src/LazyStack.js

import LazyNode from './LazyNode.js';

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

export default function LazyStack() {
	this.top = null;
	this.bottom = null;
}

LazyStack.prototype.empty = function () {
	return this.top === null;
};

LazyStack.prototype.push = function (value) {
	this.top = new LazyNode(value, this.top);

	if (this.bottom === null) this.bottom = this.top;
};

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

LazyStack.prototype.pop = function () {
	const value = this.top.value;

	this.top = this.top.next;

	if (this.top === null) this.bottom = null;

	return value;
};

LazyStack.prototype.meld = function (other) {
	if (this.bottom === null) this.top = other.top;
	else this.bottom.next = other.top;

	this.bottom = other.bottom;
};