class EventEmitter {
  constructor() { this.events = {}; }
  on(event, fn) { (this.events[event] ||= []).push(fn); }
  emit(event, data) { this.events[event]?.forEach(fn => fn(data)); }
}

class Logger {
  log(msg) { console.log(`LOG: ${msg}`); }
}

class Counter {
  constructor() { this.count = 0; }
  increment() { this.count++; console.log(this.count); }
}

function withLogging(target) {
  const logger = new Logger();
  return new Proxy(target, {
    get(obj, prop) {
      if (typeof obj[prop] === 'function') {
        return function(...args) {
          logger.log(`calling ${prop}`);
          return obj[prop].apply(obj, args);
        };
      }
      return obj[prop];
    }
  });
}

const emitter = withLogging(new EventEmitter());
const counter = new Counter();
emitter.on('tick', () => counter.increment());
emitter.emit('tick');
emitter.emit('tick');

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    return `${this.name} makes a sound`;
  }
}

class Dog extends Animal {
  speak() {
    return super.speak() + ' and barks';
  }
}

const pet = new Dog('Rex');
console.log(pet.speak());
console.log(pet instanceof Animal);
console.log(pet.constructor.name);

class DataProcessor {
  constructor(value) {
    this.value = value;
  }
  
  transform(fn) {
    return new DataProcessor(fn(this.value));
  }
  
  getValue() {
    return this.value;
  }
}

const multiply = x => x * 2;
const add = x => x + 10;
const square = x => x * x;

const result = new DataProcessor(5)
  .transform(multiply)
  .transform(add)
  .transform(square)
  .getValue();

console.log(result);

const Flyable = {
  fly() { return 'flying'; }
};

const Swimmable = {
  swim() { return 'swimming'; }
};

function mixin(Base, ...mixins) {
  mixins.forEach(mixin => Object.assign(Base.prototype, mixin));
  return Base;
}

class Bird {}
class Fish {}

const FlyingFish = mixin(class extends Fish {}, Flyable, Swimmable);
const instance = new FlyingFish();
console.log(instance.swim());
console.log(instance.fly());
console.log(instance instanceof Fish);

function createCounter() {
  let count = 0;
  return function(increment = 1) {
    count += increment;
    return count;
  };
}

const counter1 = createCounter();
const counter2 = createCounter();

console.log(counter1());
console.log(counter1(5));
console.log(counter2(3));
console.log(counter1());
console.log(counter2());

const arr = [1, 2, 3, 4, 5];
const obj = { a: 1, b: 2, c: 3 };

const result1 = Object.keys(obj).length;
const result2 = arr.length;

delete obj.b;
obj.d = 4;

const result3 = Object.keys(obj).length;
const result4 = arr.push(6);

arr.length = 3;

const result5 = arr.length;
const result6 = Object.keys(obj).join('');

console.log(result1, result2, result3, result4, result5, result6);

const multiply = (a) => (b) => (c) => a * b * c;

const step1 = multiply(2);
const step2 = step1(3);
const result1 = step2(4);

const partialMult = multiply(5)(6);
const result2 = partialMult(7);

const directResult = multiply(1)(8)(9);

console.log(result1);
console.log(result2);
console.log(directResult);
console.log(typeof step1);
console.log(typeof partialMult);

function mystery(arr, depth = 0) {
  if (arr.length <= 1) return arr;
  
  const mid = Math.floor(arr.length / 2);
  const left = mystery(arr.slice(0, mid), depth + 1);
  const right = mystery(arr.slice(mid), depth + 1);
  
  const result = [];
  let i = 0, j = 0;
  
  while (i < left.length && j < right.length) {
    result.push(left[i] <= right[j] ? left[i++] : right[j++]);
  }
  
  return result.concat(left.slice(i)).concat(right.slice(j));
}

const arr = [3, 1, 4, 1, 5];
console.log(mystery(arr));

const ws = new WeakSet();
const obj1 = { name: 'first' };
const obj2 = { name: 'second' };
const obj3 = obj1;

ws.add(obj1);
ws.add(obj2);
ws.add(obj3);

console.log(ws.has(obj1));
console.log(ws.has(obj3));
console.log(ws.has({ name: 'first' }));
console.log(ws.size);