// Simple doubly linked list (https://en.wikipedia.org/wiki/Doubly_linked_list) implementation
// used for queues. This implementation assumes that the node provided by the user can be modified
// to adjust the next and last properties. We implement only the minimal functionality
// for queue support.
export default function DLL() {
    this.head = this.tail = null;
    this.length = 0;
}

function setInitial(dll, node) {
    dll.length = 1;
    dll.head = dll.tail = node;
}

DLL.prototype.removeLink = function(node) {
    if (node.prev) node.prev.next = node.next;
    else this.head = node.next
    if (node.next) node.next.prev = node.prev;
    else this.tail = node.prev;

    node.prev = node.next = null;
    this.length -= 1;
    return node;
}

DLL.prototype.empty = DLL;

DLL.prototype.insertAfter = function(node, newNode) {
    newNode.prev = node;
    newNode.next = node.next;
    if (node.next) node.next.prev = newNode;
    else this.tail = newNode;
    node.next = newNode;
    this.length += 1;
}

DLL.prototype.insertBefore = function(node, newNode) {
    newNode.prev = node.prev;
    newNode.next = node;
    if (node.prev) node.prev.next = newNode;
    else this.head = newNode;
    node.prev = newNode;
    this.length += 1;
}

DLL.prototype.unshift = function(node) {
    if (this.head) this.insertBefore(this.head, node);
    else setInitial(this, node);
};

DLL.prototype.push = function(node) {
    if (this.tail) this.insertAfter(this.tail, node);
    else setInitial(this, node);
};

DLL.prototype.shift = function() {
    return this.head && this.removeLink(this.head);
};

DLL.prototype.pop = function() {
    return this.tail && this.removeLink(this.tail);
};