/**
 * Created by fangjiejie on 2017/8/19.
 */
"use strict";

var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    w = canvas.width = window.innerWidth,
    h = canvas.height = window.innerHeight,

    hue = 217,
    stars = [],
    count = 0,
    maxStars = 1400;

// Cache gradient
var canvas2 = document.createElement('canvas'),
    ctx2 = canvas2.getContext('2d');
canvas2.width = 100;
canvas2.height = 100;
var half = canvas2.width/2,
    gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
gradient2.addColorStop(0.025, '#fff');
gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
gradient2.addColorStop(1, 'transparent');

ctx2.fillStyle = gradient2;
ctx2.beginPath();
ctx2.arc(half, half, half, 0, Math.PI * 2);
ctx2.fill();

// End cache

function random(min, max) {
    if (arguments.length < 2) {
        max = min;
        min = 0;
    }

    if (min > max) {
        var hold = max;
        max = min;
        min = hold;
    }

    return Math.floor(Math.random() * (max - min + 1)) + min;
}

var Star = function() {

    this.orbitRadius = random(w / 2 - 50);
    this.radius = random(100, this.orbitRadius) / 10;
    this.orbitX = w / 2;
    this.orbitY = h / 2;
    this.timePassed = random(0, maxStars);
    this.speed = random(this.orbitRadius) / 100000;
    this.alpha = random(2, 10) / 10;

    count++;
    stars[count] = this;
}

Star.prototype.draw = function() {
    var x = Math.sin(this.timePassed + 1) * this.orbitRadius + this.orbitX,
        y = Math.cos(this.timePassed) * this.orbitRadius/2 + this.orbitY,
        twinkle = random(10);

    if (twinkle === 1 && this.alpha > 0) {
        this.alpha -= 0.05;
    } else if (twinkle === 2 && this.alpha < 1) {
        this.alpha += 0.05;
    }

    ctx.globalAlpha = this.alpha;
    ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
    this.timePassed += this.speed;
}

for (var i = 0; i < maxStars; i++) {
    new Star();
}

function animation() {
    ctx.globalCompositeOperation = 'source-over';
    ctx.globalAlpha = 0.8;
    ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
    ctx.fillRect(0, 0, w, h)

    ctx.globalCompositeOperation = 'lighter';
    for (var i = 1, l = stars.length; i < l; i++) {
        stars[i].draw();
    };

    window.requestAnimationFrame(animation);
}

animation();