src/NodeQueue.js

import Node from './Node' ;

/**
 * NodeQueue#peek only valid if NodeQueue#empty is false.
 * NodeQueue#shift only valid if NodeQueue#empty is false.
 */

export default function NodeQueue ( ) {

	this.front = new Node( null , null ) ;
	this.back = this.front ;

}

NodeQueue.prototype.empty = function ( ) {

	return this.front === this.back ;

} ;

/**
 * Only valid if NodeQueue#empty is false.
 */

NodeQueue.prototype.peek = function ( ) {

	return this.front.next.value ;

} ;

NodeQueue.prototype.push = function ( value ) {

	this.back = this.back.next = new Node( value , null ) ;

} ;

/**
 * Only valid if NodeQueue#empty is false.
 */

NodeQueue.prototype.shift = function ( ) {

	const node = this.front.next ;

	this.front.next = node.next ;

	if ( this.back === node ) this.back = this.front ;

	return node.value ;

} ;