在画布上画不想交的圆

<canvas  id="my_canvas" width="500" height="400">
        your browser does not support canvas
    </canvas>
    <button id="my_btn">Another Circle</button>

var context=document.getElementById("my_canvas");
        context=context.getContext("2d");
    var circles=[];
    var colors=["red","green","blue","pink"];
    var width=500;
    var height=400;
    var max_radius=30;
    var min_radius=20;
    var count=0;
    window.onload=function(){
        var btn=document.getElementById("my_btn");
        btn.onclick=function(){
            var time=new Date();
             start=time.getTime();
            make_circle();
        }
    }
    function Circle(x,y,r){
        this.x=x;
        this.y=y;
        this.r=r;
    }
    function make_circle(){
        var x=Math.floor(Math.random()*width)+1;
        var y=Math.floor(Math.random()*height)+1;
        var r=Math.floor(Math.random()*(max_radius-min_radius))+min_radius;
        var circle=new Circle(x,y,r);
        if(test1(circle)&&test2(circle)){
        circles.push(circle);
        context.strokeStyle="red";
        context.beginPath();
        context.arc(x,y,r,0,Math.PI*2,true);
        context.closePath();
        context.stroke();
        count=0;
        }
        else{
            count++;
            if(count>10000){//if it loops too many times,we can assume that there is no space for new circle
                alert("not any more circles");
                return false;
            }
            make_circle();
        }
    }
    function test1(circle){//test if the new circle intersects with the others
        var len=circles.length;
        for(var i=0;i<len;i++){
            var x1=circles[i].x;
            var y1=circles[i].y;
            var r1=circles[i].r;
            var x2=circle.x;
            var y2=circle.y;
            var r2=circle.r;
            if((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)<(r2+r1)*(r2+r1)){
                return false;
            }
        }
        return true;
    }
    function test2(circle){//test if the new circle touchs the border 
        if((circle.x+circle.r)>width||(circle.y+circle.r)>height||(circle.x-circle.r)<0||(circle.y-circle.r)<0){
            return false;
        }
        else{
            return true;
        }
    }