<script>
  
  class Map{
    constructor(){
      this.w = 800;
      this.h = 400;
      this.c = "#ccc";
      this.createEle();
    }
    createEle(){
      this.mapEle = document.createElement("div");
      this.mapEle.style.cssText = `width:${this.w}px;height:${this.h}px;background:${this.c};margin:0 auto;position:relative;border:solid 10px black;`;
      document.body.appendChild(this.mapEle);
    }
  }
  class Food{
    constructor(){
      this.w = 20;
      this.h = 20;
      this.c = "red";
      this.x = 0;
      this.y = 0;
      this.createEle();
    }
    createEle(){
      this.foodEle = document.createElement("div");
      this.foodEle.style.cssText = `width:${this.w}px;height:${this.h}px;background:${this.c};
position:absolute;left:${this.x * this.w}px;top:${this.y * this.h}px;`;
      m.mapEle.appendChild(this.foodEle);
    }
    randomPos(){
      this.x = random(0,(m.w-this.w) / this.w);
      this.y = random(0,(m.h-this.h) / this.h);
      this.foodEle.style.left = this.x * this.w + "px";
      this.foodEle.style.top = this.y * this.h + "px";
    }
  }
  class Snake{
    constructor(){
      this.w = 20;
      this.h = 20;
      this.body = [{
        ele:null,
        x:4,
        y:3,
        c:randomColor()
      },{
        ele:null,
        x:3,
        y:3,
        c:randomColor()
      },{
        ele:null,
        x:2,
        y:3,
        c:randomColor()
      }];
 
      this.d = "right";
 
      this.createEle();
    }
    createEle(){
      for(var i=0;i<this.body.length;i++){
        if(!this.body[i].ele){
          this.body[i].ele = document.createElement("div");
          m.mapEle.appendChild(this.body[i].ele);
        }
        this.body[i].ele.style.cssText = `width:${this.w}px;height:${this.h}px;background:${this.body[i].c};
position:absolute;left:${this.body[i].x * this.w}px;top:${this.body[i].y * this.h}px;`;
      }
      this.body[0].ele.innerHTML = "0";
 
      setTimeout(()=>{
        this.move();
      },300);
    }
    move(){
      for(var i=this.body.length-1; i>0; i--){
        this.body[i].x = this.body[i-1].x;
        this.body[i].y = this.body[i-1].y;
      }
      switch(this.d){
        case "left":
          this.body[0].x -= 1;
          break;
        case "right":
          this.body[0].x += 1;
          break;
        case "top":
          this.body[0].y -= 1;
          break;
        case "bottom":
          this.body[0].y += 1;
          break;
      }
      
      if(this.body[0].x < 0 || this.body[0].y < 0 || this.body[0].x > ((m.w-this.w) / this.w) || this.body[0].y > ((m.h-this.h) / this.h)){
        alert("撞墻了");
        return;
      }
 
      if(this.body[0].x === f.x && this.body[0].y === f.y){
        this.body.push({
          ele:null,
          x:this.body[this.body.length-1].x,
          y:this.body[this.body.length-1].y,
          c:randomColor()
        })
        f.randomPos();
      }
 
      for(var i=1;i<this.body.length;i++){
        if(this.body[0].x == this.body[i].x && this.body[0].y == this.body[i].y){
          alert("撞到自己了");
          return;
        }
      }
 
      this.createEle();
    }
    direct(type){
      
      switch(type){
        case 37:
          if(this.d === "right") break;
          this.d = "left";
          break;
        case 38:
          if(this.d === "bottom") break;
          this.d = "top";
          break;
        case 39:
          if(this.d === "left") break;
          this.d = "right";
          break;
        case 40:
          if(this.d === "top") break;
          this.d = "bottom";
          break;
      }
    }
  }
 
  function random(a,b){
    return Math.round(Math.random()*(a-b)+b)
  }
  function randomColor(){
    return `rgb(${random(0,255)},${random(0,255)},${random(0,255)})`
  }
 
  var m = new Map();
  
  var f = new Food();
  
    f.randomPos();
 
  var s = new Snake();
  document.onkeydown = function(eve){
    var e = eve || window.event;
    var code = e.keyCode || e.which;
    s.direct(code);
  }
 
</script>