Live Note

Remain optimistic

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codePointAt()

JavaScript-16 的格式储存，每个字符固定为两个字节。需要四个字节储存的字符，JavaScript 会认为那是两个字符。
ES6 提供了 codePointAt 方法，能够正确处理四个字节储存的字符，返回一个字符的码点。

String.fromCodePoint()

识别 UTF-32 字符。

includes()、startsWith()、endsWith()

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var s = "hello world"
s.startWith("hello") //true
s.endsWith("d") //true
s.includes("o") //true

s.startsWith("world", 6) //true;
s.includes("hello", 6) //false

repeat()

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"x".repeat(3) //'xxx'
"na".repeat(0) //''

"na".repeat(2.9) //取整 -> 'nana'

"na".repeat(Infinity) //Error
"na".repeat(-1) //Error

"na".repeat(-0.9) //''
"na".repeat(NaN) //''
"na".repeat("3") //'nanana'
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尾调用

Tail Call 是函数式编程的一个重要概念，就是指某个函数的最后一步是调用另一个函数。

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function f(x) {
  return g(x)
}

尾调用不一定出现在函数尾部，只要是最后一步操作即可。

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function f(x) {
  if (x > 0) return t(x)
  return m(x)
}

尾调用优化

函数调用会在内存中形成一个 call frame，保存调用位置和内部变量等信息。所有的 call frame 形成一个 call stack。
尾调用由于是函数的最后一步操作，所以不需要保留外层函数的 call frame，这就叫 Tail Call Optimization，即只保留内层函数的调用帧。

尾递归

递归非常耗内存，因为需要同时保存多个 call frame，很容易发生 stack overflow。但对于尾递归来说，由于只存在一个 call frame，所以不会发生溢出。

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function Fibonacci(n) {
  if (n <= 1) return 1
  return Fibonacci(n - 1) + Fibonacci(n - 2)
}
console.log(Fibonacci(10)) //89
console.log(Fibonacci(100)) //overflow
console.log(Fibonacci(500)) //overflow
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工厂模式

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let Basketball = function () {
  this.name = 'basketball'
}
Basketball.prototype = {
  getMember: function () {
    console.log(5)
  },
  getBallSize: function () {
    console.log('big')
  },
}

let FootBall = function () {
  this.name = 'football'
}
FootBall.prototype = {
  getMember: function () {
    console.log(11)
  },
  getBallSize: function () {
    console.log('big')
  },
}

let Tennis = function () {
  this.name = 'tennis'
}
Tennis.prototype = {
  getMember: function () {
    console.log(1)
  },
  getBallSize: function () {
    console.log('small')
  },
}

let SportFactory = function (name) {
  switch (name) {
    case 'basketball':
      return new Basketball()
    case 'football':
      return new FootBall()
    case 'tennis':
      return new Tennis()
  }
}

let footBall = new SportFactory('football')
footBall.getMember()
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// Product base attribute
interface Product {
  operation(): string
}

class ConcreteProduct1 implements Product {
  public operation(): string {
    return 'This is Product1'
  }
}
class ConcreteProduct2 implements Product {
  public operation(): string {
    return 'This is Product2'
  }
}

abstract class Creator {
  public abstract factoryMethod(): Product

  public operation(): string {
    const product = this.factoryMethod()

    return product.operation()
  }
}

class ConcreteCreator1 extends Creator {
  public factoryMethod(): Product {
    return new ConcreteProduct1()
  }
}
class ConcreteCreator2 extends Creator {
  public factoryMethod(): Product {
    return new ConcreteProduct2()
  }
}

// usage
function clientCode(creator: Creator) {
  console.log(creator.operation())
}

clientCode(new ConcreteCreator1())
clientCode(new ConcreteCreator2())

安全工厂

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let Factory = function (type, content) {
  if (this instanceof Factory) {
    // check if is instance Factory
    return new this[type](content)
  } else {
    return new Factory(type, content)
  }
}
Factory.prototype = {
  Java: function (content) {
    console.log(content)
  },
  JavaScript: function (content) {
    console.log(content)
  },
}

let data = [
  {
    type: 'Java',
    content: 'This is Java',
  },
  {
    type: 'JavaScript',
    content: 'This is JavaScript',
  },
]

