'use strict';

function init(list) {

list._idleNext = list;

list._idlePrev = list;

}

// create a new linked list

function create() {

const list = { _idleNext: null, _idlePrev: null };

init(list);

return list;

}

// show the most idle item

function peek(list) {

if (list._idlePrev === list) return null;

return list._idlePrev;

}

// remove the most idle item from the list

function shift(list) {

const first = list._idlePrev;

remove(first);

return first;

}

// remove a item from its list

function remove(item) {

if (item._idleNext) {

item._idleNext._idlePrev = item._idlePrev;

}

if (item._idlePrev) {

item._idlePrev._idleNext = item._idleNext;

}

item._idleNext = null;

item._idlePrev = null;

}

// remove a item from its list and place at the end.

function append(list, item) {

if (item._idleNext || item._idlePrev) {

remove(item);

}

// items are linked with _idleNext -> (older) and _idlePrev -> (newer)

// Note: This linkage (next being older) may seem counter-intuitive at first.

item._idleNext = list._idleNext;

item._idlePrev = list;

// the list _idleNext points to tail (newest) and _idlePrev to head (oldest)

list._idleNext._idlePrev = item;

list._idleNext = item;

}

function isEmpty(list) {

return list._idleNext === list;

}

module.exports = {

init,

create,

peek,

shift,

remove,

append,

isEmpty

};