XOR Linked List – A Memory Efficient Doubly Linked List Part 2 - Dieptranivsr/DroneIVSR GitHub Wiki

In this post, we will discuss the implementation of memory-efficient doubly linked list. We will mainly discuss the following two simple functions.

  1. A function to insert a new node at the beginning.
  2. A function to traverse the list in forward direction.

In the following code, insert() function inserts a new node at the beginning. We need to change the head pointer of Linked List, that is why a double pointer is used (See this). Let us first discuss few things again that have been discussed in the previous post. We store XOR of next and previous nodes with every node and we call it npx, which is the only address member we have with every node. When we insert a new node at the beginning, npx of new node will always be XOR of NULL and current head. And npx of the current head must be changed to XOR of new node and node next to the current head.

printList() traverses the list in forward direction. It prints data values from every node. To traverse the list, we need to get pointer to the next node at every point. We can get the address of next node by keeping track of current node and previous node. If we do XOR of curr->npx and prev, we get the address of next node.

/* C++ Implementation of Memory
efficient Doubly Linked List */
#include <bits/stdc++.h>
#include <cinttypes>
using namespace std;

// Node structure of a memory
// efficient doubly linked list
class Node
{
	public:
	int data;
	Node* npx; /* XOR of next and previous node */
};

/* returns XORed value of the node addresses */
Node* XOR (Node *a, Node *b)
{
	return reinterpret_cast<Node *>(
	reinterpret_cast<uintptr_t>(a) ^
	reinterpret_cast<uintptr_t>(b));
}

/* Insert a node at the beginning of the
XORed linked list and makes the newly
inserted node as head */
void insert(Node **head_ref, int data)
{
	// Allocate memory for new node
	Node *new_node = new Node();
	new_node->data = data;

	/* Since new node is being inserted at the
	beginning, npx of new node will always be
	XOR of current head and NULL */
	new_node->npx = *head_ref;

	/* If linked list is not empty, then npx of
	current head node will be XOR of new node
	and node next to current head */
	if (*head_ref != NULL)
	{
		// *(head_ref)->npx is XOR of NULL and next.
		// So if we do XOR of it with NULL, we get next
		(*head_ref)->npx = XOR(new_node, (*head_ref)->npx);
	}

	// Change head
	*head_ref = new_node;
}

// prints contents of doubly linked
// list in forward direction
void printList (Node *head)
{
	Node *curr = head;
	Node *prev = NULL;
	Node *next;

	cout << "Following are the nodes of Linked List: \n";

	while (curr != NULL)
	{
		// print current node
		cout<<curr->data<<" ";

		// get address of next node: curr->npx is
		// next^prev, so curr->npx^prev will be
		// next^prev^prev which is next
		next = XOR (prev, curr->npx);

		// update prev and curr for next iteration
		prev = curr;
		curr = next;
	}
}

// Driver code
int main ()
{
	/* Create following Doubly Linked List
	head-->40<-->30<-->20<-->10 */
	Node *head = NULL;
	insert(&head, 10);
	insert(&head, 20);
	insert(&head, 30);
	insert(&head, 40);

	// print the created list
	printList (head);

	return (0);
}

Output

  Following are the nodes of Linked List: 
  40 30 20 10

Note that XOR of pointers is not defined by C/C++ standard. So the above implementation may not work on all platforms.

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