Question

我刚刚浏览了three.js github页面上的示例源代码，我遇到了这个ImprovedNoise类，它基本上是一个Perlin噪声脚本：

https://github.com/mrdoob/three.js/blob/master/examples/js/ImprovedNoise.js

在ImprovedNoise函数的最顶层是：

var p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10,
         23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87,
         174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211,
         133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208,
         89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5,
         202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119,
         248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232,
         178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249,
         14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205,
         93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180];

for (var i=0; i < 256 ; i++) {

    p[256+i] = p[i];

}

你会注意到p填充了一个随机排序的数字0到255的数组。一旦p数组建立，脚本就会在for循环遍历每个位置。数组并有效地从位置256到511锁存自身的副本。顺序相同，但索引移动了256。

所以我的问题是：在JavaScript中循环这样的数组或者只是简单地执行它是否更快..

p = p.concat(p);

Answer 1

依赖于浏览器：

Firefox 4让concat大约快1/2。
Chrome 11使concat方法的速度提高了10倍。
IE9使concat方法的速度大约为3/4
Safari 5使concat方法的速度提高了1/3。
Opera 11使concat方法的速度提高了1/4。

亲自试试：

http://jsperf.com/concat-vs-forloop

Answer 2

好吧，我要为你准备好时间。将编辑以包含更多浏览器（1000000执行任一代码）。也很好的浏览器测试！机器规格是win7 64bit上的i5 430M和4GB RAM

测试代码：

<script>

function do1(){
p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10,
         23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87,
         174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211,
         133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208,
         89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5,
         202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119,
         248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232,
         178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249,
         14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205,
         93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180];
for (var i=0; i < 256 ; i++) {

    p[256+i] = p[i];

}
}

function do2(){
p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10,
         23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87,
         174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211,
         133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208,
         89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5,
         202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119,
         248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232,
         178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249,
         14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205,
         93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180];
p = p.concat(p);
}

function timeit(func){
var date1 = new Date(); 
var start = date1.getTime();
for (i=0;i<1000000;i++) func();
var date2 = new Date();
var end = date2.getTime();
alert(end-start);
}

timeit(do1);
//timeit(do2); // uncomment to activate

</script>

Chrome 11

方法1（循环）：

4669ms
4809ms
5103ms
5025ms
4786ms

方法2（concat）：

387ms
370ms
494ms
640毫秒
394ms

Opera 11.1（3次测试......需要很长时间）

方法1（循环）

7884 ms
7621 ms
7546 ms

注意：对于我的一次跑步，我得到了＆gt; 98000ms .. IDK发生了什么。

方法2（concat）

27684 ms
28479 ms
23539 ms

IE 9

方法1（循环）

6065ms
6026ms
6214ms

方法2（concat）

8064 ms
8105 ms
7954 ms

Answer 3

您根本不需要复制数组，只需使用p [i％256]来访问其数字成员。

是否将数组连接到自身比循环遍历数组以创建更多索引更快？

3 个答案: