constructor() {

this.head = null;

this.tail = null;

this.length = 0;

}

push(v) {

const entry = { data: v, next: null };

if (this.length > 0)

this.tail.next = entry;

else

this.head = entry;

this.tail = entry;

++this.length;

}

unshift(v) {

const entry = { data: v, next: this.head };

if (this.length === 0)

this.tail = entry;

this.head = entry;

++this.length;

}

shift() {

if (this.length === 0)

return;

const ret = this.head.data;

if (this.length === 1)

this.head = this.tail = null;

else

this.head = this.head.next;

--this.length;

return ret;

}

clear() {

this.head = this.tail = null;

this.length = 0;

}

join(s) {

if (this.length === 0)

return '';

var p = this.head;

var ret = '' + p.data;

while (p = p.next)

ret += s + p.data;

return ret;

}

concat(n) {

if (this.length === 0)

return Buffer.alloc(0);

if (this.length === 1)

return this.head.data;

const ret = Buffer.allocUnsafe(n >>> 0);

var p = this.head;

var i = 0;

while (p) {

p.data.copy(ret, i);

i += p.data.length;

p = p.next;

}

return ret;

}