// for test
for (let i = 0; i < data.length; i++) Factory(data[i].type, data[i].content)

抽象工厂

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/**
 * to extend superType
 * @param {*} subType
 * @param {*} superType
 */
let VehicleFactory = (subType, superType) => {
  if (typeof VehicleFactory[superType] === 'function') {
    function F() {}
    F.prototype = new VehicleFactory[superType]()
    subType.constuctor = subType
    subType.prototype = new F()
  } else {
    throw new Error("don' not have this abstract class")
  }
}

// create base class
VehicleFactory.Car = function () {
  this.type = 'car'
}
VehicleFactory.Car.prototype = {
  getPrice: function () {
    return new Error('abstract')
  },
  getSpeed: function () {
    return new Error('abstract')
  },
}

let BMW = function (price, speed) {
  this.price = price
  this.speed = speed
}
VehicleFactory(BMW, 'Car') // interface
BMW.prototype = {
  getPrice: function () {
    return this.price
  },
  getSpeed: function () {
    return this.speed
  },
}

// for test
let myBmw = new BMW(10000, 100)
console.log(myBmw.getPrice())
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// product interface
interface AbstractProductA {
  functionA(): string
}

interface AbstractProductB {
  /**
   * base operation
   * @return {string}
   */
  functionA(): string

  /**
   * inject some operation
   * @param {AbstractProductA} product
   * @return {string}
   */
  functionB(product: AbstractProductA): string
}

// concrete production
class ConcreteProductA1 implements AbstractProductA {
  functionA(): string {
    return 'This is functionA from ProductA1'
  }
}
class ConcreteProductA2 implements AbstractProductA {
  functionA(): string {
    return 'This is functionA from ProductA2'
  }
}
class ConcreteProductB1 implements AbstractProductB {
  functionA(): string {
    return 'This is functionA from ProductB1'
  }

  functionB(product: AbstractProductA): string {
    return 'Function B2: ' + product.functionA()
  }
}
class ConcreteProductB2 implements AbstractProductB {
  functionA(): string {
    return 'This is functionA from ProductB2'
  }

  functionB(product: AbstractProductA): string {
    return 'Function B2: ' + product.functionA()
  }
}

// factory interface
interface AbstractFactory {
  createProductA(): AbstractProductA
  createProductB(): AbstractProductB
}

// concrete factory
class ConcreteFactory1 implements AbstractFactory {
  createProductA(): AbstractProductA {
    return new ConcreteProductA1()
  }

  createProductB(): AbstractProductB {
    return new ConcreteProductB1()
  }
}
class ConcreteFactory2 implements AbstractFactory {
  createProductA(): AbstractProductA {
    return new ConcreteProductA2()
  }

  createProductB(): AbstractProductB {
    return new ConcreteProductB2()
  }
}

// usage
/**
 * use with different facotry.
 * @param {AbstractFactory} factory
 */
function clientCode(factory: AbstractFactory) {
  const productA = factory.createProductA()
  const productB = factory.createProductB()

  console.log(productB.functionA())
  console.log(productB.functionB(productA))
}

clientCode(new ConcreteFactory1())
clientCode(new ConcreteFactory2())

Posted on Edited on

又一次的见证历史。

卡西莫多失去了他心爱的姑娘，也失去了他心爱的钟楼。

大火照片1

大火照片2

大火照片3

废墟中的十字架依旧闪发着光芒

废墟

PS： 看到一条推，说你看大火都没有把十字架给损毁，难道你还不相信上帝吗？下面回：因为金的熔点比木高很多。XD

Posted on Edited on In

并查集

用集合中的某个元素来代表这个集合，该元素称为集合的代表元。
一个集合内的所有元素组织成以代表元为根的树形结构。
对于每一个元素parent[x]指向 x 在树形结构上的父亲节点。如果 x 是根节点，则令parent[x] = x。
对于查找操作，假设需要确定 x 所在的的集合，也就是确定集合的代表元。可以沿着parent[x]不断在树形结构中向上移动，直到到达根节点。

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