将数组连接到自身是否比循环数组创建更多索引更快？

Question

我只是浏览了 Three.js github 页面上示例的源代码，我发现了这个 ImprovementNoise 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函数的最顶部是这样的：

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 填充有数字 0 到 255 的随机排序数组。建立 p 数组后，脚本会对数组中的每个位置执行 for 循环并有效地从位置 256 到 511 锁存自身的副本。顺序是相同的，但索引移动了 256。

所以我的问题是：在 JavaScript 中循环这样的数组更快还是简单地执行...

p = p.concat(p);

Answer 1

它取决于浏览器：

• Firefox 4 的 concat 速度大约是 1/2。
• Chrome 11 使 concat 方法速度提高了 10 倍。
• IE9 给出的 concat 方法的速度约为 3/4，
• Safari 5 给出的 concat 方法的速度约为 1/3。
• Opera 11 给出的 concat 方法速度大约为 1/4。

亲自尝试一下：

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

Answer 2

好的，我会为你安排时间。将进行编辑以包含更多浏览器（任一代码执行 1000000 次）。浏览器测试也很好！机器规格为 i5 430M 和 win7 64 位上的 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（连续）：

• 387ms
• 370ms
• 494ms
• 640ms
• 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：对于我的一次运行，我得到了 > 98000ms ..不知道发生了什么。

方法 2（连续）

• 27684 ms
• 28479 ms
• 23539 ms

IE 9

方法 1（循环）

• 6065ms
• 6026ms
• 6214ms

方法 2（连续）

• 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] 即可访问其数字成员。