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Commit 11ae76d

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Badacadabra
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Add Flyweight (ES5 + ES6 + CoffeeScript)
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‎doc/GoF/Structural/Flyweight/README.md

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# Synopsis
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I am a fan of GNU/Linux distributions and I want to test the big names: Debian, RedHat, Slackware. However, I am too busy to test all versions available. The latest version of each distro would be just fine.
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# Problem
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Everytime we use "new", we create a new object in JavaScript. But if we know the instance we need will always be the same (e.g. a very specific version of Debian), this is not optimal to create multiple instances that represent the same thing. Each instantiation operation has a cost in terms of performance and memory which should be minimized as much as possible.
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# Solution
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We can use Flyweight here, which is actually tied to Factory. A Factory handles flyweights when it reuses instances instead of constantly creating new ones. Here we have:
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* An abstract representation of a Linux distro (abstract class or interface)
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* Concrete implementations of this abstraction (Debian, RedHat, Slackware)
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* A "class" with a factory method which will be responsible for instantiation (contrary to a simple Factory method, some extra code is required to test if the wanted instance already exists or not)
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"use strict"
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# ==============================
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# ABSTRACT GNU/LINUX DISTRO
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# ==============================
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class LinuxDistro
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constructor: (name) ->
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throw new Error "You cannot instantiate an abstract class!" if @constructor is LinuxDistro
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@name = name
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boot: ->
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"#{@name} is booting..." # string interpolation with template literal
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# ==============================
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# CONCRETE GNU/LINUX DISTROS
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# ==============================
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# Let's say that each instance of a Linux distro will have the name of its constructor...
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class Debian extends LinuxDistro
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constructor: ->
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super "Debian"
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class RedHat extends LinuxDistro
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constructor: ->
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super "RedHat"
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class Slackware extends LinuxDistro
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constructor: ->
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super "Slackware"
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# ==============================
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# FACTORY OF GNU/LINUX DISTROS
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# ==============================
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class LinuxDistrosFactory
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@DEBIAN: 0
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@REDHAT: 1
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@SLACKWARE: 2
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# Flyweights!
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@activeDistros: Object.create(null) # new object without prototype, like a basic map
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@getLinuxDistro: (id) ->
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# If the distro has never been instantiated, we will have to create a new object
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unless @activeDistros[id]
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switch id
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when @DEBIAN then @activeDistros[id] = new Debian()
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when @REDHAT then @activeDistros[id] = new RedHat()
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when @SLACKWARE then @activeDistros[id] = new Slackware()
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else throw new Error "The Linux distribution you are looking for has not been found"
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# If the distro has already been instantiated, we return the initial instance
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@activeDistros[id];
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# ==============================
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# CLIENT CODE
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# ==============================
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# Creation of our objects through the factory
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debian = LinuxDistrosFactory.getLinuxDistro LinuxDistrosFactory.DEBIAN
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debianAgain = LinuxDistrosFactory.getLinuxDistro LinuxDistrosFactory.DEBIAN
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console.log debian.boot() # Debian is booting...
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console.log debianAgain.boot() # Debian is booting...
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console.log debian is debianAgain # true (the same instance has been reused)
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'use strict';
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// ==============================
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// ABSTRACT GNU/LINUX DISTRO
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// ==============================
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var LinuxDistro = (function () {
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function LinuxDistro(name) {
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if (this.constructor === LinuxDistro) {
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throw new Error("You cannot instantiate an abstract class!");
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}
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this.name = name;
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}
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LinuxDistro.prototype.boot = function () {
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return this.name + " is booting...";
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};
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return LinuxDistro;
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})();
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// ==============================
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// CONCRETE GNU/LINUX DISTROS
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// ==============================
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// Let's say that each instance of a Linux distro will have the name of its constructor...
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var Debian = (function () {
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function Debian() {
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LinuxDistro.call(this, "Debian");
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}
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Debian.prototype = Object.create(LinuxDistro.prototype);
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Debian.prototype.constructor = Debian;
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return Debian;
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})();
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var RedHat = (function () {
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function RedHat() {
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LinuxDistro.call(this, "RedHat");
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}
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RedHat.prototype = Object.create(LinuxDistro.prototype);
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RedHat.prototype.constructor = RedHat;
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return RedHat;
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})();
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var Slackware = (function () {
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function Slackware() {
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LinuxDistro.call(this, "Slackware");
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}
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Slackware.prototype = Object.create(LinuxDistro.prototype);
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Slackware.prototype.constructor = Slackware;
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return Slackware;
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})();
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// ==============================
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// FACTORY OF GNU/LINUX DISTROS
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// ==============================
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var LinuxDistrosFactory = (function () {
