroutine (implemented in the C programming language) for looking up a given search key in a singly linked list. The first one uses the sentinel value NULL, and the second one a (pointer to the) sentinel node Sentinel, as the end-of-list indicator. The declarations of the singly linked list data structure and the outcomes of both subroutines are the same.

struct sll_node {                          // one node of the singly linked list
   int key;
   struct sll_node *next;                  // end-of-list indicator or -> next node
} sll, *first;

First version using NULL as an end-of-list indicator

 
// global initialization
first = NULL;                              // before the first insertion (not shown)

struct sll_node *Search(struct sll_node *first, int search_key) {
    struct sll_node *node;
    for (node = first; 
        node != NULL; 
        node = node->next)
    {
        if (node->key == search_key)
            return node;                   // found
    }
    // not found
    return NULL;
}

The for-loop contains two tests (yellow lines) per iteration:

  • node != NULL;
  • if (node->key == search_key).

Second version using a sentinel node

The globally available pointer sentinel to the deliberately prepared data structure Sentinel is used as end-of-list indicator. 

// global variable
sll_node Sentinel, *sentinel = &Sentinel;
// global initialization
sentinel->next = sentinel;
first = sentinel;                          // before the first insertion (not shown)
// Note that the pointer  sentinel  has always to be kept at the END of the list.

struct sll_node *SearchWithSentinelnode(struct sll_node *first, int search_key) {
    struct sll_node *node;
    sentinel->key = search_key;
    for (node = first; 
        node->key != search_key; 
        node = node->next)
    {
    }
    if (node != sentinel)
        return node;                       // found
    // not found
    return NULL;
}

The for-loop contains only one test (yellow line) per iteration:

  • node->key != search_key;.

Linked list implementation

Linked list implementations, especially one of a circular, doubly-linked list, can be simplified remarkably using a sentinel node to demarcate the beginning and end of the list.

  • The list starts out with a single node, the sentinel node which has the next and previous pointers point to itself. This condition determines if the list is empty.
  • In a non-empty list, the sentinel node's next pointer gives the head of the list, and the previous pointer gives the tail of the list.

Following is a Python implementation of a circular doubly-linked list:

class Node:
    def __init__(self, data, next=None, prev=None) -> None:
        self.data = data
        self.next = next
        self.prev = prev

    def __repr__(self) -> str:
        return 'Node(data={})'.format(self.data)

class LinkedList:
    def __init__(self) -> None:
        self._sentinel = Node(data=None)
        self._sentinel.next = self._sentinel
        self._sentinel.prev = self._sentinel

    def pop_left(self):
        return self.remove_by_ref(self._sentinel.next)

    def pop(self):
        return self.remove_by_ref(self._sentinel.prev)

    def append_nodeleft(self, node) -> None:
        self.add_node(self._sentinel, node)

    def append_node(self, node -> None):
        self.add_node(self._sentinel.prev, node)

    def append_left(self, data) -> None:
        node = Node(data=data)
        self.append_nodeleft(node)

    def append(self, data) -> None:
        node = Node(data=data)
        self.append_node(node)

    def remove_by_ref(self, node):
        if node is self._sentinel:
            raise Exception('Can never remove sentinel.')
        node.prev.next = node.next
        node.next.prev = node.prev
        node.prev = None
        node.next = None
        return node

    def add_node(self, curnode, newnode) -> None:
        newnode.next = curnode.next
        newnode.prev = curnode
        curnode.next.prev = newnode
        curnode.next = newnode

    def search(self, value):
        self._sentinel.data = value
        node = self._sentinel.next
        while node.data != value:
            node = node.next
        self._sentinel.data = None
        if node is self._sentinel:
            return None
        return node

    def __iter__(self):
        node = self._sentinel.next
        while node is not self._sentinel:
            yield node.data
            node = node.next

    def reviter(self):
        node = self._sentinel.prev
        while node is not self._sentinel:
            yield node.data
            node = node.prev

Notice how the add_node() method takes the node that will be displaced by the new node in the parameter curnode. For appending to the left, this is the head of a non-empty list, while for appending to right, it is the tail. But because of how the linkage is setup to refer back to the sentinel, the code just works for empty lists as well, where curnode will be the sentinel node.

See also

References
  1. Ignatchenko, Sergey (1998), "STL Implementations and Thread Safety", C++ Report
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