本文主要是介绍压缩速率追踪，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
Flash Player 11.3添加了一个压缩和解压ByteArray的新方法：LZMA算法。这个算法很有用，因为和现存的zlib和deflate算法相比，LZMA通常能压缩到更小的程度。但这会导致压缩速度减少多少呢？今天这篇文章就是为了找出这个问题的答案。
首先，如果你感兴趣的话，可以很容易切换到LZMA。所有你需要做的就是将CompressionAlgorithm.LZMA传递给ByteArray.compress或ByteArray.uncompress。当然你还需要建立好Flash Player 11.3，并且保证你的用户至少在使用这个版本。
至于你将得到的额外压缩，这取决于你要压缩的数据，但大约比gzip要好50%，可以有效地减少下载时间！因此，在压缩速率这篇文章的测试应用程序的基础上，我们通过下面的测试程序来看看LZMA与zlib和Deflate性能上的区别：
package
{import flash.display.*;import flash.utils.*;import flash.text.*;public class CompressionSpeedFollowup extends Sprite{private var logger:TextField = new TextField();private function row(...cols): void{logger.appendText(cols.join(",") + "");}public function CompressionSpeedFollowup(){logger.autoSize = TextFieldAutoSize.LEFT;addChild(logger);row("Size","deflate (compress)","zlib (compress)","LZMA (compress)","deflate (uncompress)","zlib (uncompress)","LZMA (compress)");runTests("1 KB", 1024, 1024);runTests("1 MB", 1024*1024, 1);}private function runTests(label:String, size:int, reps:int): void{var beforeTime:int;var afterTime:int;var emptyTime:int;var deflateTimeCompress:int;var zlibTimeCompress:int;var lzmaTimeCompress:int;var deflateTimeUncompress:int;var zlibTimeUncompress:int;var lzmaTimeUncompress:int;var bytes:ByteArray = new ByteArray();var originalBytes:ByteArray = new ByteArray();var compressedBytes:ByteArray = new ByteArray();var i:int;var zlib:String = CompressionAlgorithm.ZLIB;var deflate:String = CompressionAlgorithm.DEFLATE;var lzma:String = CompressionAlgorithm.LZMA;fillBytes(originalBytes, size);// EmptybeforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(originalBytes, bytes);}afterTime = getTimer();emptyTime = afterTime - beforeTime;// CompressbeforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(originalBytes, bytes);bytes.compress(deflate);}afterTime = getTimer();deflateTimeCompress = afterTime - beforeTime - emptyTime;beforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(originalBytes, bytes);bytes.compress(zlib);}afterTime = getTimer();zlibTimeCompress = afterTime - beforeTime - emptyTime;beforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(originalBytes, bytes);bytes.compress(lzma);}afterTime = getTimer();lzmaTimeCompress = afterTime - beforeTime - emptyTime;// UncompresscopyBytes(originalBytes, compressedBytes);compressedBytes.compress(deflate);beforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(compressedBytes, bytes);bytes.uncompress(deflate);}afterTime = getTimer();deflateTimeUncompress = afterTime - beforeTime - emptyTime;copyBytes(originalBytes, compressedBytes);compressedBytes.compress(zlib);beforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(compressedBytes, bytes);bytes.uncompress(zlib);}afterTime = getTimer();zlibTimeUncompress = afterTime - beforeTime - emptyTime;copyBytes(originalBytes, compressedBytes);compressedBytes.compress(lzma);beforeTime = getTimer();for (i = 0; i < reps; ++i){copyBytes(compressedBytes, bytes);bytes.uncompress(lzma);}afterTime = getTimer();lzmaTimeUncompress = afterTime - beforeTime - emptyTime;row(label,deflateTimeCompress,zlibTimeCompress,lzmaTimeCompress,deflateTimeUncompress,zlibTimeUncompress,lzmaTimeUncompress);}private function fillBytes(bytes:ByteArray, size:int): void{bytes.length = 0;bytes.position = 0;for (var i:int; i < size; ++i){bytes.writeByte(Math.random()*256);}bytes.position = 0;}private function copyBytes(bytes:ByteArray, into:ByteArray): void{bytes.position = 0;into.position = 0;into.length = 0;into.writeBytes(bytes);bytes.position = 0;into.position = 0;}}
}
我是在以下环境中运行的测试应用程序：
· Flex SDK（MXMLC）4.6.0.23201，在发布模式下编译（没有调试或冗长的堆栈跟踪）
· Flash Player 11.4.402.287
· 2.3 GHz的英特尔酷睿i7处理器
· Mac OS X 10.8.2
下面是我得到的结果：（最下面的LZMA标签改为LZMA（uncompress））
正如图中所描述的，LZMA的压缩和解压速率要比zlib和Deflate慢很多。大约压缩慢了28倍，解压慢了16倍，至少我使用的随机测试数据是这样。然而，与大多数编程任务一样，这里还是有个权衡的。如果你需要提高下载速度的话，你现在就可以使用LZMA压缩，但下载之后就要花费额外的处理能力来解压。这可能是你愿意做的一个权衡了，但你一定要注意上面的结果：你的CPU将遇到麻烦了！
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