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function LinuxDistrosFactory() {}
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// The only way to declare real constants in ES5 (non writable and non configurable)
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Object.defineProperties(LinuxDistrosFactory, {
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"DEBIAN": {
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value: 0,
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enumerable: true
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},
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"REDHAT": {
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value: 1,
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enumerable: true
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},
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"SLACKWARE": {
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value: 2,
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enumerable: true
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},
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"activeDistros": { // Flyweights!
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value: Object.create(null), // new object without prototype, like a basic map
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enumerable: true
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}
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});
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// Static method to get an instance of the given type
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LinuxDistrosFactory.getLinuxDistro = function (id) {
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// If the distro has never been instantiated, we will have to create a new object
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if (!this.activeDistros[id]) {
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switch (id) {
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case this.DEBIAN:
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this.activeDistros[id] = new Debian();
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break;
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case this.REDHAT:
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this.activeDistros[id] = new RedHat();
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break;
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case this.SLACKWARE:
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this.activeDistros[id] = new Slackware();
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break;
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default:
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throw new Error("The Linux distribution you are looking for has not been found");
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}
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}
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// If the distro has already been instantiated, we return the initial instance
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return this.activeDistros[id];
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};
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return LinuxDistrosFactory;
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})();
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// ==============================
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// CLIENT CODE
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// ==============================
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// Creation of our objects through the factory
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var debian = LinuxDistrosFactory.getLinuxDistro(LinuxDistrosFactory.DEBIAN),
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debianAgain = LinuxDistrosFactory.getLinuxDistro(LinuxDistrosFactory.DEBIAN);
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console.log(debian.boot()); // Debian is booting...
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console.log(debianAgain.boot()); // Debian is booting...
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console.log(debian === debianAgain); // true (the same instance has been reused)
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// ==============================
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// ABSTRACT GNU/LINUX DISTRO
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// ==============================
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class LinuxDistro {
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constructor(name) {
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if (new.target !== undefined) {
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throw new Error("You cannot instantiate an abstract class!");
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}
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this._name = name;
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}
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boot() {
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return `${this._name} is booting...`; // string interpolation with template literal
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}
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}
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// ==============================
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// CONCRETE GNU/LINUX DISTROS
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// ==============================
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// Let's say that each instance of a Linux distro will have the name of its constructor...
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class Debian extends LinuxDistro {
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constructor() {
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super(Debian.name);
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}
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}
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class RedHat extends LinuxDistro {
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constructor() {
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super(RedHat.name);
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}
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}
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class Slackware extends LinuxDistro {
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constructor() {
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super(Slackware.name);
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}
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}
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// ==============================
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// FACTORY OF GNU/LINUX DISTROS
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// ==============================
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class LinuxDistrosFactory {
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// We cannot use "const" inside a class in ES6.
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// Only methods are supported.
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static get DEBIAN() { return 0; }
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static get REDHAT() { return 1; }
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static get SLACKWARE() { return 2; }
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static getLinuxDistro(id) {
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// If the distro has never been instantiated, we will have to create a new object
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if (!this.activeDistros.has(id)) {
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switch (id) {
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case this.DEBIAN:
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this.activeDistros.set(id, new Debian());
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break;
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case this.REDHAT:
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this.activeDistros.set(id, new RedHat());
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break;
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case this.SLACKWARE:
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this.activeDistros.set(id, new Slackware());
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break;
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default:
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throw new Error("The Linux distribution you are looking for has not been found");
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}
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}
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// If the distro has already been instantiated, we return the initial instance
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return this.activeDistros.get(id);
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}
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}
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LinuxDistrosFactory.activeDistros = new Map(); // Flyweights!
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// ==============================
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// CLIENT CODE
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// ==============================
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// Creation of our objects through the factory
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let debian = LinuxDistrosFactory.getLinuxDistro(LinuxDistrosFactory.DEBIAN),
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debianAgain = LinuxDistrosFactory.getLinuxDistro(LinuxDistrosFactory.DEBIAN);
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console.log(debian.boot()); // Debian is booting...
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console.log(debianAgain.boot()); // Debian is booting...
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console.log(debian === debianAgain); // true (the same instance has been reused)

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