// ─────────────────────────────────────────────────────────────────────────────
// MEDIATOR                                                          [Behavioral]
// ─────────────────────────────────────────────────────────────────────────────
// INTENT  : Define an object that encapsulates how a set of objects interact; promotes loose coupling by
//           keeping objects from referring to each other explicitly.
// PROBLEM : A checkout form: country select resets the region select, zip validates against country, submit
//           enables only when all valid — N widgets each holding references to the others is a hairball.
// SAY     : Participants: Mediator (interface: notify(sender, event)), ConcreteMediator (knows all
//           colleagues, owns the coordination rules), Colleague (talks only to the mediator). Pitch: "hub
//           and spoke — peers never reference each other; all traffic goes through the coordinator."
// SEEN IN : React "lift state up" — a parent coordinating sibling components via props/callbacks; Flux
//           Dispatcher / Redux store as the hub components dispatch into; Socket.io server & rooms (clients
//           never talk peer-to-peer); react-hook-form `control` coordinating registered fields; chat servers.
// VS      : Observer = one Subject broadcasting to anonymous listeners; Mediator = many colleagues talking
//           THROUGH one object that holds the LOGIC (often built with Observer underneath). Facade is one-way
//           simplification of a subsystem; Mediator is two-way coordination among peers.
// ─────────────────────────────────────────────────────────────────────────────
'use strict';
const assert = require('node:assert/strict');

// ── Classic shape (classes, so the participant names are visible)
class Widget {                                            // Colleague base — knows only its mediator
  constructor(name) { this.name = name; this.mediator = null; this.enabled = true; }
  changed(event, payload) { this.mediator?.notify(this, event, payload); }
}
class Select extends Widget {
  constructor(name, options = []) { super(name); this.options = options; this.value = null; }
  setOptions(opts) { this.options = opts; this.value = null; }
  select(v) { this.value = v; this.changed('select', v); }
}
class TextInput extends Widget {
  constructor(name) { super(name); this.value = ''; this.error = null; }
  type(v) { this.value = v; this.changed('input', v); }
}
class Button extends Widget {
  constructor(name) { super(name); this.enabled = false; this.clicks = 0; }
  click() { if (this.enabled) { this.clicks += 1; this.changed('click'); } }
}

class CheckoutFormMediator {                              // ConcreteMediator — ALL cross-widget rules live here
  static regions = { US: ['CA', 'NY'], CA: ['ON', 'BC'] };
  static zipRe = { US: /^\d{5}$/, CA: /^[A-Z]\d[A-Z] ?\d[A-Z]\d$/ };
  constructor({ country, region, zip, submit }) {
    Object.assign(this, { country, region, zip, submit, submitted: [] });
    for (const w of [country, region, zip, submit]) w.mediator = this;
    region.enabled = false;
  }
  notify(sender, event, payload) {
    if (sender === this.country && event === 'select') {
      this.region.setOptions(CheckoutFormMediator.regions[payload] ?? []);
      this.region.enabled = this.region.options.length > 0;
      this.zip.value = ''; this.zip.error = null;
    }
    if (sender === this.zip && event === 'input') {
      const re = CheckoutFormMediator.zipRe[this.country.value];
      this.zip.error = re && !re.test(payload) ? `invalid postal code for ${this.country.value}` : null;
    }
    if (sender === this.submit && event === 'click') {
      this.submitted.push({ country: this.country.value, region: this.region.value, zip: this.zip.value });
    }
    this.submit.enabled = Boolean(this.country.value && this.region.value && this.zip.value && !this.zip.error);
  }
}

const country = new Select('country', ['US', 'CA']);
const region = new Select('region');
const zip = new TextInput('zip');
const submit = new Button('submit');
const form = new CheckoutFormMediator({ country, region, zip, submit });

assert.equal(region.enabled, false);
country.select('US');
assert.deepEqual(region.options, ['CA', 'NY']);          // country reconfigured region — they never met
assert.equal(region.enabled, true);
region.select('NY');
zip.type('1234');
assert.equal(zip.error, 'invalid postal code for US');
assert.equal(submit.enabled, false);
zip.type('10001');
assert.equal(zip.error, null);
assert.equal(submit.enabled, true);
submit.click();
assert.deepEqual(form.submitted, [{ country: 'US', region: 'NY', zip: '10001' }]);
country.select('CA');                                     // switching country resets dependents
assert.equal(region.value, null);
assert.equal(zip.value, '');
assert.equal(submit.enabled, false);
submit.click();
assert.equal(submit.clicks, 1);                           // disabled button ignored the click

// ── Idiomatic JS shape: a chat room — participants hold a handle to the room, never to each other
const createChatRoom = () => {
  const members = new Map();                              // name → inbox
  return {
    join(name) {
      const inbox = [];
      members.set(name, inbox);
      return {
        inbox,
        say: (text) => { for (const [n, box] of members) if (n !== name) box.push(`${name}: ${text}`); },
        dm: (to, text) => {
          if (!members.has(to)) throw new Error(`${to} not in room`);
          members.get(to).push(`${name} (dm): ${text}`);
        },
        leave: () => members.delete(name),
      };
    },
    get size() { return members.size; },
  };
};
const room = createChatRoom();
const ada = room.join('ada');
const bob = room.join('bob');
const cy = room.join('cy');
ada.say('deploy at 5?');
bob.dm('ada', 'yes');
assert.deepEqual(bob.inbox, ['ada: deploy at 5?']);
assert.deepEqual(cy.inbox, ['ada: deploy at 5?']);
assert.deepEqual(ada.inbox, ['bob (dm): yes']);           // sender doesn't receive own broadcast
cy.leave();
bob.say('ok');
assert.equal(cy.inbox.length, 1);                         // departed members stop receiving
assert.throws(() => ada.dm('cy', 'x'), /cy not in room/);
assert.equal(room.size, 2);

console.log('ok: design-patterns/behavioral/mediator.js');