/*
 * Simple doubly linked list implementation derived from Cormen, et al.,
 * "Introduction to Algorithms".
 */

module.exports = List;

function List() {
  var sentinel = {};
  sentinel._next = sentinel._prev = sentinel;
  this._sentinel = sentinel;
}

List.prototype.dequeue = function() {
  var sentinel = this._sentinel;
  var entry = sentinel._prev;
  if (entry !== sentinel) {
    unlink(entry);
    return entry;
  }
};

List.prototype.enqueue = function(entry) {
  var sentinel = this._sentinel;
  if (entry._prev && entry._next) {
    unlink(entry);
  }
  entry._next = sentinel._next;
  sentinel._next._prev = entry;
  sentinel._next = entry;
  entry._prev = sentinel;
};

List.prototype.toString = function() {
  var strs = [];
  var sentinel = this._sentinel;
  var curr = sentinel._prev;
  while (curr !== sentinel) {
    strs.push(JSON.stringify(curr, filterOutLinks));
    curr = curr._prev;
  }
  return "[" + strs.join(", ") + "]";
};

function unlink(entry) {
  entry._prev._next = entry._next;
  entry._next._prev = entry._prev;
  delete entry._next;
  delete entry._prev;
}

function filterOutLinks(k, v) {
  if (k !== "_next" && k !== "_prev") {
    return v;
  }
}